organize std lib concurrency primitives and add RwLock

 * move concurrency primitives that always operate on kernel threads to
   the std.Thread namespace
 * remove std.SpinLock. Nobody should use this in a non-freestanding
   environment; the other primitives are always preferable. In
   freestanding, it will be necessary to put custom spin logic in there,
   so there are no use cases for a std lib version.
 * move some std lib files to the top level fields convention
 * add std.Thread.spinLoopHint
 * add std.Thread.Condition
 * add std.Thread.Semaphore
 * new implementation of std.Thread.Mutex for Windows and non-pthreads Linux
 * add std.Thread.RwLock

Implementations provided by @kprotty

34 files changed, 1746 insertions, 1261 deletions

diff --git a/lib/std/Progress.zig b/lib/std/Progress.zig
index ca9fb8ea1f..b75ad106a4 100644
--- a/lib/std/Progress.zig
+++ b/lib/std/Progress.zig
@@ -50,7 +50,7 @@ done: bool = true,
 /// Protects the `refresh` function, as well as `node.recently_updated_child`.
 /// Without this, callsites would call `Node.end` and then free `Node` memory
 /// while it was still being accessed by the `refresh` function.
-update_lock: std.Mutex = .{},
+update_lock: std.Thread.Mutex = .{},
 
 /// Keeps track of how many columns in the terminal have been output, so that
 /// we can move the cursor back later.
diff --git a/lib/std/SpinLock.zig b/lib/std/SpinLock.zig
deleted file mode 100644
index a16cfa930b..0000000000
--- a/lib/std/SpinLock.zig
+++ /dev/null
@@ -1,86 +0,0 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-//! A mutually exclusive lock that grinds the CPU rather than interacting with
-//! the operating system. It does however yield to the OS scheduler while
-//! spinning, when targeting an OS that supports it.
-//! This struct can be initialized directly and statically initialized. The
-//! default state is unlocked.
-
-state: State = State.Unlocked,
-
-const std = @import("std.zig");
-const builtin = @import("builtin");
-const SpinLock = @This();
-
-const State = enum(u8) {
-    Unlocked,
-    Locked,
-};
-
-pub const Held = struct {
-    spinlock: *SpinLock,
-
-    pub fn release(self: Held) void {
-        @atomicStore(State, &self.spinlock.state, .Unlocked, .Release);
-    }
-};
-
-pub fn tryAcquire(self: *SpinLock) ?Held {
-    return switch (@atomicRmw(State, &self.state, .Xchg, .Locked, .Acquire)) {
-        .Unlocked => Held{ .spinlock = self },
-        .Locked => null,
-    };
-}
-
-pub fn acquire(self: *SpinLock) Held {
-    while (true) {
-        return self.tryAcquire() orelse {
-            yield();
-            continue;
-        };
-    }
-}
-
-pub fn yield() void {
-    // On native windows, SwitchToThread is too expensive,
-    // and yielding for 380-410 iterations was found to be
-    // a nice sweet spot. Posix systems on the other hand,
-    // especially linux, perform better by yielding the thread.
-    switch (builtin.os.tag) {
-        .windows => loopHint(400),
-        else => std.os.sched_yield() catch loopHint(1),
-    }
-}
-
-/// Hint to the cpu that execution is spinning
-/// for the given amount of iterations.
-pub fn loopHint(iterations: usize) void {
-    var i = iterations;
-    while (i != 0) : (i -= 1) {
-        switch (builtin.arch) {
-            // these instructions use a memory clobber as they
-            // flush the pipeline of any speculated reads/writes.
-            .i386, .x86_64 => asm volatile ("pause"
-                :
-                :
-                : "memory"
-            ),
-            .arm, .aarch64 => asm volatile ("yield"
-                :
-                :
-                : "memory"
-            ),
-            else => std.os.sched_yield() catch {},
-        }
-    }
-}
-
-test "basic usage" {
-    var lock: SpinLock = .{};
-
-    const held = lock.acquire();
-    defer held.release();
-}
diff --git a/lib/std/Thread.zig b/lib/std/Thread.zig
new file mode 100644
index 0000000000..f878f43539
--- /dev/null
+++ b/lib/std/Thread.zig
@@ -0,0 +1,558 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! This struct represents a kernel thread, and acts as a namespace for concurrency
+//! primitives that operate on kernel threads. For concurrency primitives that support
+//! both evented I/O and async I/O, see the respective names in the top level std namespace.
+
+data: Data,
+
+pub const AutoResetEvent = @import("Thread/AutoResetEvent.zig");
+pub const ResetEvent = @import("Thread/ResetEvent.zig");
+pub const StaticResetEvent = @import("Thread/StaticResetEvent.zig");
+pub const Mutex = @import("Thread/Mutex.zig");
+pub const Semaphore = @import("Thread/Semaphore.zig");
+pub const Condition = @import("Thread/Condition.zig");
+
+pub const use_pthreads = std.Target.current.os.tag != .windows and builtin.link_libc;
+
+const Thread = @This();
+const std = @import("std.zig");
+const builtin = std.builtin;
+const os = std.os;
+const mem = std.mem;
+const windows = std.os.windows;
+const c = std.c;
+const assert = std.debug.assert;
+
+const bad_startfn_ret = "expected return type of startFn to be 'u8', 'noreturn', 'void', or '!void'";
+
+/// Represents a kernel thread handle.
+/// May be an integer or a pointer depending on the platform.
+/// On Linux and POSIX, this is the same as Id.
+pub const Handle = if (use_pthreads)
+    c.pthread_t
+else switch (std.Target.current.os.tag) {
+    .linux => i32,
+    .windows => windows.HANDLE,
+    else => void,
+};
+
+/// Represents a unique ID per thread.
+/// May be an integer or pointer depending on the platform.
+/// On Linux and POSIX, this is the same as Handle.
+pub const Id = switch (std.Target.current.os.tag) {
+    .windows => windows.DWORD,
+    else => Handle,
+};
+
+pub const Data = if (use_pthreads)
+    struct {
+        handle: Thread.Handle,
+        memory: []u8,
+    }
+else switch (std.Target.current.os.tag) {
+    .linux => struct {
+        handle: Thread.Handle,
+        memory: []align(mem.page_size) u8,
+    },
+    .windows => struct {
+        handle: Thread.Handle,
+        alloc_start: *c_void,
+        heap_handle: windows.HANDLE,
+    },
+    else => struct {},
+};
+
+/// Signals the processor that it is inside a busy-wait spin-loop ("spin lock").
+pub fn spinLoopHint() void {
+    switch (std.Target.current.cpu.arch) {
+        .i386, .x86_64 => asm volatile ("pause"
+            :
+            :
+            : "memory"
+        ),
+        .arm, .aarch64 => asm volatile ("yield"
+            :
+            :
+            : "memory"
+        ),
+        else => {},
+    }
+}
+
+/// Returns the ID of the calling thread.
+/// Makes a syscall every time the function is called.
+/// On Linux and POSIX, this Id is the same as a Handle.
+pub fn getCurrentId() Id {
+    if (use_pthreads) {
+        return c.pthread_self();
+    } else
+        return switch (std.Target.current.os.tag) {
+            .linux => os.linux.gettid(),
+            .windows => windows.kernel32.GetCurrentThreadId(),
+            else => @compileError("Unsupported OS"),
+        };
+}
+
+/// Returns the handle of this thread.
+/// On Linux and POSIX, this is the same as Id.
+/// On Linux, it is possible that the thread spawned with `spawn`
+/// finishes executing entirely before the clone syscall completes. In this
+/// case, this function will return 0 rather than the no-longer-existing thread's
+/// pid.
+pub fn handle(self: Thread) Handle {
+    return self.data.handle;
+}
+
+pub fn wait(self: *Thread) void {
+    if (use_pthreads) {
+        const err = c.pthread_join(self.data.handle, null);
+        switch (err) {
+            0 => {},
+            os.EINVAL => unreachable,
+            os.ESRCH => unreachable,
+            os.EDEADLK => unreachable,
+            else => unreachable,
+        }
+        std.heap.c_allocator.free(self.data.memory);
+        std.heap.c_allocator.destroy(self);
+    } else switch (std.Target.current.os.tag) {
+        .linux => {
+            while (true) {
+                const pid_value = @atomicLoad(i32, &self.data.handle, .SeqCst);
+                if (pid_value == 0) break;
+                const rc = os.linux.futex_wait(&self.data.handle, os.linux.FUTEX_WAIT, pid_value, null);
+                switch (os.linux.getErrno(rc)) {
+                    0 => continue,
+                    os.EINTR => continue,
+                    os.EAGAIN => continue,
+                    else => unreachable,
+                }
+            }
+            os.munmap(self.data.memory);
+        },
+        .windows => {
+            windows.WaitForSingleObjectEx(self.data.handle, windows.INFINITE, false) catch unreachable;
+            windows.CloseHandle(self.data.handle);
+            windows.HeapFree(self.data.heap_handle, 0, self.data.alloc_start);
+        },
+        else => @compileError("Unsupported OS"),
+    }
+}
+
+pub const SpawnError = error{
+    /// A system-imposed limit on the number of threads was encountered.
+    /// There are a number of limits that may trigger this error:
+    /// *  the  RLIMIT_NPROC soft resource limit (set via setrlimit(2)),
+    ///    which limits the number of processes and threads for  a  real
+    ///    user ID, was reached;
+    /// *  the kernel's system-wide limit on the number of processes and
+    ///    threads,  /proc/sys/kernel/threads-max,  was   reached   (see
+    ///    proc(5));
+    /// *  the  maximum  number  of  PIDs, /proc/sys/kernel/pid_max, was
+    ///    reached (see proc(5)); or
+    /// *  the PID limit (pids.max) imposed by the cgroup "process  num‐
+    ///    ber" (PIDs) controller was reached.
+    ThreadQuotaExceeded,
+
+    /// The kernel cannot allocate sufficient memory to allocate a task structure
+    /// for the child, or to copy those parts of the caller's context that need to
+    /// be copied.
+    SystemResources,
+
+    /// Not enough userland memory to spawn the thread.
+    OutOfMemory,
+
+    /// `mlockall` is enabled, and the memory needed to spawn the thread
+    /// would exceed the limit.
+    LockedMemoryLimitExceeded,
+
+    Unexpected,
+};
+
+/// caller must call wait on the returned thread
+/// fn startFn(@TypeOf(context)) T
+/// where T is u8, noreturn, void, or !void
+/// caller must call wait on the returned thread
+pub fn spawn(context: anytype, comptime startFn: anytype) SpawnError!*Thread {
+    if (builtin.single_threaded) @compileError("cannot spawn thread when building in single-threaded mode");
+    // TODO compile-time call graph analysis to determine stack upper bound
+    // https://github.com/ziglang/zig/issues/157
+    const default_stack_size = 16 * 1024 * 1024;
+
+    const Context = @TypeOf(context);
+    comptime assert(@typeInfo(@TypeOf(startFn)).Fn.args[0].arg_type.? == Context);
+
+    if (std.Target.current.os.tag == .windows) {
+        const WinThread = struct {
+            const OuterContext = struct {
+                thread: Thread,
+                inner: Context,
+            };
+            fn threadMain(raw_arg: windows.LPVOID) callconv(.C) windows.DWORD {
+                const arg = if (@sizeOf(Context) == 0) {} else @ptrCast(*Context, @alignCast(@alignOf(Context), raw_arg)).*;
+
+                switch (@typeInfo(@typeInfo(@TypeOf(startFn)).Fn.return_type.?)) {
+                    .NoReturn => {
+                        startFn(arg);
+                    },
+                    .Void => {
+                        startFn(arg);
+                        return 0;
+                    },
+                    .Int => |info| {
+                        if (info.bits != 8) {
+                            @compileError(bad_startfn_ret);
+                        }
+                        return startFn(arg);
+                    },
+                    .ErrorUnion => |info| {
+                        if (info.payload != void) {
+                            @compileError(bad_startfn_ret);
+                        }
+                        startFn(arg) catch |err| {
+                            std.debug.warn("error: {s}\n", .{@errorName(err)});
+                            if (@errorReturnTrace()) |trace| {
+                                std.debug.dumpStackTrace(trace.*);
+                            }
+                        };
+                        return 0;
+                    },
+                    else => @compileError(bad_startfn_ret),
+                }
+            }
+        };
+
+        const heap_handle = windows.kernel32.GetProcessHeap() orelse return error.OutOfMemory;
+        const byte_count = @alignOf(WinThread.OuterContext) + @sizeOf(WinThread.OuterContext);
+        const bytes_ptr = windows.kernel32.HeapAlloc(heap_handle, 0, byte_count) orelse return error.OutOfMemory;
+        errdefer assert(windows.kernel32.HeapFree(heap_handle, 0, bytes_ptr) != 0);
+        const bytes = @ptrCast([*]u8, bytes_ptr)[0..byte_count];
+        const outer_context = std.heap.FixedBufferAllocator.init(bytes).allocator.create(WinThread.OuterContext) catch unreachable;
+        outer_context.* = WinThread.OuterContext{
+            .thread = Thread{
+                .data = Thread.Data{
+                    .heap_handle = heap_handle,
+                    .alloc_start = bytes_ptr,
+                    .handle = undefined,
+                },
+            },
+            .inner = context,
+        };
+
+        const parameter = if (@sizeOf(Context) == 0) null else @ptrCast(*c_void, &outer_context.inner);
+        outer_context.thread.data.handle = windows.kernel32.CreateThread(null, default_stack_size, WinThread.threadMain, parameter, 0, null) orelse {
+            switch (windows.kernel32.GetLastError()) {
+                else => |err| return windows.unexpectedError(err),
+            }
+        };
+        return &outer_context.thread;
+    }
+
+    const MainFuncs = struct {
+        fn linuxThreadMain(ctx_addr: usize) callconv(.C) u8 {
+            const arg = if (@sizeOf(Context) == 0) {} else @intToPtr(*const Context, ctx_addr).*;
+
+            switch (@typeInfo(@typeInfo(@TypeOf(startFn)).Fn.return_type.?)) {
+                .NoReturn => {
+                    startFn(arg);
+                },
+                .Void => {
+                    startFn(arg);
+                    return 0;
+                },
+                .Int => |info| {
+                    if (info.bits != 8) {
+                        @compileError(bad_startfn_ret);
+                    }
+                    return startFn(arg);
+                },
+                .ErrorUnion => |info| {
+                    if (info.payload != void) {
+                        @compileError(bad_startfn_ret);
+                    }
+                    startFn(arg) catch |err| {
+                        std.debug.warn("error: {s}\n", .{@errorName(err)});
+                        if (@errorReturnTrace()) |trace| {
+                            std.debug.dumpStackTrace(trace.*);
+                        }
+                    };
+                    return 0;
+                },
+                else => @compileError(bad_startfn_ret),
+            }
+        }
+        fn posixThreadMain(ctx: ?*c_void) callconv(.C) ?*c_void {
+            const arg = if (@sizeOf(Context) == 0) {} else @ptrCast(*Context, @alignCast(@alignOf(Context), ctx)).*;
+
+            switch (@typeInfo(@typeInfo(@TypeOf(startFn)).Fn.return_type.?)) {
+                .NoReturn => {
+                    startFn(arg);
+                },
+                .Void => {
+                    startFn(arg);
+                    return null;
+                },
+                .Int => |info| {
+                    if (info.bits != 8) {
+                        @compileError(bad_startfn_ret);
+                    }
+                    // pthreads don't support exit status, ignore value
+                    _ = startFn(arg);
+                    return null;
+                },
+                .ErrorUnion => |info| {
+                    if (info.payload != void) {
