std.mem.Allocator: introduce `remap` function to the interface

This one changes the size of an allocation, allowing it to be relocated.
However, the implementation will still return `null` if it would be
equivalent to

new = alloc
memcpy(new, old)
free(old)

Mainly this prepares for taking advantage of `mremap` which I thought
would be a bigger deal but apparently is only available on Linux. Still,
we should use it on Linux.

7 files changed, 389 insertions, 217 deletions

diff --git a/lib/std/array_list.zig b/lib/std/array_list.zig
index 5eb527e742..55b6e3cc40 100644
--- a/lib/std/array_list.zig
+++ b/lib/std/array_list.zig
@@ -105,21 +105,19 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
             return result;
         }
 
-        /// The caller owns the returned memory. Empties this ArrayList,
-        /// Its capacity is cleared, making deinit() safe but unnecessary to call.
+        /// The caller owns the returned memory. Empties this ArrayList.
+        /// Its capacity is cleared, making `deinit` safe but unnecessary to call.
         pub fn toOwnedSlice(self: *Self) Allocator.Error!Slice {
             const allocator = self.allocator;
 
             const old_memory = self.allocatedSlice();
-            if (allocator.resize(old_memory, self.items.len)) {
-                const result = self.items;
+            if (allocator.remap(old_memory, self.items.len)) |new_items| {
                 self.* = init(allocator);
-                return result;
+                return new_items;
             }
 
             const new_memory = try allocator.alignedAlloc(T, alignment, self.items.len);
             @memcpy(new_memory, self.items);
-            @memset(self.items, undefined);
             self.clearAndFree();
             return new_memory;
         }
@@ -185,8 +183,9 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
             // extra capacity.
             const new_capacity = growCapacity(self.capacity, new_len);
             const old_memory = self.allocatedSlice();
-            if (self.allocator.resize(old_memory, new_capacity)) {
-                self.capacity = new_capacity;
+            if (self.allocator.remap(old_memory, new_capacity)) |new_memory| {
+                self.items.ptr = new_memory.ptr;
+                self.capacity = new_memory.len;
                 return addManyAtAssumeCapacity(self, index, count);
             }
 
@@ -468,8 +467,9 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
             // the allocator implementation would pointlessly copy our
             // extra capacity.
             const old_memory = self.allocatedSlice();
-            if (self.allocator.resize(old_memory, new_capacity)) {
-                self.capacity = new_capacity;
+            if (self.allocator.remap(old_memory, new_capacity)) |new_memory| {
+                self.items.ptr = new_memory.ptr;
+                self.capacity = new_memory.len;
             } else {
                 const new_memory = try self.allocator.alignedAlloc(T, alignment, new_capacity);
                 @memcpy(new_memory[0..self.items.len], self.items);
@@ -707,15 +707,13 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
         /// Its capacity is cleared, making deinit() safe but unnecessary to call.
         pub fn toOwnedSlice(self: *Self, allocator: Allocator) Allocator.Error!Slice {
             const old_memory = self.allocatedSlice();
-            if (allocator.resize(old_memory, self.items.len)) {
-                const result = self.items;
+            if (allocator.remap(old_memory, self.items.len)) |new_items| {
                 self.* = .empty;
-                return result;
+                return new_items;
             }
 
             const new_memory = try allocator.alignedAlloc(T, alignment, self.items.len);
             @memcpy(new_memory, self.items);
-            @memset(self.items, undefined);
             self.clearAndFree(allocator);
             return new_memory;
         }
@@ -1031,9 +1029,9 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
             }
 
             const old_memory = self.allocatedSlice();
-            if (allocator.resize(old_memory, new_len)) {
-                self.capacity = new_len;
-                self.items.len = new_len;
+            if (allocator.remap(old_memory, new_len)) |new_items| {
+                self.capacity = new_items.len;
+                self.items = new_items;
                 return;
             }
 
@@ -1099,8 +1097,9 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
             // the allocator implementation would pointlessly copy our
             // extra capacity.
             const old_memory = self.allocatedSlice();
-            if (allocator.resize(old_memory, new_capacity)) {
-                self.capacity = new_capacity;
+            if (allocator.remap(old_memory, new_capacity)) |new_memory| {
+                self.items.ptr = new_memory.ptr;
+                self.capacity = new_memory.len;
             } else {
                 const new_memory = try allocator.alignedAlloc(T, alignment, new_capacity);
                 @memcpy(new_memory[0..self.items.len], self.items);
diff --git a/lib/std/heap/FixedBufferAllocator.zig b/lib/std/heap/FixedBufferAllocator.zig
index 5995dfe154..0951dd3bcc 100644
--- a/lib/std/heap/FixedBufferAllocator.zig
+++ b/lib/std/heap/FixedBufferAllocator.zig
@@ -9,7 +9,7 @@ end_index: usize,
 buffer: []u8,
 
