std.debug: reorg and clarify API goals

After this commit:

`std.debug.SelfInfo` is a cross-platform abstraction for the current
executable's own debug information, with a goal of minimal code bloat
and compilation speed penalty.

`std.debug.Dwarf` does not assume the current executable is itself the
thing being debugged, however, it does assume the debug info has the
same CPU architecture and OS as the current executable. It is planned to
remove this limitation.

1 files changed, 33 insertions, 94 deletions

diff --git a/lib/std/debug/Dwarf/abi.zig b/lib/std/debug/Dwarf/abi.zig
index 1a47625ae7..e87f023d72 100644
--- a/lib/std/debug/Dwarf/abi.zig
+++ b/lib/std/debug/Dwarf/abi.zig
@@ -1,8 +1,9 @@
 const builtin = @import("builtin");
+
 const std = @import("../../std.zig");
 const mem = std.mem;
-const native_os = builtin.os.tag;
 const posix = std.posix;
+const Arch = std.Target.Cpu.Arch;
 
 pub fn supportsUnwinding(target: std.Target) bool {
     return switch (target.cpu.arch) {
@@ -26,8 +27,8 @@ pub fn supportsUnwinding(target: std.Target) bool {
     };
 }
 
-pub fn ipRegNum() u8 {
-    return switch (builtin.cpu.arch) {
+pub fn ipRegNum(arch: Arch) u8 {
+    return switch (arch) {
         .x86 => 8,
         .x86_64 => 16,
         .arm => 15,
@@ -36,9 +37,10 @@ pub fn ipRegNum() u8 {
     };
 }
 
-pub fn fpRegNum(reg_context: RegisterContext) u8 {
-    return switch (builtin.cpu.arch) {
-        // GCC on OS X historically did the opposite of ELF for these registers (only in .eh_frame), and that is now the convention for MachO
+pub fn fpRegNum(arch: Arch, reg_context: RegisterContext) u8 {
+    return switch (arch) {
+        // GCC on OS X historically did the opposite of ELF for these registers
+        // (only in .eh_frame), and that is now the convention for MachO
         .x86 => if (reg_context.eh_frame and reg_context.is_macho) 4 else 5,
         .x86_64 => 6,
         .arm => 11,
@@ -47,8 +49,8 @@ pub fn fpRegNum(reg_context: RegisterContext) u8 {
     };
 }
 
-pub fn spRegNum(reg_context: RegisterContext) u8 {
-    return switch (builtin.cpu.arch) {
+pub fn spRegNum(arch: Arch, reg_context: RegisterContext) u8 {
+    return switch (arch) {
         .x86 => if (reg_context.eh_frame and reg_context.is_macho) 5 else 4,
         .x86_64 => 7,
         .arm => 13,
@@ -57,33 +59,12 @@ pub fn spRegNum(reg_context: RegisterContext) u8 {
     };
 }
 
-/// Some platforms use pointer authentication - the upper bits of instruction pointers contain a signature.
-/// This function clears these signature bits to make the pointer usable.
-pub inline fn stripInstructionPtrAuthCode(ptr: usize) usize {
-    if (builtin.cpu.arch == .aarch64) {
-        // `hint 0x07` maps to `xpaclri` (or `nop` if the hardware doesn't support it)
-        // The save / restore is because `xpaclri` operates on x30 (LR)
-        return asm (
-            \\mov x16, x30
-            \\mov x30, x15
-            \\hint 0x07
-            \\mov x15, x30
-            \\mov x30, x16
-            : [ret] "={x15}" (-> usize),
-            : [ptr] "{x15}" (ptr),
-            : "x16"
-        );
-    }
-
-    return ptr;
-}
-
 pub const RegisterContext = struct {
     eh_frame: bool,
     is_macho: bool,
 };
 
-pub const AbiError = error{
+pub const RegBytesError = error{
     InvalidRegister,
     UnimplementedArch,
     UnimplementedOs,
@@ -91,55 +72,21 @@ pub const AbiError = error{
     ThreadContextNotSupported,
 };
 
-fn RegValueReturnType(comptime ContextPtrType: type, comptime T: type) type {
-    const reg_bytes_type = comptime RegBytesReturnType(ContextPtrType);
-    const info = @typeInfo(reg_bytes_type).Pointer;
-    return @Type(.{
-        .Pointer = .{
-            .size = .One,
-            .is_const = info.is_const,
-            .is_volatile = info.is_volatile,
-            .is_allowzero = info.is_allowzero,
-            .alignment = info.alignment,
-            .address_space = info.address_space,
-            .child = T,
-            .sentinel = null,
-        },
-    });
-}
-
-/// Returns a pointer to a register stored in a ThreadContext, preserving the pointer attributes of the context.
-pub fn regValueNative(
-    comptime T: type,
-    thread_context_ptr: anytype,
-    reg_number: u8,
-    reg_context: ?RegisterContext,
-) !RegValueReturnType(@TypeOf(thread_context_ptr), T) {
-    const reg_bytes = try regBytes(thread_context_ptr, reg_number, reg_context);
-    if (@sizeOf(T) != reg_bytes.len) return error.IncompatibleRegisterSize;
-    return mem.bytesAsValue(T, reg_bytes[0..@sizeOf(T)]);
-}
-
-fn RegBytesReturnType(comptime ContextPtrType: type) type {
-    const info = @typeInfo(ContextPtrType);
-    if (info != .Pointer or info.Pointer.child != std.debug.ThreadContext) {
-        @compileError("Expected a pointer to std.debug.ThreadContext, got " ++ @typeName(@TypeOf(ContextPtrType)));
-    }
-
-    return if (info.Pointer.is_const) return []const u8 else []u8;
-}
-
 /// Returns a slice containing the backing storage for `reg_number`.
 ///
+/// This function assumes the Dwarf information corresponds not necessarily to
+/// the current executable, but at least with a matching CPU architecture and
+/// OS. It is planned to lift this limitation with a future enhancement.
+///
 /// `reg_context` describes in what context the register number is used, as it can have different
 /// meanings depending on the DWARF container. It is only required when getting the stack or
 /// frame pointer register on some architectures.
 pub fn regBytes(
-    thread_context_ptr: anytype,
+    thread_context_ptr: *std.debug.ThreadContext,
     reg_number: u8,
     reg_context: ?RegisterContext,
-) AbiError!RegBytesReturnType(@TypeOf(thread_context_ptr)) {
-    if (native_os == .windows) {
+) RegBytesError![]u8 {
+    if (builtin.os.tag == .windows) {
         return switch (builtin.cpu.arch) {
             .x86 => switch (reg_number) {
                 0 => mem.asBytes(&thread_context_ptr.Eax),
@@ -194,7 +141,7 @@ pub fn regBytes(
 