+                        @compileError(bad_startfn_ret);
+                    }
+                    startFn(arg) catch |err| {
+                        std.debug.warn("error: {s}\n", .{@errorName(err)});
+                        if (@errorReturnTrace()) |trace| {
+                            std.debug.dumpStackTrace(trace.*);
+                        }
+                    };
+                    return null;
+                },
+                else => @compileError(bad_startfn_ret),
+            }
+        }
+    };
+
+    if (Thread.use_pthreads) {
+        var attr: c.pthread_attr_t = undefined;
+        if (c.pthread_attr_init(&attr) != 0) return error.SystemResources;
+        defer assert(c.pthread_attr_destroy(&attr) == 0);
+
+        const thread_obj = try std.heap.c_allocator.create(Thread);
+        errdefer std.heap.c_allocator.destroy(thread_obj);
+        if (@sizeOf(Context) > 0) {
+            thread_obj.data.memory = try std.heap.c_allocator.allocAdvanced(
+                u8,
+                @alignOf(Context),
+                @sizeOf(Context),
+                .at_least,
+            );
+            errdefer std.heap.c_allocator.free(thread_obj.data.memory);
+            mem.copy(u8, thread_obj.data.memory, mem.asBytes(&context));
+        } else {
+            thread_obj.data.memory = @as([*]u8, undefined)[0..0];
+        }
+
+        // Use the same set of parameters used by the libc-less impl.
+        assert(c.pthread_attr_setstacksize(&attr, default_stack_size) == 0);
+        assert(c.pthread_attr_setguardsize(&attr, mem.page_size) == 0);
+
+        const err = c.pthread_create(
+            &thread_obj.data.handle,
+            &attr,
+            MainFuncs.posixThreadMain,
+            thread_obj.data.memory.ptr,
+        );
+        switch (err) {
+            0 => return thread_obj,
+            os.EAGAIN => return error.SystemResources,
+            os.EPERM => unreachable,
+            os.EINVAL => unreachable,
+            else => return os.unexpectedErrno(@intCast(usize, err)),
+        }
+
+        return thread_obj;
+    }
+
+    var guard_end_offset: usize = undefined;
+    var stack_end_offset: usize = undefined;
+    var thread_start_offset: usize = undefined;
+    var context_start_offset: usize = undefined;
+    var tls_start_offset: usize = undefined;
+    const mmap_len = blk: {
+        var l: usize = mem.page_size;
+        // Allocate a guard page right after the end of the stack region
+        guard_end_offset = l;
+        // The stack itself, which grows downwards.
+        l = mem.alignForward(l + default_stack_size, mem.page_size);
+        stack_end_offset = l;
+        // Above the stack, so that it can be in the same mmap call, put the Thread object.
+        l = mem.alignForward(l, @alignOf(Thread));
+        thread_start_offset = l;
+        l += @sizeOf(Thread);
+        // Next, the Context object.
+        if (@sizeOf(Context) != 0) {
+            l = mem.alignForward(l, @alignOf(Context));
+            context_start_offset = l;
+            l += @sizeOf(Context);
+        }
+        // Finally, the Thread Local Storage, if any.
+        l = mem.alignForward(l, os.linux.tls.tls_image.alloc_align);
+        tls_start_offset = l;
+        l += os.linux.tls.tls_image.alloc_size;
+        // Round the size to the page size.
+        break :blk mem.alignForward(l, mem.page_size);
+    };
+
+    const mmap_slice = mem: {
+        // Map the whole stack with no rw permissions to avoid
+        // committing the whole region right away
+        const mmap_slice = os.mmap(
+            null,
+            mmap_len,
+            os.PROT_NONE,
+            os.MAP_PRIVATE | os.MAP_ANONYMOUS,
+            -1,
+            0,
+        ) catch |err| switch (err) {
+            error.MemoryMappingNotSupported => unreachable,
+            error.AccessDenied => unreachable,
+            error.PermissionDenied => unreachable,
+            else => |e| return e,
+        };
+        errdefer os.munmap(mmap_slice);
+
+        // Map everything but the guard page as rw
+        os.mprotect(
+            mmap_slice[guard_end_offset..],
+            os.PROT_READ | os.PROT_WRITE,
+        ) catch |err| switch (err) {
+            error.AccessDenied => unreachable,
+            else => |e| return e,
+        };
+
+        break :mem mmap_slice;
+    };
+
+    const mmap_addr = @ptrToInt(mmap_slice.ptr);
+
+    const thread_ptr = @alignCast(@alignOf(Thread), @intToPtr(*Thread, mmap_addr + thread_start_offset));
+    thread_ptr.data.memory = mmap_slice;
+
+    var arg: usize = undefined;
+    if (@sizeOf(Context) != 0) {
+        arg = mmap_addr + context_start_offset;
+        const context_ptr = @alignCast(@alignOf(Context), @intToPtr(*Context, arg));
+        context_ptr.* = context;
+    }
+
+    if (std.Target.current.os.tag == .linux) {
+        const flags: u32 = os.CLONE_VM | os.CLONE_FS | os.CLONE_FILES |
+            os.CLONE_SIGHAND | os.CLONE_THREAD | os.CLONE_SYSVSEM |
+            os.CLONE_PARENT_SETTID | os.CLONE_CHILD_CLEARTID |
+            os.CLONE_DETACHED | os.CLONE_SETTLS;
+        // This structure is only needed when targeting i386
+        var user_desc: if (std.Target.current.cpu.arch == .i386) os.linux.user_desc else void = undefined;
+
+        const tls_area = mmap_slice[tls_start_offset..];
+        const tp_value = os.linux.tls.prepareTLS(tls_area);
+
+        const newtls = blk: {
+            if (std.Target.current.cpu.arch == .i386) {
+                user_desc = os.linux.user_desc{
+                    .entry_number = os.linux.tls.tls_image.gdt_entry_number,
+                    .base_addr = tp_value,
+                    .limit = 0xfffff,
+                    .seg_32bit = 1,
+                    .contents = 0, // Data
+                    .read_exec_only = 0,
+                    .limit_in_pages = 1,
+                    .seg_not_present = 0,
+                    .useable = 1,
+                };
+                break :blk @ptrToInt(&user_desc);
+            } else {
+                break :blk tp_value;
+            }
+        };
+
+        const rc = os.linux.clone(
+            MainFuncs.linuxThreadMain,
+            mmap_addr + stack_end_offset,
+            flags,
+            arg,
+            &thread_ptr.data.handle,
+            newtls,
+            &thread_ptr.data.handle,
+        );
+        switch (os.errno(rc)) {
+            0 => return thread_ptr,
+            os.EAGAIN => return error.ThreadQuotaExceeded,
+            os.EINVAL => unreachable,
+            os.ENOMEM => return error.SystemResources,
+            os.ENOSPC => unreachable,
+            os.EPERM => unreachable,
+            os.EUSERS => unreachable,
+            else => |err| return os.unexpectedErrno(err),
+        }
+    } else {
+        @compileError("Unsupported OS");
+    }
+}
+
+pub const CpuCountError = error{
+    PermissionDenied,
+    SystemResources,
+    Unexpected,
+};
+
+pub fn cpuCount() CpuCountError!usize {
+    if (std.Target.current.os.tag == .linux) {
+        const cpu_set = try os.sched_getaffinity(0);
+        return @as(usize, os.CPU_COUNT(cpu_set)); // TODO should not need this usize cast
+    }
+    if (std.Target.current.os.tag == .windows) {
+        return os.windows.peb().NumberOfProcessors;
+    }
+    if (std.Target.current.os.tag == .openbsd) {
+        var count: c_int = undefined;
+        var count_size: usize = @sizeOf(c_int);
+        const mib = [_]c_int{ os.CTL_HW, os.HW_NCPUONLINE };
+        os.sysctl(&mib, &count, &count_size, null, 0) catch |err| switch (err) {
+            error.NameTooLong, error.UnknownName => unreachable,
+            else => |e| return e,
+        };
+        return @intCast(usize, count);
+    }
+    var count: c_int = undefined;
+    var count_len: usize = @sizeOf(c_int);
+    const name = if (comptime std.Target.current.isDarwin()) "hw.logicalcpu" else "hw.ncpu";
+    os.sysctlbynameZ(name, &count, &count_len, null, 0) catch |err| switch (err) {
+        error.NameTooLong, error.UnknownName => unreachable,
+        else => |e| return e,
+    };
+    return @intCast(usize, count);
+}
+
+pub fn getCurrentThreadId() u64 {
+    switch (std.Target.current.os.tag) {
+        .linux => {
+            // Use the syscall directly as musl doesn't provide a wrapper.
+            return @bitCast(u32, os.linux.gettid());
+        },
+        .windows => {
+            return os.windows.kernel32.GetCurrentThreadId();
+        },
+        .macos, .ios, .watchos, .tvos => {
+            var thread_id: u64 = undefined;
+            // Pass thread=null to get the current thread ID.
+            assert(c.pthread_threadid_np(null, &thread_id) == 0);
+            return thread_id;
+        },
+        .netbsd => {
+            return @bitCast(u32, c._lwp_self());
+        },
+        .freebsd => {
+            return @bitCast(u32, c.pthread_getthreadid_np());
+        },
+        .openbsd => {
+            return @bitCast(u32, c.getthrid());
+        },
+        else => {
+            @compileError("getCurrentThreadId not implemented for this platform");
+        },
+    }
+}
+
+test "" {
+    std.testing.refAllDecls(@This());
+}
diff --git a/lib/std/Thread/AutoResetEvent.zig b/lib/std/Thread/AutoResetEvent.zig
new file mode 100644
index 0000000000..8b8b5658bf
--- /dev/null
+++ b/lib/std/Thread/AutoResetEvent.zig
@@ -0,0 +1,228 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! Similar to `StaticResetEvent` but on `set()` it also (atomically) does `reset()`.
+//! Unlike StaticResetEvent, `wait()` can only be called by one thread (MPSC-like).
+//!
+//! AutoResetEvent has 3 possible states:
+//! - UNSET: the AutoResetEvent is currently unset
+//! - SET: the AutoResetEvent was notified before a wait() was called
+//! - <StaticResetEvent pointer>: there is an active waiter waiting for a notification.
+//!
+//! When attempting to wait:
+//!  if the event is unset, it registers a ResetEvent pointer to be notified when the event is set
+//!  if the event is already set, then it consumes the notification and resets the event.
+//!
+//! When attempting to notify:
+//!  if the event is unset, then we set the event
+//!  if theres a waiting ResetEvent, then we unset the event and notify the ResetEvent
+//!
+//! This ensures that the event is automatically reset after a wait() has been issued
+//! and avoids the race condition when using StaticResetEvent in the following scenario:
+//!  thread 1                  | thread 2
+//!  StaticResetEvent.wait()   |
+//!                            | StaticResetEvent.set()
+//!                            | StaticResetEvent.set()
+//!  StaticResetEvent.reset()  |
+//!  StaticResetEvent.wait()   | (missed the second .set() notification above)
+
+state: usize = UNSET,
+
+const std = @import("../std.zig");
+const builtin = @import("builtin");
+const testing = std.testing;
+const assert = std.debug.assert;
+const StaticResetEvent = std.Thread.StaticResetEvent;
+const AutoResetEvent = @This();
+
+const UNSET = 0;
+const SET = 1;
+
+/// the minimum alignment for the `*StaticResetEvent` created by wait*()
+const event_align = std.math.max(@alignOf(StaticResetEvent), 2);
+
+pub fn wait(self: *AutoResetEvent) void {
+    self.waitFor(null) catch unreachable;
+}
+
+pub fn timedWait(self: *AutoResetEvent, timeout: u64) error{TimedOut}!void {
+    return self.waitFor(timeout);
+}
+
+fn waitFor(self: *AutoResetEvent, timeout: ?u64) error{TimedOut}!void {
+    // lazily initialized StaticResetEvent
+    var reset_event: StaticResetEvent align(event_align) = undefined;
+    var has_reset_event = false;
+
+    var state = @atomicLoad(usize, &self.state, .SeqCst);
+    while (true) {
+        // consume a notification if there is any
+        if (state == SET) {
+            @atomicStore(usize, &self.state, UNSET, .SeqCst);
+            return;
+        }
+
+        // check if theres currently a pending ResetEvent pointer already registered
+        if (state != UNSET) {
+            unreachable; // multiple waiting threads on the same AutoResetEvent
+        }
+
+        // lazily initialize the ResetEvent if it hasn't been already
+        if (!has_reset_event) {
+            has_reset_event = true;
+            reset_event = .{};
+        }
+
+        // Since the AutoResetEvent currently isnt set,
+        // try to register our ResetEvent on it to wait
+        // for a set() call from another thread.
+        if (@cmpxchgWeak(
+            usize,
+            &self.state,
+            UNSET,
+            @ptrToInt(&reset_event),
+            .SeqCst,
+            .SeqCst,
+        )) |new_state| {
+            state = new_state;
+            continue;
+        }
+
+        // if no timeout was specified, then just wait forever
+        const timeout_ns = timeout orelse {
+            reset_event.wait();
+            return;
+        };
+
+        // wait with a timeout and return if signalled via set()
+        switch (reset_event.timedWait(timeout_ns)) {
+            .event_set => return,
+            .timed_out => {},
+        }
+
+        // If we timed out, we need to transition the AutoResetEvent back to UNSET.
+        // If we don't, then when we return, a set() thread could observe a pointer to an invalid ResetEvent.
+        state = @cmpxchgStrong(
+            usize,
+            &self.state,
+            @ptrToInt(&reset_event),
+            UNSET,
+            .SeqCst,
+            .SeqCst,
+        ) orelse return error.TimedOut;
+
+        // We didn't manage to unregister ourselves from the state.
+        if (state == SET) {
+            unreachable; // AutoResetEvent notified without waking up the waiting thread
+        } else if (state != UNSET) {
+            unreachable; // multiple waiting threads on the same AutoResetEvent observed when timing out
+        }
+
+        // This menas a set() thread saw our ResetEvent pointer, acquired it, and is trying to wake it up.
+        // We need to wait for it to wake up our ResetEvent before we can return and invalidate it.
+        // We don't return error.TimedOut here as it technically notified us while we were "timing out".
+        reset_event.wait();
+        return;
+    }
+}
+
+pub fn set(self: *AutoResetEvent) void {
+    var state = @atomicLoad(usize, &self.state, .SeqCst);
+    while (true) {
+        // If the AutoResetEvent is already set, there is nothing else left to do
+        if (state == SET) {
+            return;
+        }
+
+        // If the AutoResetEvent isn't set,
+        // then try to leave a notification for the wait() thread that we set() it.
+        if (state == UNSET) {
+            state = @cmpxchgWeak(
+                usize,
+                &self.state,
+                UNSET,
+                SET,
+                .SeqCst,
+                .SeqCst,
+            ) orelse return;
+            continue;
+        }
+
+        // There is a ResetEvent pointer registered on the AutoResetEvent event thats waiting.
+        // Try to acquire ownership of it so that we can wake it up.
+        // This also resets the AutoResetEvent so that there is no race condition as defined above.
+        if (@cmpxchgWeak(
+            usize,
+            &self.state,
+            state,
+            UNSET,
+            .SeqCst,
+            .SeqCst,
+        )) |new_state| {
+            state = new_state;
+            continue;
+        }
+
+        const reset_event = @intToPtr(*align(event_align) StaticResetEvent, state);
+        reset_event.set();
+        return;
+    }
+}
+
+test "basic usage" {
+    // test local code paths
+    {
+        var event = AutoResetEvent{};
+        testing.expectError(error.TimedOut, event.timedWait(1));
+        event.set();
+        event.wait();
+    }
+
+    // test cross-thread signaling
+    if (builtin.single_threaded)
+        return;
+
+    const Context = struct {
+        value: u128 = 0,
+        in: AutoResetEvent = AutoResetEvent{},