 pub fn init(buffer: []u8) FixedBufferAllocator {
-    return FixedBufferAllocator{
+    return .{
         .buffer = buffer,
         .end_index = 0,
     };
@@ -22,6 +22,7 @@ pub fn allocator(self: *FixedBufferAllocator) Allocator {
         .vtable = &.{
             .alloc = alloc,
             .resize = resize,
+            .remap = remap,
             .free = free,
         },
     };
@@ -36,6 +37,7 @@ pub fn threadSafeAllocator(self: *FixedBufferAllocator) Allocator {
         .vtable = &.{
             .alloc = threadSafeAlloc,
             .resize = Allocator.noResize,
+            .remap = Allocator.noRemap,
             .free = Allocator.noFree,
         },
     };
@@ -57,10 +59,10 @@ pub fn isLastAllocation(self: *FixedBufferAllocator, buf: []u8) bool {
     return buf.ptr + buf.len == self.buffer.ptr + self.end_index;
 }
 
-pub fn alloc(ctx: *anyopaque, n: usize, log2_ptr_align: u8, ra: usize) ?[*]u8 {
+pub fn alloc(ctx: *anyopaque, n: usize, alignment: mem.Alignment, ra: usize) ?[*]u8 {
     const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
     _ = ra;
-    const ptr_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_ptr_align));
+    const ptr_align = alignment.toByteUnits();
     const adjust_off = mem.alignPointerOffset(self.buffer.ptr + self.end_index, ptr_align) orelse return null;
     const adjusted_index = self.end_index + adjust_off;
     const new_end_index = adjusted_index + n;
@@ -72,12 +74,12 @@ pub fn alloc(ctx: *anyopaque, n: usize, log2_ptr_align: u8, ra: usize) ?[*]u8 {
 pub fn resize(
     ctx: *anyopaque,
     buf: []u8,
-    log2_buf_align: u8,
+    alignment: mem.Alignment,
     new_size: usize,
     return_address: usize,
 ) bool {
     const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
-    _ = log2_buf_align;
+    _ = alignment;
     _ = return_address;
     assert(@inComptime() or self.ownsSlice(buf));
 
@@ -99,14 +101,24 @@ pub fn resize(
     return true;
 }
 
+pub fn remap(
+    context: *anyopaque,
+    memory: []u8,
+    alignment: mem.Alignment,
+    new_len: usize,
+    return_address: usize,
+) ?[*]u8 {
+    return if (resize(context, memory, alignment, new_len, return_address)) memory.ptr else null;
+}
+
 pub fn free(
     ctx: *anyopaque,
     buf: []u8,
-    log2_buf_align: u8,
+    alignment: mem.Alignment,
     return_address: usize,
 ) void {
     const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
-    _ = log2_buf_align;
+    _ = alignment;
     _ = return_address;
     assert(@inComptime() or self.ownsSlice(buf));
 
@@ -115,10 +127,10 @@ pub fn free(
     }
 }
 
-fn threadSafeAlloc(ctx: *anyopaque, n: usize, log2_ptr_align: u8, ra: usize) ?[*]u8 {
+fn threadSafeAlloc(ctx: *anyopaque, n: usize, alignment: mem.Alignment, ra: usize) ?[*]u8 {
     const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
     _ = ra;
-    const ptr_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_ptr_align));
+    const ptr_align = alignment.toByteUnits();
     var end_index = @atomicLoad(usize, &self.end_index, .seq_cst);
     while (true) {
         const adjust_off = mem.alignPointerOffset(self.buffer.ptr + end_index, ptr_align) orelse return null;
diff --git a/lib/std/heap/PageAllocator.zig b/lib/std/heap/PageAllocator.zig
index 732760b98f..59b0c6015d 100644
--- a/lib/std/heap/PageAllocator.zig
+++ b/lib/std/heap/PageAllocator.zig
@@ -12,18 +12,18 @@ const page_size_min = std.heap.page_size_min;
 pub const vtable: Allocator.VTable = .{
     .alloc = alloc,
     .resize = resize,
+    .remap = remap,
     .free = free,
 };
 