     const ucontext_ptr = thread_context_ptr;
     return switch (builtin.cpu.arch) {
-        .x86 => switch (native_os) {
+        .x86 => switch (builtin.os.tag) {
             .linux, .netbsd, .solaris, .illumos => switch (reg_number) {
                 0 => mem.asBytes(&ucontext_ptr.mcontext.gregs[posix.REG.EAX]),
                 1 => mem.asBytes(&ucontext_ptr.mcontext.gregs[posix.REG.ECX]),
@@ -229,7 +176,7 @@ pub fn regBytes(
             },
             else => error.UnimplementedOs,
         },
-        .x86_64 => switch (native_os) {
+        .x86_64 => switch (builtin.os.tag) {
             .linux, .solaris, .illumos => switch (reg_number) {
                 0 => mem.asBytes(&ucontext_ptr.mcontext.gregs[posix.REG.RAX]),
                 1 => mem.asBytes(&ucontext_ptr.mcontext.gregs[posix.REG.RDX]),
@@ -248,7 +195,7 @@ pub fn regBytes(
                 14 => mem.asBytes(&ucontext_ptr.mcontext.gregs[posix.REG.R14]),
                 15 => mem.asBytes(&ucontext_ptr.mcontext.gregs[posix.REG.R15]),
                 16 => mem.asBytes(&ucontext_ptr.mcontext.gregs[posix.REG.RIP]),
-                17...32 => |i| if (native_os.isSolarish())
+                17...32 => |i| if (builtin.os.tag.isSolarish())
                     mem.asBytes(&ucontext_ptr.mcontext.fpregs.chip_state.xmm[i - 17])
                 else
                     mem.asBytes(&ucontext_ptr.mcontext.fpregs.xmm[i - 17]),
@@ -318,7 +265,7 @@ pub fn regBytes(
             },
             else => error.UnimplementedOs,
         },
-        .arm => switch (native_os) {
+        .arm => switch (builtin.os.tag) {
             .linux => switch (reg_number) {
                 0 => mem.asBytes(&ucontext_ptr.mcontext.arm_r0),
                 1 => mem.asBytes(&ucontext_ptr.mcontext.arm_r1),
@@ -341,7 +288,7 @@ pub fn regBytes(
             },
             else => error.UnimplementedOs,
         },
-        .aarch64 => switch (native_os) {
+        .aarch64 => switch (builtin.os.tag) {
             .macos, .ios => switch (reg_number) {
                 0...28 => mem.asBytes(&ucontext_ptr.mcontext.ss.regs[reg_number]),
                 29 => mem.asBytes(&ucontext_ptr.mcontext.ss.fp),
@@ -389,22 +336,14 @@ pub fn regBytes(
     };
 }
 
-/// Returns the ABI-defined default value this register has in the unwinding table
-/// before running any of the CIE instructions. The DWARF spec defines these as having
-/// the .undefined rule by default, but allows ABI authors to override that.
-pub fn getRegDefaultValue(reg_number: u8, context: *std.debug.Dwarf.UnwindContext, out: []u8) !void {
-    switch (builtin.cpu.arch) {
-        .aarch64 => {
-            // Callee-saved registers are initialized as if they had the .same_value rule
-            if (reg_number >= 19 and reg_number <= 28) {
-                const src = try regBytes(context.thread_context, reg_number, context.reg_context);
-                if (src.len != out.len) return error.RegisterSizeMismatch;
-                @memcpy(out, src);
-                return;
-            }
-        },
-        else => {},
-    }
-
-    @memset(out, undefined);
+/// Returns a pointer to a register stored in a ThreadContext, preserving the
+/// pointer attributes of the context.
+pub fn regValueNative(
+    thread_context_ptr: *std.debug.ThreadContext,
+    reg_number: u8,
+    reg_context: ?RegisterContext,
+) !*align(1) usize {
+    const reg_bytes = try regBytes(thread_context_ptr, reg_number, reg_context);
+    if (@sizeOf(usize) != reg_bytes.len) return error.IncompatibleRegisterSize;
+    return mem.bytesAsValue(usize, reg_bytes[0..@sizeOf(usize)]);
 }