+        out: AutoResetEvent = AutoResetEvent{},
+
+        const Self = @This();
+
+        fn sender(self: *Self) void {
+            testing.expect(self.value == 0);
+            self.value = 1;
+            self.out.set();
+
+            self.in.wait();
+            testing.expect(self.value == 2);
+            self.value = 3;
+            self.out.set();
+
+            self.in.wait();
+            testing.expect(self.value == 4);
+        }
+
+        fn receiver(self: *Self) void {
+            self.out.wait();
+            testing.expect(self.value == 1);
+            self.value = 2;
+            self.in.set();
+
+            self.out.wait();
+            testing.expect(self.value == 3);
+            self.value = 4;
+            self.in.set();
+        }
+    };
+
+    var context = Context{};
+    const send_thread = try std.Thread.spawn(&context, Context.sender);
+    const recv_thread = try std.Thread.spawn(&context, Context.receiver);
+
+    send_thread.wait();
+    recv_thread.wait();
+}
diff --git a/lib/std/Thread/Condition.zig b/lib/std/Thread/Condition.zig
new file mode 100644
index 0000000000..2379d264d1
--- /dev/null
+++ b/lib/std/Thread/Condition.zig
@@ -0,0 +1,182 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! A condition provides a way for a kernel thread to block until it is signaled
+//! to wake up. Spurious wakeups are possible.
+//! This API supports static initialization and does not require deinitialization.
+
+impl: Impl,
+
+const std = @import("../std.zig");
+const Condition = @This();
+const windows = std.os.windows;
+const linux = std.os.linux;
+const Mutex = std.Thread.Mutex;
+const assert = std.debug.assert;
+
+const Impl = if (std.builtin.single_threaded)
+    SingleThreadedCondition
+else if (std.Target.current.os.tag == .windows)
+    WindowsCondition
+else if (std.Thread.use_pthreads)
+    PthreadCondition
+else
+    AtomicCondition;
+
+pub const SingleThreadedCondition = struct {
+    pub fn wait(cond: *SingleThreadedCondition, mutex: *Mutex) void {
+        unreachable; // deadlock detected
+    }
+
+    pub fn signal(cond: *SingleThreadedCondition) void {}
+
+    pub fn broadcast(cond: *SingleThreadedCondition) void {}
+};
+
+pub const WindowsCondition = struct {
+    cond: windows.CONDITION_VARIABLE = windows.CONDITION_VARIABLE_INIT,
+
+    pub fn wait(cond: *WindowsCondition, mutex: *Mutex) void {
+        const rc = windows.SleepConditionVariableSRW(
+            &cond.cond,
+            &mutex.srwlock,
+            windows.INFINITE,
+            @as(windows.ULONG, 0),
+        );
+        assert(rc != windows.FALSE);
+    }
+
+    pub fn signal(cond: *WindowsCondition) void {
+        windows.WakeConditionVariable(&cond.cond);
+    }
+
+    pub fn broadcast(cond: *WindowsCondition) void {
+        windows.WakeAllConditionVariable(&cond.cond);
+    }
+};
+
+pub const PthreadCondition = struct {
+    cond: std.c.pthread_cond_t = .{},
+
+    pub fn wait(cond: *PthreadCondition, mutex: *Mutex) void {
+        const rc = std.c.pthread_cond_wait(&cond.cond, &mutex.mutex);
+        assert(rc == 0);
+    }
+
+    pub fn signal(cond: *PthreadCondition) void {
+        const rc = std.c.pthread_cond_signal(&cond.cond);
+        assert(rc == 0);
+    }
+
+    pub fn broadcast(cond: *PthreadCondition) void {
+        const rc = std.c.pthread_cond_broadcast(&cond.cond);
+        assert(rc == 0);
+    }
+};
+
+pub const AtomicCondition = struct {
+    pending: bool = false,
+    queue_mutex: Mutex = .{},
+    queue_list: QueueList = .{},
+
+    pub const QueueList = std.SinglyLinkedList(QueueItem);
+
+    pub const QueueItem = struct {
+        futex: i32 = 0,
+
+        fn wait(cond: *@This()) void {
+            while (@atomicLoad(i32, &cond.futex, .Acquire) == 0) {
+                switch (std.Target.current.os.tag) {
+                    .linux => {
+                        switch (linux.getErrno(linux.futex_wait(
+                            &cond.futex,
+                            linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAIT,
+                            0,
+                            null,
+                        ))) {
+                            0 => {},
+                            std.os.EINTR => {},
+                            std.os.EAGAIN => {},
+                            else => unreachable,
+                        }
+                    },
+                    else => spinLoopHint(),
+                }
+            }
+        }
+
+        fn notify(cond: *@This()) void {
+            @atomicStore(i32, &cond.futex, 1, .Release);
+
+            switch (std.Target.current.os.tag) {
+                .linux => {
+                    switch (linux.getErrno(linux.futex_wake(
+                        &cond.futex,
+                        linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAKE,
+                        1,
+                    ))) {
+                        0 => {},
+                        std.os.EFAULT => {},
+                        else => unreachable,
+                    }
+                },
+                else => {},
+            }
+        }
+    };
+
+    pub fn wait(cond: *AtomicCondition, mutex: *Mutex) void {
+        var waiter = QueueList.Node{ .data = .{} };
+
+        {
+            const held = cond.queue_mutex.acquire();
+            defer held.release();
+
+            cond.queue_list.prepend(&waiter);
+            @atomicStore(bool, &cond.pending, true, .SeqCst);
+        }
+
+        mutex.unlock();
+        waiter.data.wait();
+        mutex.lock();
+    }
+
+    pub fn signal(cond: *AtomicCondition) void {
+        if (@atomicLoad(bool, &cond.pending, .SeqCst) == false)
+            return;
+
+        const maybe_waiter = blk: {
+            const held = cond.queue_mutex.acquire();
+            defer held.release();
+
+            const maybe_waiter = cond.queue_list.popFirst();
+            @atomicStore(bool, &cond.pending, cond.queue_list.first != null, .SeqCst);
+            break :blk maybe_waiter;
+        };
+
+        if (maybe_waiter) |waiter|
+            waiter.data.notify();
+    }
+
+    pub fn broadcast(cond: *AtomicCondition) void {
+        if (@atomicLoad(bool, &cond.pending, .SeqCst) == false)
+            return;
+
+        @atomicStore(bool, &cond.pending, false, .SeqCst);
+
+        var waiters = blk: {
+            const held = cond.queue_mutex.acquire();
+            defer held.release();
+
+            const waiters = cond.queue_list;
+            cond.queue_list = .{};
+            break :blk waiters;
+        };
+
+        while (waiters.popFirst()) |waiter|
+            waiter.data.notify();
+    }
+};
diff --git a/lib/std/Thread/Mutex.zig b/lib/std/Thread/Mutex.zig
new file mode 100644
index 0000000000..128eb0be80
--- /dev/null
+++ b/lib/std/Thread/Mutex.zig
@@ -0,0 +1,303 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! Lock may be held only once. If the same thread tries to acquire
+//! the same mutex twice, it deadlocks.  This type supports static
+//! initialization and is at most `@sizeOf(usize)` in size.  When an
+//! application is built in single threaded release mode, all the
+//! functions are no-ops. In single threaded debug mode, there is
+//! deadlock detection.
+//!
+//! Example usage:
+//! var m = Mutex{};
+//!
+//! const lock = m.acquire();
+//! defer lock.release();
+//! ... critical code
+//!
+//! Non-blocking:
+//! if (m.tryAcquire) |lock| {
+//!     defer lock.release();
+//!     // ... critical section
+//! } else {
+//!     // ... lock not acquired
+//! }
+
+impl: Impl = .{},
+
+const Mutex = @This();
+const std = @import("../std.zig");
+const builtin = std.builtin;
+const os = std.os;
+const assert = std.debug.assert;
+const windows = os.windows;
+const linux = os.linux;
+const testing = std.testing;
+const StaticResetEvent = std.thread.StaticResetEvent;
+
+pub const Held = struct {
+    impl: *Impl,
+
+    pub fn release(held: Held) void {
+        held.impl.release();
+    }
+};
+
+/// Try to acquire the mutex without blocking. Returns null if
+/// the mutex is unavailable. Otherwise returns Held. Call
+/// release on Held.
+pub fn tryAcquire(m: *Mutex) ?Held {
+    if (m.impl.tryAcquire()) {
+        return Held{ .impl = &m.impl };
+    } else {
+        return null;
+    }
+}
+
+/// Acquire the mutex. Deadlocks if the mutex is already
+/// held by the calling thread.
+pub fn acquire(m: *Mutex) Held {
+    m.impl.acquire();
+    return .{ .impl = &m.impl };
+}
+
+const Impl = if (builtin.single_threaded)
+    Dummy
+else if (builtin.os.tag == .windows)
+    WindowsMutex
+else if (std.Thread.use_pthreads)
+    PthreadMutex
+else
+    AtomicMutex;
+
+pub const AtomicMutex = struct {
+    state: State = .unlocked,
+
+    const State = enum(i32) {
+        unlocked,
+        locked,
+        waiting,
+    };
+
+    pub fn tryAcquire(self: *AtomicMutex) bool {
+        return @cmpxchgStrong(
+            State,
+            &self.state,
+            .unlocked,
+            .locked,
+            .Acquire,
+            .Monotonic,
+        ) == null;
+    }
+
+    pub fn acquire(self: *AtomicMutex) void {
+        switch (@atomicRmw(State, &self.state, .Xchg, .locked, .Acquire)) {
+            .unlocked => {},
+            else => |s| self.lockSlow(s),
+        }
+    }
+
+    fn lockSlow(self: *AtomicMutex, current_state: State) void {
+        @setCold(true);
+        var new_state = current_state;
+
+        var spin: u8 = 0;
+        while (spin < 100) : (spin += 1) {
+            const state = @cmpxchgWeak(
+                State,
+                &self.state,
+                .unlocked,
+                new_state,
+                .Acquire,
+                .Monotonic,
+            ) orelse return;
+
+            switch (state) {
+                .unlocked => {},
+                .locked => {},
+                .waiting => break,
+            }
+
+            var iter = std.math.min(32, spin + 1);
+            while (iter > 0) : (iter -= 1)
+                std.Thread.spinLoopHint();
+        }
+
+        new_state = .waiting;
+        while (true) {
+            switch (@atomicRmw(State, &self.state, .Xchg, new_state, .Acquire)) {
+                .unlocked => return,
+                else => {},
+            }
+            switch (std.Target.current.os.tag) {
+                .linux => {
+                    switch (linux.getErrno(linux.futex_wait(
+                        @ptrCast(*const i32, &self.state),
+                        linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAIT,
+                        @enumToInt(new_state),
+                        null,
+                    ))) {
+                        0 => {},
+                        std.os.EINTR => {},
+                        std.os.EAGAIN => {},
+                        else => unreachable,
+                    }
+                },
+                else => std.Thread.spinLoopHint(),
+            }
+        }
+    }
+
+    pub fn release(self: *AtomicMutex) void {
+        switch (@atomicRmw(State, &self.state, .Xchg, .unlocked, .Release)) {
+            .unlocked => unreachable,
+            .locked => {},
+            .waiting => self.unlockSlow(),
+        }
+    }
+
+    fn unlockSlow(self: *AtomicMutex) void {
+        @setCold(true);
+
+        switch (std.Target.current.os.tag) {
+            .linux => {
+                switch (linux.getErrno(linux.futex_wake(
+                    @ptrCast(*const i32, &self.state),
+                    linux.FUTEX_PRIVATE_FLAG | linux.FUTEX_WAKE,
+                    1,
+                ))) {
+                    0 => {},
+                    std.os.EFAULT => {},
+                    else => unreachable,
+                }
+            },
+            else => {},
+        }
+    }
+};
+
+pub const PthreadMutex = struct {
+    pthread_mutex: std.c.pthread_mutex_t = .{},
+
+    /// Try to acquire the mutex without blocking. Returns null if
+    /// the mutex is unavailable. Otherwise returns Held. Call
+    /// release on Held.
+    pub fn tryAcquire(self: *PthreadMutex) bool {
+        return std.c.pthread_mutex_trylock(&self.pthread_mutex) == 0;
+    }
+
+    /// Acquire the mutex. Will deadlock if the mutex is already
+    /// held by the calling thread.
+    pub fn acquire(self: *PthreadMutex) void {
+        switch (std.c.pthread_mutex_lock(&self.pthread_mutex)) {
+            0 => return,
+            std.c.EINVAL => unreachable,
+            std.c.EBUSY => unreachable,
+            std.c.EAGAIN => unreachable,
+            std.c.EDEADLK => unreachable,
+            std.c.EPERM => unreachable,
+            else => unreachable,
+        }
+    }
+
+    pub fn release(self: *PthreadMutex) void {
+        switch (std.c.pthread_mutex_unlock(&self.pthread_mutex)) {
+            0 => return,
+            std.c.EINVAL => unreachable,
+            std.c.EAGAIN => unreachable,
+            std.c.EPERM => unreachable,
+            else => unreachable,
+        }
+    }
+};
+
+/// This has the sematics as `Mutex`, however it does not actually do any
+/// synchronization. Operations are safety-checked no-ops.
+pub const Dummy = struct {
+    lock: @TypeOf(lock_init) = lock_init,
+
+    const lock_init = if (std.debug.runtime_safety) false else {};
+
+    /// Try to acquire the mutex without blocking. Returns null if
+    /// the mutex is unavailable. Otherwise returns Held. Call
+    /// release on Held.
+    pub fn tryAcquire(self: *Dummy) bool {
+        if (std.debug.runtime_safety) {
+            if (self.lock) return false;
+            self.lock = true;
+        }
+        return true;
+    }
+
+    /// Acquire the mutex. Will deadlock if the mutex is already
+    /// held by the calling thread.
+    pub fn acquire(self: *Dummy) void {
+        return self.tryAcquire() orelse @panic("deadlock detected");
+    }
+
+    pub fn release(self: *Dummy) void {
+        if (std.debug.runtime_safety) {
+            self.mutex.lock = false;
+        }
+    }
+};
+
+const WindowsMutex = struct {
+    srwlock: windows.SRWLOCK = windows.SRWLOCK_INIT,
+
+    pub fn tryAcquire(self: *WindowsMutex) bool {
+        return TryAcquireSRWLockExclusive(&self.srwlock) != system.FALSE;
+    }
+
+    pub fn acquire(self: *WindowsMutex) void {
+        AcquireSRWLockExclusive(&self.srwlock);
+    }
+
+    pub fn release(self: *WindowsMutex) void {
+        ReleaseSRWLockExclusive(&self.srwlock);
+    }
+};
+
+const TestContext = struct {
+    mutex: *Mutex,
+    data: i128,
+
+    const incr_count = 10000;
+};
+
+test "basic usage" {
+    var mutex = Mutex{};
+
+    var context = TestContext{
+        .mutex = &mutex,
+        .data = 0,
+    };
+
+    if (builtin.single_threaded) {
+        worker(&context);
+        testing.expect(context.data == TestContext.incr_count);
+    } else {
+        const thread_count = 10;
+        var threads: [thread_count]*std.Thread = undefined;
+        for (threads) |*t| {
+            t.* = try std.Thread.spawn(&context, worker);
+        }
+        for (threads) |t|
+            t.wait();
+
+        testing.expect(context.data == thread_count * TestContext.incr_count);
+    }
+}
+
+fn worker(ctx: *TestContext) void {
+    var i: usize = 0;
+    while (i != TestContext.incr_count) : (i += 1) {
+        const held = ctx.mutex.acquire();
+        defer held.release();
+
+        ctx.data += 1;
+    }
+}
diff --git a/lib/std/ResetEvent.zig b/lib/std/Thread/ResetEvent.zig
index 4443fdcdfb..622f9be98e 100644
--- a/lib/std/ResetEvent.zig
+++ b/lib/std/Thread/ResetEvent.zig
@@ -9,11 +9,11 @@
 //! This API requires being initialized at runtime, and initialization
 //! can fail. Once initialized, the core operations cannot fail.
 //! If you need an abstraction that cannot fail to be initialized, see
-//! `std.StaticResetEvent`. However if you can handle initialization failure,
+//! `std.Thread.StaticResetEvent`. However if you can handle initialization failure,
 //! it is preferred to use `ResetEvent`.
 
 const ResetEvent = @This();
-const std = @import("std.zig");
+const std = @import("../std.zig");
 const builtin = std.builtin;
 const testing = std.testing;
 const assert = std.debug.assert;
@@ -24,13 +24,13 @@ const time = std.time;
 impl: Impl,
 
 pub const Impl = if (builtin.single_threaded)
-    std.StaticResetEvent.DebugEvent
+    std.Thread.StaticResetEvent.DebugEvent
 else if (std.Target.current.isDarwin())
     DarwinEvent
 else if (std.Thread.use_pthreads)
     PosixEvent
 else
-    std.StaticResetEvent.AtomicEvent;
+    std.Thread.StaticResetEvent.AtomicEvent;
 
 pub const InitError = error{SystemResources};
 
diff --git a/lib/std/Thread/RwLock.zig b/lib/std/Thread/RwLock.zig
new file mode 100644
index 0000000000..1d606a9cf1
--- /dev/null
+++ b/lib/std/Thread/RwLock.zig
@@ -0,0 +1,308 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! A lock that supports one writer or many readers.
+//! This API is for kernel threads, not evented I/O.
+//! This API requires being initialized at runtime, and initialization
+//! can fail. Once initialized, the core operations cannot fail.
+
+impl: Impl,
+
+const RwLock = @This();
+const std = @import("../std.zig");
+const builtin = std.builtin;
+const assert = std.debug.assert;
+const Mutex = std.Thread.Mutex;
+const Semaphore = std.Semaphore;
+const CondVar = std.CondVar;
+
+pub const Impl = if (builtin.single_threaded)
+    SingleThreadedRwLock
+else if (std.Thread.use_pthreads)
+    PthreadRwLock
+else
+    DefaultRwLock;
+
+pub fn init(rwl: *RwLock) void {
+    return rwl.impl.init();
+}
+
+pub fn deinit(rwl: *RwLock) void {
+    return rwl.impl.deinit();
+}
+
+/// Attempts to obtain exclusive lock ownership.
+/// Returns `true` if the lock is obtained, `false` otherwise.
+pub fn tryLock(rwl: *RwLock) bool {
+    return rwl.impl.tryLock();
+}
+
+/// Blocks until exclusive lock ownership is acquired.
+pub fn lock(rwl: *RwLock) void {
+    return rwl.impl.lock();
+}
+
+/// Releases a held exclusive lock.
+/// Asserts the lock is held exclusively.
+pub fn unlock(rwl: *RwLock) void {
+    return rwl.impl.unlock();
+}
+
+/// Attempts to obtain shared lock ownership.
+/// Returns `true` if the lock is obtained, `false` otherwise.
+pub fn tryLockShared(rwl: *RwLock) bool {
+    return rwl.impl.tryLockShared();
+}
+
+/// Blocks until shared lock ownership is acquired.
+pub fn lockShared(rwl: *RwLock) void {
+    return rwl.impl.lockShared();
+}
+
+/// Releases a held shared lock.
+pub fn unlockShared(rwl: *RwLock) void {
+    return rwl.impl.unlockShared();
+}
+
+/// Single-threaded applications use this for deadlock checks in
+/// debug mode, and no-ops in release modes.
+pub const SingleThreadedRwLock = struct {
+    state: enum { unlocked, locked_exclusive, locked_shared },
+    shared_count: usize,
+
+    pub fn init(rwl: *SingleThreadedRwLock) void {
+        rwl.* = .{
+            .state = .unlocked,
+            .shared_count = 0,
+        };
+    }
+
+    pub fn deinit(rwl: *SingleThreadedRwLock) void {
+        assert(rwl.state == .unlocked);
+        assert(rwl.shared_count == 0);
+    }
+
+    /// Attempts to obtain exclusive lock ownership.
+    /// Returns `true` if the lock is obtained, `false` otherwise.
+    pub fn tryLock(rwl: *SingleThreadedRwLock) bool {
+        switch (rwl.state) {
+            .unlocked => {
+                assert(rwl.shared_count == 0);
+                rwl.state = .locked_exclusive;
+                return true;
+            },
+            .locked_exclusive, .locked_shared => return false,
+        }
+    }
+
+    /// Blocks until exclusive lock ownership is acquired.
+    pub fn lock(rwl: *SingleThreadedRwLock) void {
+        assert(rwl.state == .unlocked); // deadlock detected
+        assert(rwl.shared_count == 0); // corrupted state detected
+        rwl.state = .locked_exclusive;
+    }
+
+    /// Releases a held exclusive lock.
+    /// Asserts the lock is held exclusively.
+    pub fn unlock(rwl: *SingleThreadedRwLock) void {
+        assert(rwl.state == .locked_exclusive);
+        assert(rwl.shared_count == 0); // corrupted state detected
+        rwl.state = .unlocked;
+    }
+
+    /// Attempts to obtain shared lock ownership.
+    /// Returns `true` if the lock is obtained, `false` otherwise.
+    pub fn tryLockShared(rwl: *SingleThreadedRwLock) bool {
+        switch (rwl.state) {
+            .unlocked => {
+                rwl.state = .locked_shared;
+                assert(rwl.shared_count == 0);
+                rwl.shared_count = 1;
+                return true;
+            },
+            .locked_exclusive, .locked_shared => return false,
+        }
+    }
+
+    /// Blocks until shared lock ownership is acquired.
+    pub fn lockShared(rwl: *SingleThreadedRwLock) void {
+        switch (rwl.state) {
+            .unlocked => {
+                rwl.state = .locked_shared;
+                assert(rwl.shared_count == 0);
+                rwl.shared_count = 1;
+            },
+            .locked_shared => {
+                rwl.shared_count += 1;
+            },
+            .locked_exclusive => unreachable, // deadlock detected
+        }
+    }
+
+    /// Releases a held shared lock.
+    pub fn unlockShared(rwl: *SingleThreadedRwLock) void {
+        switch (rwl.state) {
+            .unlocked => unreachable, // too many calls to `unlockShared`
+            .locked_exclusive => unreachable, // exclusively held lock
+            .locked_shared => {
+                rwl.shared_count -= 1;
+                if (rwl.shared_count == 0) {
+                    rwl.state = .unlocked;
+                }
+            },
+        }
+    }
+};
+
+pub const PthreadRwLock = struct {
+    rwlock: pthread_rwlock_t,
+
+    pub fn init(rwl: *PthreadRwLock) void {
+        rwl.* = .{ .rwlock = .{} };
+    }
+
+    pub fn deinit(rwl: *PthreadRwLock) void {
+        const safe_rc = switch (std.builtin.os.tag) {
+            .dragonfly, .netbsd => std.os.EAGAIN,
+            else => 0,
+        };
+
+        const rc = std.c.pthread_rwlock_destroy(&rwl.rwlock);
+        assert(rc == 0 or rc == safe_rc);
+
+        rwl.* = undefined;
+    }
+
+    pub fn tryLock(rwl: *PthreadRwLock) bool {
+        return pthread_rwlock_trywrlock(&rwl.rwlock) == 0;
+    }
+
+    pub fn lock(rwl: *PthreadRwLock) void {
+        const rc = pthread_rwlock_wrlock(&rwl.rwlock);
+        assert(rc == 0);
+    }
+
+    pub fn unlock(rwl: *PthreadRwLock) void {
+        const rc = pthread_rwlock_unlock(&rwl.rwlock);
+        assert(rc == 0);
+    }
+
+    pub fn tryLockShared(rwl: *PthreadRwLock) bool {
+        return pthread_rwlock_tryrdlock(&rwl.rwlock) == 0;
+    }
+
+    pub fn lockShared(rwl: *PthreadRwLock) void {
+        const rc = pthread_rwlock_rdlock(&rwl.rwlock);
+        assert(rc == 0);
+    }
+
+    pub fn unlockShared(rwl: *PthreadRwLock) void {
+        const rc = pthread_rwlock_unlock(&rwl.rwlock);
+        assert(rc == 0);
+    }
+};
+
+pub const DefaultRwLock = struct {
+    state: usize,
+    mutex: Mutex,
+    semaphore: Semaphore,
+
+    const IS_WRITING: usize = 1;
+    const WRITER: usize = 1 << 1;
+    const READER: usize = 1 << (1 + std.meta.bitCount(Count));
+    const WRITER_MASK: usize = std.math.maxInt(Count) << @ctz(usize, WRITER);
+    const READER_MASK: usize = std.math.maxInt(Count) << @ctz(usize, READER);
+    const Count = std.meta.Int(.unsigned, @divFloor(std.meta.bitCount(usize) - 1, 2));
+
+    pub fn init(rwl: *DefaultRwLock) void {
+        rwl.* = .{
+            .state = 0,
+            .mutex = Mutex.init(),
+            .semaphore = Semaphore.init(0),
+        };
+    }
+
+    pub fn deinit(rwl: *DefaultRwLock) void {
+        rwl.semaphore.deinit();
+        rwl.mutex.deinit();
+        rwl.* = undefined;
+    }
+
+    pub fn tryLock(rwl: *DefaultRwLock) bool {
+        if (rwl.mutex.tryLock()) {
+            const state = @atomicLoad(usize, &rwl.state, .SeqCst);
+            if (state & READER_MASK == 0) {
+                _ = @atomicRmw(usize, &rwl.state, .Or, IS_WRITING, .SeqCst);
+                return true;
+            }
+
+            rwl.mutex.unlock();
+        }
+
+        return false;
+    }
+
+    pub fn lock(rwl: *DefaultRwLock) void {
+        _ = @atomicRmw(usize, &rwl.state, .Add, WRITER, .SeqCst);
+        rwl.mutex.lock();
+
+        const state = @atomicRmw(usize, &rwl.state, .Or, IS_WRITING, .SeqCst);
+        if (state & READER_MASK != 0)
+            rwl.semaphore.wait();
+    }
+
+    pub fn unlock(rwl: *DefaultRwLock) void {
+        _ = @atomicRmw(usize, &rwl.state, .And, ~IS_WRITING, .SeqCst);
+        rwl.mutex.unlock();
+    }
+
+    pub fn tryLockShared(rwl: *DefaultRwLock) bool {
+        const state = @atomicLoad(usize, &rwl.state, .SeqCst);
+        if (state & (IS_WRITING | WRITER_MASK) == 0) {
+            _ = @cmpxchgStrong(
+                usize,
+                &rwl.state,
+                state,
+                state + READER,
+                .SeqCst,
+                .SeqCst,
+            ) orelse return true;
+        }
+
+        if (rwl.mutex.tryLock()) {
+            _ = @atomicRmw(usize, &rwl.state, .Add, READER, .SeqCst);
+            rwl.mutex.unlock();
+            return true;
+        }
+
+        return false;
+    }
+
+    pub fn lockShared(rwl: *DefaultRwLock) void {
+        var state = @atomicLoad(usize, &rwl.state, .SeqCst);
+        while (state & (IS_WRITING | WRITER_MASK) == 0) {
+            state = @cmpxchgWeak(
+                usize,
+                &rwl.state,
+                state,
+                state + READER,
+                .SeqCst,
+                .SeqCst,
+            ) orelse return;
+        }
+
+        rwl.mutex.lock();
+        _ = @atomicRmw(usize, &rwl.state, .Add, READER, .SeqCst);
+        rwl.mutex.unlock();
+    }
+
+    pub fn unlockShared(rwl: *DefaultRwLock) void {
+        const state = @atomicRmw(usize, &rwl.state, .Sub, READER, .SeqCst);
+
+        if ((state & READER_MASK == READER) and (state & IS_WRITING != 0))
+            rwl.semaphore.post();
+    }
+};
diff --git a/lib/std/Thread/Semaphore.zig b/lib/std/Thread/Semaphore.zig
new file mode 100644
index 0000000000..77a278b355
--- /dev/null
+++ b/lib/std/Thread/Semaphore.zig
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2015-2021 Zig Contributors
+// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
+// The MIT license requires this copyright notice to be included in all copies
+// and substantial portions of the software.
+
+//! A semaphore is an unsigned integer that blocks the kernel thread if
+//! the number would become negative.
+//! This API supports static initialization and does not require deinitialization.
+
+mutex: Mutex = .{},
+cond: Condition = .{},
+//! It is OK to initialize this field to any value.
+permits: usize = 0,
+
+const RwLock = @This();
+const std = @import("../std.zig");
+const Mutex = std.Thread.Mutex;
+const Condition = std.Thread.Condition;
+
+pub fn wait(sem: *Semaphore) void {
+    const held = sem.mutex.acquire();
+    defer held.release();
+
+    while (sem.permits == 0)
+        sem.cond.wait(&sem.mutex);
+
+    sem.permits -= 1;
+    if (sem.permits > 0)
+        sem.cond.signal();
+}
+
+pub fn post(sem: *Semaphore) void {
+    const held = sem.mutex.acquire();
+    defer held.release();
+
+    sem.permits += 1;
+    sem.cond.signal();
+}
diff --git a/lib/std/StaticResetEvent.zig b/lib/std/Thread/StaticResetEvent.zig
index 4e551a565e..414583e477 100644
--- a/lib/std/StaticResetEvent.zig
+++ b/lib/std/Thread/StaticResetEvent.zig
@@ -10,10 +10,10 @@
 //! and it requires no deinitialization. The downside is that it may not
 //! integrate as cleanly into other synchronization APIs, or, in a worst case,
 //! may be forced to fall back on spin locking. As a rule of thumb, prefer
-//! to use `std.ResetEvent` when possible, and use `StaticResetEvent` when
+//! to use `std.Thread.ResetEvent` when possible, and use `StaticResetEvent` when
 //! the logic needs stronger API guarantees.
 
-const std = @import("std.zig");
+const std = @import("../std.zig");
 const StaticResetEvent = @This();
 const SpinLock = std.SpinLock;
 const assert = std.debug.assert;
@@ -53,7 +53,7 @@ pub fn reset(ev: *StaticResetEvent) void {
     return ev.impl.reset();
 }
 
-pub const TimedWaitResult = std.ResetEvent.TimedWaitResult;
+pub const TimedWaitResult = std.Thread.ResetEvent.TimedWaitResult;
 
 /// Wait for the event to be set by blocking the current thread.
 /// A timeout in nanoseconds can be provided as a hint for how
@@ -78,13 +78,13 @@ pub const DebugEvent = struct {
     };
 
     /// This function is provided so that this type can be re-used inside
-    /// `std.ResetEvent`.
+    /// `std.Thread.ResetEvent`.
     pub fn init(ev: *DebugEvent) void {
         ev.* = .{};
     }
 
     /// This function is provided so that this type can be re-used inside
-    /// `std.ResetEvent`.
+    /// `std.Thread.ResetEvent`.
     pub fn deinit(ev: *DebugEvent) void {
         ev.* = undefined;
     }
@@ -125,13 +125,13 @@ pub const AtomicEvent = struct {
     const WAIT = 1 << 1;
 
     /// This function is provided so that this type can be re-used inside
-    /// `std.ResetEvent`.
+    /// `std.Thread.ResetEvent`.
     pub fn init(ev: *AtomicEvent) void {
         ev.* = .{};
     }
 
     /// This function is provided so that this type can be re-used inside
-    /// `std.ResetEvent`.
+    /// `std.Thread.ResetEvent`.
     pub fn deinit(ev: *AtomicEvent) void {
         ev.* = undefined;
     }
diff --git a/lib/std/atomic/queue.zig b/lib/std/atomic/queue.zig
index fa3711cd9f..f5f63944ab 100644
--- a/lib/std/atomic/queue.zig
+++ b/lib/std/atomic/queue.zig
@@ -16,7 +16,7 @@ pub fn Queue(comptime T: type) type {
     return struct {
         head: ?*Node,
         tail: ?*Node,
-        mutex: std.Mutex,
+        mutex: std.Thread.Mutex,
 
         pub const Self = @This();
         pub const Node = std.TailQueue(T).Node;
@@ -27,7 +27,7 @@ pub fn Queue(comptime T: type) type {
             return Self{
                 .head = null,
                 .tail = null,
-                .mutex = std.Mutex{},
+                .mutex = std.Thread.Mutex{},
             };
         }
 
diff --git a/lib/std/auto_reset_event.zig b/lib/std/auto_reset_event.zig
deleted file mode 100644
index 39cd184a68..0000000000
--- a/lib/std/auto_reset_event.zig
+++ /dev/null
@@ -1,226 +0,0 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-const std = @import("std.zig");
-const builtin = @import("builtin");
-const testing = std.testing;
-const assert = std.debug.assert;
-const StaticResetEvent = std.StaticResetEvent;
-
-/// Similar to `StaticResetEvent` but on `set()` it also (atomically) does `reset()`.
-/// Unlike StaticResetEvent, `wait()` can only be called by one thread (MPSC-like).
-pub const AutoResetEvent = struct {
-    /// AutoResetEvent has 3 possible states:
-    /// - UNSET: the AutoResetEvent is currently unset
-    /// - SET: the AutoResetEvent was notified before a wait() was called
-    /// - <StaticResetEvent pointer>: there is an active waiter waiting for a notification.
-    ///
-    /// When attempting to wait:
-    ///  if the event is unset, it registers a ResetEvent pointer to be notified when the event is set
-    ///  if the event is already set, then it consumes the notification and resets the event.
-    ///
-    /// When attempting to notify:
-    ///  if the event is unset, then we set the event
-    ///  if theres a waiting ResetEvent, then we unset the event and notify the ResetEvent
-    ///
-    /// This ensures that the event is automatically reset after a wait() has been issued
-    /// and avoids the race condition when using StaticResetEvent in the following scenario:
-    ///  thread 1                  | thread 2
-    ///  StaticResetEvent.wait()   |
-    ///                            | StaticResetEvent.set()
-    ///                            | StaticResetEvent.set()
-    ///  StaticResetEvent.reset()  |
-    ///  StaticResetEvent.wait()   | (missed the second .set() notification above)
-    state: usize = UNSET,
-
-    const UNSET = 0;
-    const SET = 1;
-
-    /// the minimum alignment for the `*StaticResetEvent` created by wait*()
-    const event_align = std.math.max(@alignOf(StaticResetEvent), 2);
-
-    pub fn wait(self: *AutoResetEvent) void {
-        self.waitFor(null) catch unreachable;
-    }
-
-    pub fn timedWait(self: *AutoResetEvent, timeout: u64) error{TimedOut}!void {
-        return self.waitFor(timeout);
-    }
-
-    fn waitFor(self: *AutoResetEvent, timeout: ?u64) error{TimedOut}!void {
-        // lazily initialized StaticResetEvent
-        var reset_event: StaticResetEvent align(event_align) = undefined;
-        var has_reset_event = false;
-
-        var state = @atomicLoad(usize, &self.state, .SeqCst);
-        while (true) {
-            // consume a notification if there is any
-            if (state == SET) {
-                @atomicStore(usize, &self.state, UNSET, .SeqCst);
-                return;
-            }
-
-            // check if theres currently a pending ResetEvent pointer already registered
-            if (state != UNSET) {
-                unreachable; // multiple waiting threads on the same AutoResetEvent
-            }
-
-            // lazily initialize the ResetEvent if it hasn't been already
-            if (!has_reset_event) {
-                has_reset_event = true;
-                reset_event = .{};
-            }
-
-            // Since the AutoResetEvent currently isnt set,
-            // try to register our ResetEvent on it to wait
-            // for a set() call from another thread.
-            if (@cmpxchgWeak(
-                usize,
-                &self.state,
-                UNSET,
-                @ptrToInt(&reset_event),
-                .SeqCst,
-                .SeqCst,
-            )) |new_state| {
-                state = new_state;
-                continue;
-            }
-
-            // if no timeout was specified, then just wait forever
-            const timeout_ns = timeout orelse {
-                reset_event.wait();
-                return;
-            };
-
-            // wait with a timeout and return if signalled via set()
-            switch (reset_event.timedWait(timeout_ns)) {
-                .event_set => return,
-                .timed_out => {},
-            }
-
-            // If we timed out, we need to transition the AutoResetEvent back to UNSET.
-            // If we don't, then when we return, a set() thread could observe a pointer to an invalid ResetEvent.
-            state = @cmpxchgStrong(
-                usize,
-                &self.state,
-                @ptrToInt(&reset_event),
-                UNSET,
-                .SeqCst,
-                .SeqCst,
-            ) orelse return error.TimedOut;
-
-            // We didn't manage to unregister ourselves from the state.
-            if (state == SET) {
-                unreachable; // AutoResetEvent notified without waking up the waiting thread
-            } else if (state != UNSET) {
-                unreachable; // multiple waiting threads on the same AutoResetEvent observed when timing out
-            }
-
-            // This menas a set() thread saw our ResetEvent pointer, acquired it, and is trying to wake it up.
-            // We need to wait for it to wake up our ResetEvent before we can return and invalidate it.
-            // We don't return error.TimedOut here as it technically notified us while we were "timing out".
-            reset_event.wait();
-            return;
-        }
-    }
-
-    pub fn set(self: *AutoResetEvent) void {
-        var state = @atomicLoad(usize, &self.state, .SeqCst);
-        while (true) {
-            // If the AutoResetEvent is already set, there is nothing else left to do
-            if (state == SET) {
-                return;
-            }
-
-            // If the AutoResetEvent isn't set,
-            // then try to leave a notification for the wait() thread that we set() it.
-            if (state == UNSET) {
-                state = @cmpxchgWeak(
-                    usize,
-                    &self.state,
-                    UNSET,
-                    SET,
-                    .SeqCst,
-                    .SeqCst,
-                ) orelse return;
-                continue;
-            }
-
-            // There is a ResetEvent pointer registered on the AutoResetEvent event thats waiting.
-            // Try to acquire ownership of it so that we can wake it up.
-            // This also resets the AutoResetEvent so that there is no race condition as defined above.
-            if (@cmpxchgWeak(
-                usize,
-                &self.state,
-                state,
-                UNSET,
-                .SeqCst,
-                .SeqCst,
-            )) |new_state| {
-                state = new_state;
-                continue;
-            }
-
-            const reset_event = @intToPtr(*align(event_align) StaticResetEvent, state);
-            reset_event.set();
-            return;
-        }
-    }
-};
-
-test "std.AutoResetEvent" {
-    // test local code paths
-    {
-        var event = AutoResetEvent{};
-        testing.expectError(error.TimedOut, event.timedWait(1));
-        event.set();
-        event.wait();
-    }
-
-    // test cross-thread signaling
-    if (builtin.single_threaded)
-        return;
-
-    const Context = struct {
-        value: u128 = 0,
-        in: AutoResetEvent = AutoResetEvent{},
-        out: AutoResetEvent = AutoResetEvent{},
-
-        const Self = @This();
-
-        fn sender(self: *Self) void {
-            testing.expect(self.value == 0);
-            self.value = 1;
-            self.out.set();
-
-            self.in.wait();
-            testing.expect(self.value == 2);
-            self.value = 3;
-            self.out.set();
-
-            self.in.wait();
-            testing.expect(self.value == 4);
-        }
-
-        fn receiver(self: *Self) void {
-            self.out.wait();
-            testing.expect(self.value == 1);
-            self.value = 2;
-            self.in.set();
-
-            self.out.wait();
-            testing.expect(self.value == 3);
-            self.value = 4;
-            self.in.set();
-        }
-    };
-
-    var context = Context{};
-    const send_thread = try std.Thread.spawn(&context, Context.sender);
-    const recv_thread = try std.Thread.spawn(&context, Context.receiver);
-
-    send_thread.wait();
-    recv_thread.wait();
-}
diff --git a/lib/std/c.zig b/lib/std/c.zig
index b7a412339e..1e86bfbd8c 100644
--- a/lib/std/c.zig
+++ b/lib/std/c.zig
@@ -338,6 +338,13 @@ pub extern "c" fn pthread_cond_signal(cond: *pthread_cond_t) c_int;
 pub extern "c" fn pthread_cond_broadcast(cond: *pthread_cond_t) c_int;
 pub extern "c" fn pthread_cond_destroy(cond: *pthread_cond_t) c_int;
 
+pub extern "c" fn pthread_rwlock_destroy(rwl: *pthread_rwlock_t) callconv(.C) c_int;
+pub extern "c" fn pthread_rwlock_rdlock(rwl: *pthread_rwlock_t) callconv(.C) c_int;
+pub extern "c" fn pthread_rwlock_wrlock(rwl: *pthread_rwlock_t) callconv(.C) c_int;
+pub extern "c" fn pthread_rwlock_tryrdlock(rwl: *pthread_rwlock_t) callconv(.C) c_int;
+pub extern "c" fn pthread_rwlock_trywrlock(rwl: *pthread_rwlock_t) callconv(.C) c_int;
+pub extern "c" fn pthread_rwlock_unlock(rwl: *pthread_rwlock_t) callconv(.C) c_int;
+
 pub const pthread_t = *opaque {};
 pub const FILE = opaque {};
 
diff --git a/lib/std/c/darwin.zig b/lib/std/c/darwin.zig
index 7527752527..b5c3fbf977 100644
--- a/lib/std/c/darwin.zig
+++ b/lib/std/c/darwin.zig
@@ -177,6 +177,10 @@ pub const pthread_cond_t = extern struct {
     __sig: c_long = 0x3CB0B1BB,
     __opaque: [__PTHREAD_COND_SIZE__]u8 = [_]u8{0} ** __PTHREAD_COND_SIZE__,
 };
+pub const pthread_rwlock_t = extern struct {
+    __sig: c_long = 0x2DA8B3B4,
+    __opaque: [192]u8 = [_]u8{0} ** 192,
+};
 pub const sem_t = c_int;
 const __PTHREAD_MUTEX_SIZE__ = if (@sizeOf(usize) == 8) 56 else 40;
 const __PTHREAD_COND_SIZE__ = if (@sizeOf(usize) == 8) 40 else 24;
@@ -192,7 +196,7 @@ pub extern "c" fn pthread_threadid_np(thread: ?pthread_t, thread_id: *u64) c_int
 pub extern "c" fn arc4random_buf(buf: [*]u8, len: usize) void;
 
 // Grand Central Dispatch is exposed by libSystem.
-pub const dispatch_semaphore_t = *opaque{};
+pub const dispatch_semaphore_t = *opaque {};
 pub const dispatch_time_t = u64;
 pub const DISPATCH_TIME_NOW = @as(dispatch_time_t, 0);
 pub const DISPATCH_TIME_FOREVER = ~@as(dispatch_time_t, 0);
diff --git a/lib/std/c/emscripten.zig b/lib/std/c/emscripten.zig
index 1652975eb9..526eb9e99c 100644
--- a/lib/std/c/emscripten.zig
+++ b/lib/std/c/emscripten.zig
@@ -9,5 +9,8 @@ pub const pthread_mutex_t = extern struct {
 pub const pthread_cond_t = extern struct {
     size: [__SIZEOF_PTHREAD_COND_T]u8 align(@alignOf(usize)) = [_]u8{0} ** __SIZEOF_PTHREAD_COND_T,
 };
+pub const pthread_rwlock_t = extern struct {
+    size: [32]u8 align(4) = [_]u8{0} ** 32,
+};
 const __SIZEOF_PTHREAD_COND_T = 48;
 const __SIZEOF_PTHREAD_MUTEX_T = 28;
diff --git a/lib/std/c/freebsd.zig b/lib/std/c/freebsd.zig
index a8d11b95b2..a6c84c66fa 100644
--- a/lib/std/c/freebsd.zig
+++ b/lib/std/c/freebsd.zig
@@ -43,6 +43,9 @@ pub const pthread_mutex_t = extern struct {
 pub const pthread_cond_t = extern struct {
     inner: ?*c_void = null,
 };
+pub const pthread_rwlock_t = extern struct {
+    ptr: ?*c_void = null,
+};
 
 pub const pthread_attr_t = extern struct {
     __size: [56]u8,
diff --git a/lib/std/c/fuchsia.zig b/lib/std/c/fuchsia.zig
index bc53dc81a6..fc34f49d22 100644
--- a/lib/std/c/fuchsia.zig
+++ b/lib/std/c/fuchsia.zig
@@ -9,5 +9,8 @@ pub const pthread_mutex_t = extern struct {
 pub const pthread_cond_t = extern struct {
     size: [__SIZEOF_PTHREAD_COND_T]u8 align(@alignOf(usize)) = [_]u8{0} ** __SIZEOF_PTHREAD_COND_T,
 };
+pub const pthread_rwlock_t = extern struct {
+    size: [56]u8 align(@alignOf(usize)) = [_]u8{0} ** 56,
+};
 const __SIZEOF_PTHREAD_COND_T = 48;
 const __SIZEOF_PTHREAD_MUTEX_T = 40;
diff --git a/lib/std/c/haiku.zig b/lib/std/c/haiku.zig
index 438012c3b3..0f695a9446 100644
--- a/lib/std/c/haiku.zig
+++ b/lib/std/c/haiku.zig
@@ -17,3 +17,12 @@ pub const pthread_cond_t = extern struct {
     waiter_count: i32 = 0,
     lock: i32 = 0,
 };
+pub const pthread_rwlock_t = extern struct {
+    flags: u32 = 0,
+    owner: i32 = -1,
+    lock_sem: i32 = 0,
+    lock_count: i32 = 0,
+    reader_count: i32 = 0,
+    writer_count: i32 = 0,
+    waiters: [2]?*c_void = [_]?*c_void{ null, null },
+};
diff --git a/lib/std/c/hermit.zig b/lib/std/c/hermit.zig
index fa351bc0db..a159395ab3 100644
--- a/lib/std/c/hermit.zig
+++ b/lib/std/c/hermit.zig
@@ -9,3 +9,6 @@ pub const pthread_mutex_t = extern struct {
 pub const pthread_cond_t = extern struct {
     inner: usize = ~@as(usize, 0),
 };
+pub const pthread_rwlock_t = extern struct {
+    ptr: usize = std.math.maxInt(usize),
+};
diff --git a/lib/std/c/linux.zig b/lib/std/c/linux.zig
index db464d7a6d..fbfabdd568 100644
--- a/lib/std/c/linux.zig
+++ b/lib/std/c/linux.zig
@@ -123,6 +123,32 @@ pub const pthread_mutex_t = extern struct {
 pub const pthread_cond_t = extern struct {
     size: [__SIZEOF_PTHREAD_COND_T]u8 align(@alignOf(usize)) = [_]u8{0} ** __SIZEOF_PTHREAD_COND_T,
 };
+pub const pthread_rwlock_t = switch (std.builtin.abi) {
+    .android => switch (@sizeOf(usize)) {
+        4 => extern struct {
+            lock: std.c.pthread_mutex_t = std.c.PTHREAD_MUTEX_INITIALIZER,
+            cond: std.c.pthread_cond_t = std.c.PTHREAD_COND_INITIALIZER,
+            numLocks: c_int = 0,
+            writerThreadId: c_int = 0,
+            pendingReaders: c_int = 0,
+            pendingWriters: c_int = 0,
+            attr: i32 = 0,
+            __reserved: [12]u8 = [_]u8{0} ** 2,
+        },
+        8 => extern struct {
+            numLocks: c_int = 0,
+            writerThreadId: c_int = 0,
+            pendingReaders: c_int = 0,
+            pendingWriters: c_int = 0,
+            attr: i32 = 0,
+            __reserved: [36]u8 = [_]u8{0} ** 36,
+        },
+        else => unreachable,
+    },
+    else => extern struct {
+        size: [56]u8 align(@alignOf(usize)) = [_]u8{0} ** 56,
+    },
+};
 pub const sem_t = extern struct {
     __size: [__SIZEOF_SEM_T]u8 align(@alignOf(usize)),
 };
diff --git a/lib/std/c/netbsd.zig b/lib/std/c/netbsd.zig
index 258ee6a5c6..46a38706f4 100644
--- a/lib/std/c/netbsd.zig
+++ b/lib/std/c/netbsd.zig
@@ -54,6 +54,22 @@ pub const pthread_cond_t = extern struct {
     ptc_private: ?*c_void = null,
 };
 
+pub const pthread_rwlock_t = extern struct {
+    ptr_magic: c_uint = 0x99990009,
+    ptr_interlock: switch (std.builtin.arch) {
+        .aarch64, .sparc, .x86_64, .i386 => u8,
+        .arm, .powerpc => c_int,
+        else => unreachable,
+    } = 0,
+    ptr_rblocked_first: ?*u8 = null,
+    ptr_rblocked_last: ?*u8 = null,
+    ptr_wblocked_first: ?*u8 = null,
+    ptr_wblocked_last: ?*u8 = null,
+    ptr_nreaders: c_uint = 0,
+    ptr_owner: std.c.pthread_t = null,
+    ptr_private: ?*c_void = null,
+};
+
 const pthread_spin_t = switch (builtin.arch) {
     .aarch64, .aarch64_be, .aarch64_32 => u8,
     .mips, .mipsel, .mips64, .mips64el => u32,
diff --git a/lib/std/c/openbsd.zig b/lib/std/c/openbsd.zig
index dd89c837ff..99debf57e7 100644
--- a/lib/std/c/openbsd.zig
+++ b/lib/std/c/openbsd.zig
@@ -27,6 +27,9 @@ pub const pthread_mutex_t = extern struct {
 pub const pthread_cond_t = extern struct {
     inner: ?*c_void = null,
 };
+pub const pthread_rwlock_t = extern struct {
+    ptr: ?*c_void = null,
+};
 pub const pthread_spinlock_t = extern struct {
     inner: ?*c_void = null,
 };
diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index 15d3baa1d0..e4ef25724b 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -50,7 +50,7 @@ pub const LineInfo = struct {
     }
 };
 
-var stderr_mutex = std.Mutex{};
+var stderr_mutex = std.Thread.Mutex{};
 
 /// Deprecated. Use `std.log` functions for logging or `std.debug.print` for
 /// "printf debugging".
@@ -65,7 +65,7 @@ pub fn print(comptime fmt: []const u8, args: anytype) void {
     nosuspend stderr.print(fmt, args) catch return;
 }
 
-pub fn getStderrMutex() *std.Mutex {
+pub fn getStderrMutex() *std.Thread.Mutex {
     return &stderr_mutex;
 }
 
@@ -235,7 +235,7 @@ pub fn panic(comptime format: []const u8, args: anytype) noreturn {
 var panicking: u8 = 0;
 
 // Locked to avoid interleaving panic messages from multiple threads.
-var panic_mutex = std.Mutex{};
+var panic_mutex = std.Thread.Mutex{};
 
 /// Counts how many times the panic handler is invoked by this thread.
 /// This is used to catch and handle panics triggered by the panic handler.
@@ -280,7 +280,7 @@ pub fn panicExtra(trace: ?*const builtin.StackTrace, first_trace_addr: ?usize, c
                 // and call abort()
 
                 // Sleep forever without hammering the CPU
-                var event: std.StaticResetEvent = .{};
+                var event: std.Thread.StaticResetEvent = .{};
                 event.wait();
                 unreachable;
             }
diff --git a/lib/std/event/lock.zig b/lib/std/event/lock.zig
index 17d79c753b..d48c6c1520 100644
--- a/lib/std/event/lock.zig
+++ b/lib/std/event/lock.zig
@@ -16,7 +16,7 @@ const Loop = std.event.Loop;
 /// Allows only one actor to hold the lock.
 /// TODO: make this API also work in blocking I/O mode.
 pub const Lock = struct {
-    mutex: std.Mutex = std.Mutex{},
+    mutex: std.Thread.Mutex = std.Thread.Mutex{},
     head: usize = UNLOCKED,
 
     const UNLOCKED = 0;
diff --git a/lib/std/event/loop.zig b/lib/std/event/loop.zig
index 8101d27a55..492b7c1758 100644
--- a/lib/std/event/loop.zig
+++ b/lib/std/event/loop.zig
@@ -29,7 +29,7 @@ pub const Loop = struct {
     fs_thread: *Thread,
     fs_queue: std.atomic.Queue(Request),
     fs_end_request: Request.Node,
-    fs_thread_wakeup: std.ResetEvent,
+    fs_thread_wakeup: std.Thread.ResetEvent,
 
     /// For resources that have the same lifetime as the `Loop`.
     /// This is only used by `Loop` for the thread pool and associated resources.
@@ -785,7 +785,7 @@ pub const Loop = struct {
         timer: std.time.Timer,
         waiters: Waiters,
         thread: *std.Thread,
-        event: std.AutoResetEvent,
+        event: std.Thread.AutoResetEvent,
         is_running: bool,
 
         /// Initialize the delay queue by spawning the timer thread
@@ -796,7 +796,7 @@ pub const Loop = struct {
                 .waiters = DelayQueue.Waiters{
                     .entries = std.atomic.Queue(anyframe).init(),
                 },
-                .event = std.AutoResetEvent{},
+                .event = std.Thread.AutoResetEvent{},
                 .is_running = true,
                 // Must be last so that it can read the other state, such as `is_running`.
                 .thread = try std.Thread.spawn(self, DelayQueue.run),
diff --git a/lib/std/event/wait_group.zig b/lib/std/event/wait_group.zig
index d123f7df27..0b83c18c74 100644
--- a/lib/std/event/wait_group.zig
+++ b/lib/std/event/wait_group.zig
@@ -30,7 +30,7 @@ pub fn WaitGroupGeneric(comptime counter_size: u16) type {
     return struct {
         counter: CounterType = 0,
         max_counter: CounterType = std.math.maxInt(CounterType),
-        mutex: std.Mutex = .{},
+        mutex: std.Thread.Mutex = .{},
         waiters: ?*Waiter = null,
         const Waiter = struct {
             next: ?*Waiter,
diff --git a/lib/std/fs/test.zig b/lib/std/fs/test.zig
index 9afcc50f3c..e9f28a0b60 100644
--- a/lib/std/fs/test.zig
+++ b/lib/std/fs/test.zig
@@ -750,7 +750,7 @@ test "open file with exclusive lock twice, make sure it waits" {
     errdefer file.close();
 
     const S = struct {
-        const C = struct { dir: *fs.Dir, evt: *std.ResetEvent };
+        const C = struct { dir: *fs.Dir, evt: *std.Thread.ResetEvent };
         fn checkFn(ctx: C) !void {
             const file1 = try ctx.dir.createFile(filename, .{ .lock = .Exclusive });
             defer file1.close();
@@ -758,7 +758,7 @@ test "open file with exclusive lock twice, make sure it waits" {
         }
     };
 
-    var evt: std.ResetEvent = undefined;
+    var evt: std.Thread.ResetEvent = undefined;
     try evt.init();
     defer evt.deinit();
 
diff --git a/lib/std/heap/general_purpose_allocator.zig b/lib/std/heap/general_purpose_allocator.zig
index 415b206b6a..05f05c1da3 100644
--- a/lib/std/heap/general_purpose_allocator.zig
+++ b/lib/std/heap/general_purpose_allocator.zig
@@ -149,12 +149,12 @@ pub const Config = struct {
     thread_safe: bool = !std.builtin.single_threaded,
 
     /// What type of mutex you'd like to use, for thread safety.
-    /// when specfied, the mutex type must have the same shape as `std.Mutex` and
+    /// when specfied, the mutex type must have the same shape as `std.Thread.Mutex` and
     /// `std.mutex.Dummy`, and have no required fields. Specifying this field causes
     /// the `thread_safe` field to be ignored.
     ///
     /// when null (default):
-    /// * the mutex type defaults to `std.Mutex` when thread_safe is enabled.
+    /// * the mutex type defaults to `std.Thread.Mutex` when thread_safe is enabled.
     /// * the mutex type defaults to `std.mutex.Dummy` otherwise.
     MutexType: ?type = null,
 
@@ -187,7 +187,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
         const mutex_init = if (config.MutexType) |T|
             T{}
         else if (config.thread_safe)
-            std.Mutex{}
+            std.Thread.Mutex{}
         else
             std.mutex.Dummy{};
 
@@ -869,9 +869,9 @@ test "realloc large object to small object" {
 }
 
 test "overrideable mutexes" {
-    var gpa = GeneralPurposeAllocator(.{ .MutexType = std.Mutex }){
+    var gpa = GeneralPurposeAllocator(.{ .MutexType = std.Thread.Mutex }){
         .backing_allocator = std.testing.allocator,
-        .mutex = std.Mutex{},
+        .mutex = std.Thread.Mutex{},
     };
     defer std.testing.expect(!gpa.deinit());
     const allocator = &gpa.allocator;
diff --git a/lib/std/mutex.zig b/lib/std/mutex.zig
deleted file mode 100644
index 50bbb40bf0..0000000000
--- a/lib/std/mutex.zig
+++ /dev/null
@@ -1,379 +0,0 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-const std = @import("std.zig");
-const builtin = @import("builtin");
-const os = std.os;
-const assert = std.debug.assert;
-const windows = os.windows;
-const testing = std.testing;
-const SpinLock = std.SpinLock;
-const StaticResetEvent = std.StaticResetEvent;
-
-/// Lock may be held only once. If the same thread tries to acquire
-/// the same mutex twice, it deadlocks.  This type supports static
-/// initialization and is at most `@sizeOf(usize)` in size.  When an
-/// application is built in single threaded release mode, all the
-/// functions are no-ops. In single threaded debug mode, there is
-/// deadlock detection.
-///
-/// Example usage:
-/// var m = Mutex{};
-///
-/// const lock = m.acquire();
-/// defer lock.release();
-/// ... critical code
-///
-/// Non-blocking:
-/// if (m.tryAcquire) |lock| {
-///     defer lock.release();
-///     // ... critical section
-/// } else {
-///     // ... lock not acquired
-/// }
-pub const Mutex = if (builtin.single_threaded)
-    Dummy
-else if (builtin.os.tag == .windows)
-    WindowsMutex
-else if (std.Thread.use_pthreads)
-    PthreadMutex
-else if (builtin.link_libc or builtin.os.tag == .linux)
-    // stack-based version of https://github.com/Amanieu/parking_lot/blob/master/core/src/word_lock.rs
-    struct {
-        state: usize = 0,
-
-        /// number of times to spin trying to acquire the lock.
-        /// https://webkit.org/blog/6161/locking-in-webkit/
-        const SPIN_COUNT = 40;
-
-        const MUTEX_LOCK: usize = 1 << 0;
-        const QUEUE_LOCK: usize = 1 << 1;
-        const QUEUE_MASK: usize = ~(MUTEX_LOCK | QUEUE_LOCK);
-
-        const Node = struct {
-            next: ?*Node,
-            event: StaticResetEvent,
-        };
-
-        pub fn tryAcquire(self: *Mutex) ?Held {
-            if (@cmpxchgWeak(usize, &self.state, 0, MUTEX_LOCK, .Acquire, .Monotonic) != null)
-                return null;
-            return Held{ .mutex = self };
-        }
-
-        pub fn acquire(self: *Mutex) Held {
-            return self.tryAcquire() orelse {
-                self.acquireSlow();
-                return Held{ .mutex = self };
-            };
-        }
-
-        fn acquireSlow(self: *Mutex) void {
-            // inlining the fast path and hiding *Slow()
-            // calls behind a @setCold(true) appears to
-            // improve performance in release builds.
-            @setCold(true);
-            while (true) {
-
-                // try and spin for a bit to acquire the mutex if theres currently no queue
-                var spin_count: u32 = SPIN_COUNT;
-                var state = @atomicLoad(usize, &self.state, .Monotonic);
-                while (spin_count != 0) : (spin_count -= 1) {
-                    if (state & MUTEX_LOCK == 0) {
-                        _ = @cmpxchgWeak(usize, &self.state, state, state | MUTEX_LOCK, .Acquire, .Monotonic) orelse return;
-                    } else if (state & QUEUE_MASK == 0) {
-                        break;
-                    }
-                    SpinLock.yield();
-                    state = @atomicLoad(usize, &self.state, .Monotonic);
-                }
-
-                // create the StaticResetEvent node on the stack
-                // (faster than threadlocal on platforms like OSX)
-                var node: Node = .{
-                    .next = undefined,
-                    .event = .{},
-                };
-
-                // we've spun too long, try and add our node to the LIFO queue.
-                // if the mutex becomes available in the process, try and grab it instead.
-                while (true) {
-                    if (state & MUTEX_LOCK == 0) {
-                        _ = @cmpxchgWeak(usize, &self.state, state, state | MUTEX_LOCK, .Acquire, .Monotonic) orelse return;
-                    } else {
-                        node.next = @intToPtr(?*Node, state & QUEUE_MASK);
-                        const new_state = @ptrToInt(&node) | (state & ~QUEUE_MASK);
-                        _ = @cmpxchgWeak(usize, &self.state, state, new_state, .Release, .Monotonic) orelse {
-                            node.event.wait();
-                            break;
-                        };
-                    }
-                    SpinLock.yield();
-                    state = @atomicLoad(usize, &self.state, .Monotonic);
-                }
-            }
-        }
-
-        /// Returned when the lock is acquired. Call release to
-        /// release.
-        pub const Held = struct {
-            mutex: *Mutex,
-
-            /// Release the held lock.
-            pub fn release(self: Held) void {
-                // first, remove the lock bit so another possibly parallel acquire() can succeed.
-                // use .Sub since it can be usually compiled down more efficiency
-                // (`lock sub` on x86) vs .And ~MUTEX_LOCK (`lock cmpxchg` loop on x86)
-                const state = @atomicRmw(usize, &self.mutex.state, .Sub, MUTEX_LOCK, .Release);
-
-                // if the LIFO queue isnt locked and it has a node, try and wake up the node.
-                if ((state & QUEUE_LOCK) == 0 and (state & QUEUE_MASK) != 0)
-                    self.mutex.releaseSlow();
-            }
-        };
-
-        fn releaseSlow(self: *Mutex) void {
-            @setCold(true);
-
-            // try and lock the LFIO queue to pop a node off,
-            // stopping altogether if its already locked or the queue is empty
-            var state = @atomicLoad(usize, &self.state, .Monotonic);
-            while (true) : (SpinLock.loopHint(1)) {
-                if (state & QUEUE_LOCK != 0 or state & QUEUE_MASK == 0)
-                    return;
-                state = @cmpxchgWeak(usize, &self.state, state, state | QUEUE_LOCK, .Acquire, .Monotonic) orelse break;
-            }
-
-            // acquired the QUEUE_LOCK, try and pop a node to wake it.
-            // if the mutex is locked, then unset QUEUE_LOCK and let
-            // the thread who holds the mutex do the wake-up on unlock()
-            while (true) : (SpinLock.loopHint(1)) {
-                if ((state & MUTEX_LOCK) != 0) {
-                    state = @cmpxchgWeak(usize, &self.state, state, state & ~QUEUE_LOCK, .Release, .Acquire) orelse return;
-                } else {
-                    const node = @intToPtr(*Node, state & QUEUE_MASK);
-                    const new_state = @ptrToInt(node.next);
-                    state = @cmpxchgWeak(usize, &self.state, state, new_state, .Release, .Acquire) orelse {
-                        node.event.set();
-                        return;
-                    };
-                }
-            }
-        }
-    }
-
-    // for platforms without a known OS blocking
-    // primitive, default to SpinLock for correctness
-else
-    SpinLock;
-
-pub const PthreadMutex = struct {
-    pthread_mutex: std.c.pthread_mutex_t = init,
-
-    pub const Held = struct {
-        mutex: *PthreadMutex,
-
-        pub fn release(self: Held) void {
-            switch (std.c.pthread_mutex_unlock(&self.mutex.pthread_mutex)) {
-                0 => return,
-                std.c.EINVAL => unreachable,
-                std.c.EAGAIN => unreachable,
-                std.c.EPERM => unreachable,
-                else => unreachable,
-            }
-        }
-    };
-
-    /// Create a new mutex in unlocked state.
-    pub const init = std.c.PTHREAD_MUTEX_INITIALIZER;
-
-    /// Try to acquire the mutex without blocking. Returns null if
-    /// the mutex is unavailable. Otherwise returns Held. Call
-    /// release on Held.
-    pub fn tryAcquire(self: *PthreadMutex) ?Held {
-        if (std.c.pthread_mutex_trylock(&self.pthread_mutex) == 0) {
-            return Held{ .mutex = self };
-        } else {
-            return null;
-        }
-    }
-
-    /// Acquire the mutex. Will deadlock if the mutex is already
-    /// held by the calling thread.
-    pub fn acquire(self: *PthreadMutex) Held {
-        switch (std.c.pthread_mutex_lock(&self.pthread_mutex)) {
-            0 => return Held{ .mutex = self },
-            std.c.EINVAL => unreachable,
-            std.c.EBUSY => unreachable,
-            std.c.EAGAIN => unreachable,
-            std.c.EDEADLK => unreachable,
-            std.c.EPERM => unreachable,
-            else => unreachable,
-        }
-    }
-};
-
-/// This has the sematics as `Mutex`, however it does not actually do any
-/// synchronization. Operations are safety-checked no-ops.
-pub const Dummy = struct {
-    lock: @TypeOf(lock_init) = lock_init,
-
-    const lock_init = if (std.debug.runtime_safety) false else {};
-
-    pub const Held = struct {
-        mutex: *Dummy,
-
-        pub fn release(self: Held) void {
-            if (std.debug.runtime_safety) {
-                self.mutex.lock = false;
-            }
-        }
-    };
-
-    /// Create a new mutex in unlocked state.
-    pub const init = Dummy{};
-
-    /// Try to acquire the mutex without blocking. Returns null if
-    /// the mutex is unavailable. Otherwise returns Held. Call
-    /// release on Held.
-    pub fn tryAcquire(self: *Dummy) ?Held {
-        if (std.debug.runtime_safety) {
-            if (self.lock) return null;
-            self.lock = true;
-        }
-        return Held{ .mutex = self };
-    }
-
-    /// Acquire the mutex. Will deadlock if the mutex is already
-    /// held by the calling thread.
-    pub fn acquire(self: *Dummy) Held {
-        return self.tryAcquire() orelse @panic("deadlock detected");
-    }
-};
-
-// https://locklessinc.com/articles/keyed_events/
-const WindowsMutex = struct {
-    state: State = State{ .waiters = 0 },
-
-    const State = extern union {
-        locked: u8,
-        waiters: u32,
-    };
-
-    const WAKE = 1 << 8;
-    const WAIT = 1 << 9;
-
-    pub fn tryAcquire(self: *WindowsMutex) ?Held {
-        if (@atomicRmw(u8, &self.state.locked, .Xchg, 1, .Acquire) != 0)
-            return null;
-        return Held{ .mutex = self };
-    }
-
-    pub fn acquire(self: *WindowsMutex) Held {
-        return self.tryAcquire() orelse self.acquireSlow();
-    }
-
-    fn acquireSpinning(self: *WindowsMutex) Held {
-        @setCold(true);
-        while (true) : (SpinLock.yield()) {
-            return self.tryAcquire() orelse continue;
-        }
-    }
-
-    fn acquireSlow(self: *WindowsMutex) Held {
-        // try to use NT keyed events for blocking, falling back to spinlock if unavailable
-        @setCold(true);
-        const handle = StaticResetEvent.Impl.Futex.getEventHandle() orelse return self.acquireSpinning();
-        const key = @ptrCast(*const c_void, &self.state.waiters);
-
-        while (true) : (SpinLock.loopHint(1)) {
-            const waiters = @atomicLoad(u32, &self.state.waiters, .Monotonic);
-
-            // try and take lock if unlocked
-            if ((waiters & 1) == 0) {
-                if (@atomicRmw(u8, &self.state.locked, .Xchg, 1, .Acquire) == 0) {
-                    return Held{ .mutex = self };
-                }
-
-                // otherwise, try and update the waiting count.
-                // then unset the WAKE bit so that another unlocker can wake up a thread.
-            } else if (@cmpxchgWeak(u32, &self.state.waiters, waiters, (waiters + WAIT) | 1, .Monotonic, .Monotonic) == null) {
-                const rc = windows.ntdll.NtWaitForKeyedEvent(handle, key, windows.FALSE, null);
-                assert(rc == .SUCCESS);
-                _ = @atomicRmw(u32, &self.state.waiters, .Sub, WAKE, .Monotonic);
-            }
-        }
-    }
-
-    pub const Held = struct {
-        mutex: *WindowsMutex,
-
-        pub fn release(self: Held) void {
-            // unlock without a rmw/cmpxchg instruction
-            @atomicStore(u8, @ptrCast(*u8, &self.mutex.state.locked), 0, .Release);
-            const handle = StaticResetEvent.Impl.Futex.getEventHandle() orelse return;
-            const key = @ptrCast(*const c_void, &self.mutex.state.waiters);
-
-            while (true) : (SpinLock.loopHint(1)) {
-                const waiters = @atomicLoad(u32, &self.mutex.state.waiters, .Monotonic);
-
-                // no one is waiting
-                if (waiters < WAIT) return;
-                // someone grabbed the lock and will do the wake instead
-                if (waiters & 1 != 0) return;
-                // someone else is currently waking up
-                if (waiters & WAKE != 0) return;
-
-                // try to decrease the waiter count & set the WAKE bit meaning a thread is waking up
-                if (@cmpxchgWeak(u32, &self.mutex.state.waiters, waiters, waiters - WAIT + WAKE, .Release, .Monotonic) == null) {
-                    const rc = windows.ntdll.NtReleaseKeyedEvent(handle, key, windows.FALSE, null);
-                    assert(rc == .SUCCESS);
-                    return;
-                }
-            }
-        }
-    };
-};
-
-const TestContext = struct {
-    mutex: *Mutex,
-    data: i128,
-
-    const incr_count = 10000;
-};
-
-test "std.Mutex" {
-    var mutex = Mutex{};
-
-    var context = TestContext{
-        .mutex = &mutex,
-        .data = 0,
-    };
-
-    if (builtin.single_threaded) {
-        worker(&context);
-        testing.expect(context.data == TestContext.incr_count);
-    } else {
-        const thread_count = 10;
-        var threads: [thread_count]*std.Thread = undefined;
-        for (threads) |*t| {
-            t.* = try std.Thread.spawn(&context, worker);
-        }
-        for (threads) |t|
-            t.wait();
-
-        testing.expect(context.data == thread_count * TestContext.incr_count);
-    }
-}
-
-fn worker(ctx: *TestContext) void {
-    var i: usize = 0;
-    while (i != TestContext.incr_count) : (i += 1) {
-        const held = ctx.mutex.acquire();
-        defer held.release();
-
-        ctx.data += 1;
-    }
-}
diff --git a/lib/std/once.zig b/lib/std/once.zig
index f4ac47f8d8..efa99060d3 100644
--- a/lib/std/once.zig
+++ b/lib/std/once.zig
@@ -15,7 +15,7 @@ pub fn once(comptime f: fn () void) Once(f) {
 pub fn Once(comptime f: fn () void) type {
     return struct {
         done: bool = false,
-        mutex: std.Mutex = std.Mutex{},
+        mutex: std.Thread.Mutex = std.Thread.Mutex{},
 
         /// Call the function `f`.
         /// If `call` is invoked multiple times `f` will be executed only the
diff --git a/lib/std/os/windows/bits.zig b/lib/std/os/windows/bits.zig
index d8a2fb4a4d..8461378da0 100644
--- a/lib/std/os/windows/bits.zig
+++ b/lib/std/os/windows/bits.zig
@@ -1635,3 +1635,8 @@ pub const OBJECT_NAME_INFORMATION = extern struct {
     Name: UNICODE_STRING,
 };
 pub const POBJECT_NAME_INFORMATION = *OBJECT_NAME_INFORMATION;
+
+pub const SRWLOCK = usize;
+pub const SRWLOCK_INIT: SRWLOCK = 0;
+pub const CONDITION_VARIABLE = usize;
+pub const CONDITION_VARIABLE_INIT: CONDITION_VARIABLE = 0;
diff --git a/lib/std/os/windows/kernel32.zig b/lib/std/os/windows/kernel32.zig
index 78d2a6501e..ec4a75afa9 100644
--- a/lib/std/os/windows/kernel32.zig
+++ b/lib/std/os/windows/kernel32.zig
@@ -289,3 +289,16 @@ pub extern "kernel32" fn K32QueryWorkingSet(hProcess: HANDLE, pv: PVOID, cb: DWO
 pub extern "kernel32" fn K32QueryWorkingSetEx(hProcess: HANDLE, pv: PVOID, cb: DWORD) callconv(WINAPI) BOOL;
 
 pub extern "kernel32" fn FlushFileBuffers(hFile: HANDLE) callconv(WINAPI) BOOL;
+
+pub extern "kernel32" fn WakeAllConditionVariable(c: *CONDITION_VARIABLE) callconv(WINAPI) void;
+pub extern "kernel32" fn WakeConditionVariable(c: *CONDITION_VARIABLE) callconv(WINAPI) void;
+pub extern "kernel32" fn SleepConditionVariableSRW(
+    c: *CONDITION_VARIABLE,
+    s: *SRWLOCK,
+    t: DWORD,
+    f: ULONG,
+) callconv(WINAPI) BOOL;
+
+pub extern "kernel32" fn TryAcquireSRWLockExclusive(s: *SRWLOCK) callconv(WINAPI) BOOL;
+pub extern "kernel32" fn AcquireSRWLockExclusive(s: *SRWLOCK) callconv(WINAPI) void;
+pub extern "kernel32" fn ReleaseSRWLockExclusive(s: *SRWLOCK) callconv(WINAPI) void;
diff --git a/lib/std/std.zig b/lib/std/std.zig
index d736899a45..d085d4fc41 100644
--- a/lib/std/std.zig
+++ b/lib/std/std.zig
@@ -13,7 +13,6 @@ pub const AutoArrayHashMap = array_hash_map.AutoArrayHashMap;
 pub const AutoArrayHashMapUnmanaged = array_hash_map.AutoArrayHashMapUnmanaged;
 pub const AutoHashMap = hash_map.AutoHashMap;
 pub const AutoHashMapUnmanaged = hash_map.AutoHashMapUnmanaged;
-pub const AutoResetEvent = @import("auto_reset_event.zig").AutoResetEvent;
 pub const BufMap = @import("buf_map.zig").BufMap;
 pub const BufSet = @import("buf_set.zig").BufSet;
 pub const ChildProcess = @import("child_process.zig").ChildProcess;
@@ -21,26 +20,21 @@ pub const ComptimeStringMap = @import("comptime_string_map.zig").ComptimeStringM
 pub const DynLib = @import("dynamic_library.zig").DynLib;
 pub const HashMap = hash_map.HashMap;
 pub const HashMapUnmanaged = hash_map.HashMapUnmanaged;
-pub const mutex = @import("mutex.zig");
-pub const Mutex = mutex.Mutex;
 pub const PackedIntArray = @import("packed_int_array.zig").PackedIntArray;
 pub const PackedIntArrayEndian = @import("packed_int_array.zig").PackedIntArrayEndian;
 pub const PackedIntSlice = @import("packed_int_array.zig").PackedIntSlice;
 pub const PackedIntSliceEndian = @import("packed_int_array.zig").PackedIntSliceEndian;
 pub const PriorityQueue = @import("priority_queue.zig").PriorityQueue;
 pub const Progress = @import("Progress.zig");
-pub const ResetEvent = @import("ResetEvent.zig");
 pub const SemanticVersion = @import("SemanticVersion.zig");
 pub const SinglyLinkedList = @import("linked_list.zig").SinglyLinkedList;
-pub const SpinLock = @import("SpinLock.zig");
-pub const StaticResetEvent = @import("StaticResetEvent.zig");
 pub const StringHashMap = hash_map.StringHashMap;
 pub const StringHashMapUnmanaged = hash_map.StringHashMapUnmanaged;
 pub const StringArrayHashMap = array_hash_map.StringArrayHashMap;
 pub const StringArrayHashMapUnmanaged = array_hash_map.StringArrayHashMapUnmanaged;
 pub const TailQueue = @import("linked_list.zig").TailQueue;
 pub const Target = @import("target.zig").Target;
-pub const Thread = @import("thread.zig").Thread;
+pub const Thread = @import("Thread.zig");
 
 pub const array_hash_map = @import("array_hash_map.zig");
 pub const atomic = @import("atomic.zig");
@@ -98,12 +92,7 @@ test "" {
         // server is hitting OOM. TODO revert this after stage2 arrives.
         _ = ChildProcess;
         _ = DynLib;
-        _ = mutex;
-        _ = Mutex;
         _ = Progress;
-        _ = ResetEvent;
-        _ = SpinLock;
-        _ = StaticResetEvent;
         _ = Target;
         _ = Thread;
 
diff --git a/lib/std/thread.zig b/lib/std/thread.zig
deleted file mode 100644
index 0fee13b057..0000000000
--- a/lib/std/thread.zig
+++ /dev/null
@@ -1,526 +0,0 @@
-// SPDX-License-Identifier: MIT
-// Copyright (c) 2015-2021 Zig Contributors
-// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
-// The MIT license requires this copyright notice to be included in all copies
-// and substantial portions of the software.
-const std = @import("std.zig");
-const builtin = std.builtin;
-const os = std.os;
-const mem = std.mem;
-const windows = std.os.windows;
-const c = std.c;
-const assert = std.debug.assert;
-
-const bad_startfn_ret = "expected return type of startFn to be 'u8', 'noreturn', 'void', or '!void'";
-
-pub const Thread = struct {
-    data: Data,
-
-    pub const use_pthreads = std.Target.current.os.tag != .windows and builtin.link_libc;
-
-    /// Represents a kernel thread handle.
-    /// May be an integer or a pointer depending on the platform.
-    /// On Linux and POSIX, this is the same as Id.
-    pub const Handle = if (use_pthreads)
-        c.pthread_t
-    else switch (std.Target.current.os.tag) {
-        .linux => i32,
-        .windows => windows.HANDLE,
-        else => void,
-    };
-
-    /// Represents a unique ID per thread.
-    /// May be an integer or pointer depending on the platform.
-    /// On Linux and POSIX, this is the same as Handle.
-    pub const Id = switch (std.Target.current.os.tag) {
-        .windows => windows.DWORD,
-        else => Handle,
-    };
-
-    pub const Data = if (use_pthreads)
-        struct {
-            handle: Thread.Handle,
-            memory: []u8,
-        }
-    else switch (std.Target.current.os.tag) {
-        .linux => struct {
-            handle: Thread.Handle,
-            memory: []align(mem.page_size) u8,
-        },
-        .windows => struct {
-            handle: Thread.Handle,
-            alloc_start: *c_void,
-            heap_handle: windows.HANDLE,
-        },
-        else => struct {},
-    };
-
-    /// Returns the ID of the calling thread.
-    /// Makes a syscall every time the function is called.
-    /// On Linux and POSIX, this Id is the same as a Handle.
-    pub fn getCurrentId() Id {
-        if (use_pthreads) {
-            return c.pthread_self();
-        } else
-            return switch (std.Target.current.os.tag) {
-                .linux => os.linux.gettid(),
-                .windows => windows.kernel32.GetCurrentThreadId(),
-                else => @compileError("Unsupported OS"),
-            };
-    }
-
-    /// Returns the handle of this thread.
-    /// On Linux and POSIX, this is the same as Id.
-    /// On Linux, it is possible that the thread spawned with `spawn`
-    /// finishes executing entirely before the clone syscall completes. In this
-    /// case, this function will return 0 rather than the no-longer-existing thread's
-    /// pid.
-    pub fn handle(self: Thread) Handle {
-        return self.data.handle;
-    }
-
-    pub fn wait(self: *Thread) void {
-        if (use_pthreads) {
-            const err = c.pthread_join(self.data.handle, null);
-            switch (err) {
-                0 => {},
-                os.EINVAL => unreachable,
-                os.ESRCH => unreachable,
-                os.EDEADLK => unreachable,
-                else => unreachable,
-            }
-            std.heap.c_allocator.free(self.data.memory);
-            std.heap.c_allocator.destroy(self);
-        } else switch (std.Target.current.os.tag) {
-            .linux => {
-                while (true) {
-                    const pid_value = @atomicLoad(i32, &self.data.handle, .SeqCst);
-                    if (pid_value == 0) break;
-                    const rc = os.linux.futex_wait(&self.data.handle, os.linux.FUTEX_WAIT, pid_value, null);
-                    switch (os.linux.getErrno(rc)) {
-                        0 => continue,
-                        os.EINTR => continue,
-                        os.EAGAIN => continue,
-                        else => unreachable,
-                    }
-                }
-                os.munmap(self.data.memory);
-            },
-            .windows => {
-                windows.WaitForSingleObjectEx(self.data.handle, windows.INFINITE, false) catch unreachable;
-                windows.CloseHandle(self.data.handle);
-                windows.HeapFree(self.data.heap_handle, 0, self.data.alloc_start);
-            },
-            else => @compileError("Unsupported OS"),
-        }
-    }
-
-    pub const SpawnError = error{
-        /// A system-imposed limit on the number of threads was encountered.
-        /// There are a number of limits that may trigger this error:
-        /// *  the  RLIMIT_NPROC soft resource limit (set via setrlimit(2)),
-        ///    which limits the number of processes and threads for  a  real
-        ///    user ID, was reached;
-        /// *  the kernel's system-wide limit on the number of processes and
-        ///    threads,  /proc/sys/kernel/threads-max,  was   reached   (see
-        ///    proc(5));
-        /// *  the  maximum  number  of  PIDs, /proc/sys/kernel/pid_max, was
-        ///    reached (see proc(5)); or
-        /// *  the PID limit (pids.max) imposed by the cgroup "process  num‐
-        ///    ber" (PIDs) controller was reached.
-        ThreadQuotaExceeded,
-
-        /// The kernel cannot allocate sufficient memory to allocate a task structure
-        /// for the child, or to copy those parts of the caller's context that need to
-        /// be copied.
-        SystemResources,
-
-        /// Not enough userland memory to spawn the thread.
-        OutOfMemory,
-
-        /// `mlockall` is enabled, and the memory needed to spawn the thread
-        /// would exceed the limit.
-        LockedMemoryLimitExceeded,
-
-        Unexpected,
-    };
-
-    /// caller must call wait on the returned thread
-    /// fn startFn(@TypeOf(context)) T
-    /// where T is u8, noreturn, void, or !void
-    /// caller must call wait on the returned thread
-    pub fn spawn(context: anytype, comptime startFn: anytype) SpawnError!*Thread {
-        if (builtin.single_threaded) @compileError("cannot spawn thread when building in single-threaded mode");
-        // TODO compile-time call graph analysis to determine stack upper bound
-        // https://github.com/ziglang/zig/issues/157
-        const default_stack_size = 16 * 1024 * 1024;
-
-        const Context = @TypeOf(context);
-        comptime assert(@typeInfo(@TypeOf(startFn)).Fn.args[0].arg_type.? == Context);
-
-        if (std.Target.current.os.tag == .windows) {
-            const WinThread = struct {
-                const OuterContext = struct {
-                    thread: Thread,
-                    inner: Context,
-                };
-                fn threadMain(raw_arg: windows.LPVOID) callconv(.C) windows.DWORD {
-                    const arg = if (@sizeOf(Context) == 0) {} else @ptrCast(*Context, @alignCast(@alignOf(Context), raw_arg)).*;
-
-                    switch (@typeInfo(@typeInfo(@TypeOf(startFn)).Fn.return_type.?)) {
-                        .NoReturn => {
-                            startFn(arg);
-                        },
-                        .Void => {
-                            startFn(arg);
-                            return 0;
-                        },
-                        .Int => |info| {
-                            if (info.bits != 8) {
-                                @compileError(bad_startfn_ret);
-                            }
-                            return startFn(arg);
-                        },
-                        .ErrorUnion => |info| {
-                            if (info.payload != void) {
-                                @compileError(bad_startfn_ret);
-                            }
-                            startFn(arg) catch |err| {
-                                std.debug.warn("error: {s}\n", .{@errorName(err)});
-                                if (@errorReturnTrace()) |trace| {
-                                    std.debug.dumpStackTrace(trace.*);
-                                }
-                            };
-                            return 0;
-                        },
-                        else => @compileError(bad_startfn_ret),
-                    }
-                }
-            };
-
-            const heap_handle = windows.kernel32.GetProcessHeap() orelse return error.OutOfMemory;
-            const byte_count = @alignOf(WinThread.OuterContext) + @sizeOf(WinThread.OuterContext);
-            const bytes_ptr = windows.kernel32.HeapAlloc(heap_handle, 0, byte_count) orelse return error.OutOfMemory;
-            errdefer assert(windows.kernel32.HeapFree(heap_handle, 0, bytes_ptr) != 0);
-            const bytes = @ptrCast([*]u8, bytes_ptr)[0..byte_count];
-            const outer_context = std.heap.FixedBufferAllocator.init(bytes).allocator.create(WinThread.OuterContext) catch unreachable;
-            outer_context.* = WinThread.OuterContext{
-                .thread = Thread{
-                    .data = Thread.Data{
-                        .heap_handle = heap_handle,
-                        .alloc_start = bytes_ptr,
-                        .handle = undefined,
-                    },
-                },
-                .inner = context,
-            };
-
-            const parameter = if (@sizeOf(Context) == 0) null else @ptrCast(*c_void, &outer_context.inner);
-            outer_context.thread.data.handle = windows.kernel32.CreateThread(null, default_stack_size, WinThread.threadMain, parameter, 0, null) orelse {
-                switch (windows.kernel32.GetLastError()) {
-                    else => |err| return windows.unexpectedError(err),
-                }
-            };
-            return &outer_context.thread;
-        }
-
-        const MainFuncs = struct {
-            fn linuxThreadMain(ctx_addr: usize) callconv(.C) u8 {
-                const arg = if (@sizeOf(Context) == 0) {} else @intToPtr(*const Context, ctx_addr).*;
-
-                switch (@typeInfo(@typeInfo(@TypeOf(startFn)).Fn.return_type.?)) {
-                    .NoReturn => {
-                        startFn(arg);
-                    },
-                    .Void => {
-                        startFn(arg);
-                        return 0;
-                    },
-                    .Int => |info| {
-                        if (info.bits != 8) {
-                            @compileError(bad_startfn_ret);
-                        }
-                        return startFn(arg);
-                    },
-                    .ErrorUnion => |info| {
-                        if (info.payload != void) {
-                            @compileError(bad_startfn_ret);
-                        }
-                        startFn(arg) catch |err| {
-                            std.debug.warn("error: {s}\n", .{@errorName(err)});
-                            if (@errorReturnTrace()) |trace| {
-                                std.debug.dumpStackTrace(trace.*);
-                            }
-                        };
-                        return 0;
-                    },
-                    else => @compileError(bad_startfn_ret),
-                }
-            }
-            fn posixThreadMain(ctx: ?*c_void) callconv(.C) ?*c_void {
-                const arg = if (@sizeOf(Context) == 0) {} else @ptrCast(*Context, @alignCast(@alignOf(Context), ctx)).*;
-
-                switch (@typeInfo(@typeInfo(@TypeOf(startFn)).Fn.return_type.?)) {
-                    .NoReturn => {
-                        startFn(arg);
-                    },
-                    .Void => {
-                        startFn(arg);
-                        return null;
-                    },
-                    .Int => |info| {
-                        if (info.bits != 8) {
-                            @compileError(bad_startfn_ret);
-                        }
-                        // pthreads don't support exit status, ignore value
-                        _ = startFn(arg);
-                        return null;
-                    },
-                    .ErrorUnion => |info| {
-                        if (info.payload != void) {
-                            @compileError(bad_startfn_ret);
-                        }
-                        startFn(arg) catch |err| {
-                            std.debug.warn("error: {s}\n", .{@errorName(err)});
-                            if (@errorReturnTrace()) |trace| {
-                                std.debug.dumpStackTrace(trace.*);
-                            }
-                        };
-                        return null;
-                    },
-                    else => @compileError(bad_startfn_ret),
-                }
-            }
-        };
-
-        if (Thread.use_pthreads) {
-            var attr: c.pthread_attr_t = undefined;
-            if (c.pthread_attr_init(&attr) != 0) return error.SystemResources;
-            defer assert(c.pthread_attr_destroy(&attr) == 0);
-
-            const thread_obj = try std.heap.c_allocator.create(Thread);
-            errdefer std.heap.c_allocator.destroy(thread_obj);
-            if (@sizeOf(Context) > 0) {
-                thread_obj.data.memory = try std.heap.c_allocator.allocAdvanced(
-                    u8,
-                    @alignOf(Context),
-                    @sizeOf(Context),
-                    .at_least,
-                );
-                errdefer std.heap.c_allocator.free(thread_obj.data.memory);
-                mem.copy(u8, thread_obj.data.memory, mem.asBytes(&context));
-            } else {
-                thread_obj.data.memory = @as([*]u8, undefined)[0..0];
-            }
-
-            // Use the same set of parameters used by the libc-less impl.
-            assert(c.pthread_attr_setstacksize(&attr, default_stack_size) == 0);
-            assert(c.pthread_attr_setguardsize(&attr, mem.page_size) == 0);
-
-            const err = c.pthread_create(
-                &thread_obj.data.handle,
-                &attr,
-                MainFuncs.posixThreadMain,
-                thread_obj.data.memory.ptr,
-            );
-            switch (err) {
-                0 => return thread_obj,
-                os.EAGAIN => return error.SystemResources,
-                os.EPERM => unreachable,
-                os.EINVAL => unreachable,
-                else => return os.unexpectedErrno(@intCast(usize, err)),
-            }
-
-            return thread_obj;
-        }
-
-        var guard_end_offset: usize = undefined;
-        var stack_end_offset: usize = undefined;
-        var thread_start_offset: usize = undefined;
-        var context_start_offset: usize = undefined;
-        var tls_start_offset: usize = undefined;
-        const mmap_len = blk: {
-            var l: usize = mem.page_size;
-            // Allocate a guard page right after the end of the stack region
-            guard_end_offset = l;
-            // The stack itself, which grows downwards.
-            l = mem.alignForward(l + default_stack_size, mem.page_size);
-            stack_end_offset = l;
-            // Above the stack, so that it can be in the same mmap call, put the Thread object.
-            l = mem.alignForward(l, @alignOf(Thread));
-            thread_start_offset = l;
-            l += @sizeOf(Thread);
-            // Next, the Context object.
-            if (@sizeOf(Context) != 0) {
-                l = mem.alignForward(l, @alignOf(Context));
-                context_start_offset = l;
-                l += @sizeOf(Context);
-            }
-            // Finally, the Thread Local Storage, if any.
-            l = mem.alignForward(l, os.linux.tls.tls_image.alloc_align);
-            tls_start_offset = l;
-            l += os.linux.tls.tls_image.alloc_size;
-            // Round the size to the page size.
-            break :blk mem.alignForward(l, mem.page_size);
-        };
-
-        const mmap_slice = mem: {
-            // Map the whole stack with no rw permissions to avoid
-            // committing the whole region right away
-            const mmap_slice = os.mmap(
-                null,
-                mmap_len,
-                os.PROT_NONE,
-                os.MAP_PRIVATE | os.MAP_ANONYMOUS,
-                -1,
-                0,
-            ) catch |err| switch (err) {
-                error.MemoryMappingNotSupported => unreachable,
-                error.AccessDenied => unreachable,
-                error.PermissionDenied => unreachable,
-                else => |e| return e,
-            };
-            errdefer os.munmap(mmap_slice);
-
-            // Map everything but the guard page as rw
-            os.mprotect(
-                mmap_slice[guard_end_offset..],
-                os.PROT_READ | os.PROT_WRITE,
-            ) catch |err| switch (err) {
-                error.AccessDenied => unreachable,
-                else => |e| return e,
-            };
-
-            break :mem mmap_slice;
-        };
-
-        const mmap_addr = @ptrToInt(mmap_slice.ptr);
-
-        const thread_ptr = @alignCast(@alignOf(Thread), @intToPtr(*Thread, mmap_addr + thread_start_offset));
-        thread_ptr.data.memory = mmap_slice;
-
-        var arg: usize = undefined;
-        if (@sizeOf(Context) != 0) {
-            arg = mmap_addr + context_start_offset;
-            const context_ptr = @alignCast(@alignOf(Context), @intToPtr(*Context, arg));
-            context_ptr.* = context;
-        }
-
-        if (std.Target.current.os.tag == .linux) {
-            const flags: u32 = os.CLONE_VM | os.CLONE_FS | os.CLONE_FILES |
-                os.CLONE_SIGHAND | os.CLONE_THREAD | os.CLONE_SYSVSEM |
-                os.CLONE_PARENT_SETTID | os.CLONE_CHILD_CLEARTID |
-                os.CLONE_DETACHED | os.CLONE_SETTLS;
-            // This structure is only needed when targeting i386
-            var user_desc: if (std.Target.current.cpu.arch == .i386) os.linux.user_desc else void = undefined;
-
-            const tls_area = mmap_slice[tls_start_offset..];
-            const tp_value = os.linux.tls.prepareTLS(tls_area);
-
-            const newtls = blk: {
-                if (std.Target.current.cpu.arch == .i386) {
-                    user_desc = os.linux.user_desc{
-                        .entry_number = os.linux.tls.tls_image.gdt_entry_number,
-                        .base_addr = tp_value,
-                        .limit = 0xfffff,
-                        .seg_32bit = 1,
-                        .contents = 0, // Data
-                        .read_exec_only = 0,
-                        .limit_in_pages = 1,
-                        .seg_not_present = 0,
-                        .useable = 1,
-                    };
-                    break :blk @ptrToInt(&user_desc);
-                } else {
-                    break :blk tp_value;
-                }
-            };
-
-            const rc = os.linux.clone(
-                MainFuncs.linuxThreadMain,
-                mmap_addr + stack_end_offset,
-                flags,
-                arg,
-                &thread_ptr.data.handle,
-                newtls,
-                &thread_ptr.data.handle,
-            );
-            switch (os.errno(rc)) {
-                0 => return thread_ptr,
-                os.EAGAIN => return error.ThreadQuotaExceeded,
-                os.EINVAL => unreachable,
-                os.ENOMEM => return error.SystemResources,
-                os.ENOSPC => unreachable,
-                os.EPERM => unreachable,
-                os.EUSERS => unreachable,
-                else => |err| return os.unexpectedErrno(err),
-            }
-        } else {
-            @compileError("Unsupported OS");
-        }
-    }
-
-    pub const CpuCountError = error{
-        PermissionDenied,
-        SystemResources,
-        Unexpected,
-    };
-
-    pub fn cpuCount() CpuCountError!usize {
-        if (std.Target.current.os.tag == .linux) {
-            const cpu_set = try os.sched_getaffinity(0);
-            return @as(usize, os.CPU_COUNT(cpu_set)); // TODO should not need this usize cast
-        }
-        if (std.Target.current.os.tag == .windows) {
-            return os.windows.peb().NumberOfProcessors;
-        }
-        if (std.Target.current.os.tag == .openbsd) {
-            var count: c_int = undefined;
-            var count_size: usize = @sizeOf(c_int);
-            const mib = [_]c_int{ os.CTL_HW, os.HW_NCPUONLINE };
-            os.sysctl(&mib, &count, &count_size, null, 0) catch |err| switch (err) {
-                error.NameTooLong, error.UnknownName => unreachable,
-                else => |e| return e,
-            };
-            return @intCast(usize, count);
-        }
-        var count: c_int = undefined;
-        var count_len: usize = @sizeOf(c_int);
-        const name = if (comptime std.Target.current.isDarwin()) "hw.logicalcpu" else "hw.ncpu";
-        os.sysctlbynameZ(name, &count, &count_len, null, 0) catch |err| switch (err) {
-            error.NameTooLong, error.UnknownName => unreachable,
-            else => |e| return e,
-        };
-        return @intCast(usize, count);
-    }
-
-    pub fn getCurrentThreadId() u64 {
-        switch (std.Target.current.os.tag) {
-            .linux => {
-                // Use the syscall directly as musl doesn't provide a wrapper.
-                return @bitCast(u32, os.linux.gettid());
-            },
-            .windows => {
-                return os.windows.kernel32.GetCurrentThreadId();
-            },
-            .macos, .ios, .watchos, .tvos => {
-                var thread_id: u64 = undefined;
-                // Pass thread=null to get the current thread ID.
-                assert(c.pthread_threadid_np(null, &thread_id) == 0);
-                return thread_id;
-            },
-            .netbsd => {
-                return @bitCast(u32, c._lwp_self());
-            },
-            .freebsd => {
-                return @bitCast(u32, c.pthread_getthreadid_np());
-            },
-            .openbsd => {
-                return @bitCast(u32, c.getthrid());
-            },
-            else => {
-                @compileError("getCurrentThreadId not implemented for this platform");
-            },
-        }
-    }
-};