-fn alloc(context: *anyopaque, n: usize, log2_align: u8, ra: usize) ?[*]u8 {
-    const requested_alignment: mem.Alignment = @enumFromInt(log2_align);
+fn alloc(context: *anyopaque, n: usize, alignment: mem.Alignment, ra: usize) ?[*]u8 {
     _ = context;
     _ = ra;
     assert(n > 0);
 
     const page_size = std.heap.pageSize();
     if (n >= maxInt(usize) - page_size) return null;
-    const alignment_bytes = requested_alignment.toByteUnits();
+    const alignment_bytes = alignment.toByteUnits();
 
     if (native_os == .windows) {
         // According to official documentation, VirtualAlloc aligns to page
@@ -103,22 +103,52 @@ fn alloc(context: *anyopaque, n: usize, log2_align: u8, ra: usize) ?[*]u8 {
 
 fn resize(
     context: *anyopaque,
-    buf_unaligned: []u8,
-    log2_buf_align: u8,
-    new_size: usize,
+    memory: []u8,
+    alignment: mem.Alignment,
+    new_len: usize,
     return_address: usize,
 ) bool {
     _ = context;
-    _ = log2_buf_align;
+    _ = alignment;
     _ = return_address;
+    return realloc(memory, new_len, false) != null;
+}
+
+pub fn remap(
+    context: *anyopaque,
+    memory: []u8,
+    alignment: mem.Alignment,
+    new_len: usize,
+    return_address: usize,
+) ?[*]u8 {
+    _ = context;
+    _ = alignment;
+    _ = return_address;
+    return realloc(memory, new_len, true);
+}
+
+fn free(context: *anyopaque, slice: []u8, alignment: mem.Alignment, return_address: usize) void {
+    _ = context;
+    _ = alignment;
+    _ = return_address;
+
+    if (native_os == .windows) {
+        windows.VirtualFree(slice.ptr, 0, windows.MEM_RELEASE);
+    } else {
+        const buf_aligned_len = mem.alignForward(usize, slice.len, std.heap.pageSize());
+        posix.munmap(@alignCast(slice.ptr[0..buf_aligned_len]));
+    }
+}
+
+fn realloc(memory: []u8, new_len: usize, may_move: bool) ?[*]u8 {
     const page_size = std.heap.pageSize();
-    const new_size_aligned = mem.alignForward(usize, new_size, page_size);
+    const new_size_aligned = mem.alignForward(usize, new_len, page_size);
 
     if (native_os == .windows) {
-        if (new_size <= buf_unaligned.len) {
-            const base_addr = @intFromPtr(buf_unaligned.ptr);
-            const old_addr_end = base_addr + buf_unaligned.len;
-            const new_addr_end = mem.alignForward(usize, base_addr + new_size, page_size);
+        if (new_len <= memory.len) {
+            const base_addr = @intFromPtr(memory.ptr);
+            const old_addr_end = base_addr + memory.len;
+            const new_addr_end = mem.alignForward(usize, base_addr + new_len, page_size);
             if (old_addr_end > new_addr_end) {
                 // For shrinking that is not releasing, we will only decommit
                 // the pages not needed anymore.
@@ -128,40 +158,31 @@ fn resize(
                     windows.MEM_DECOMMIT,
                 );
             }
-            return true;
+            return memory.ptr;
         }
-        const old_size_aligned = mem.alignForward(usize, buf_unaligned.len, page_size);
+        const old_size_aligned = mem.alignForward(usize, memory.len, page_size);
         if (new_size_aligned <= old_size_aligned) {
-            return true;
+            return memory.ptr;
         }
-        return false;
+        return null;
     }
 
-    const buf_aligned_len = mem.alignForward(usize, buf_unaligned.len, page_size);
-    if (new_size_aligned == buf_aligned_len)
-        return true;
+    const page_aligned_len = mem.alignForward(usize, memory.len, page_size);
+    if (new_size_aligned == page_aligned_len)
+        return memory.ptr;
 
-    if (new_size_aligned < buf_aligned_len) {
-        const ptr = buf_unaligned.ptr + new_size_aligned;
-        // TODO: if the next_mmap_addr_hint is within the unmapped range, update it
-        posix.munmap(@alignCast(ptr[0 .. buf_aligned_len - new_size_aligned]));
-        return true;
+    const mremap_available = false; // native_os == .linux;
+    if (mremap_available) {
+        // TODO: if the next_mmap_addr_hint is within the remapped range, update it
+        return posix.mremap(memory, new_len, .{ .MAYMOVE = may_move }, null) catch return null;
     }
 
-    // TODO: call mremap
-    // TODO: if the next_mmap_addr_hint is within the remapped range, update it
-    return false;
-}
-
-fn free(context: *anyopaque, slice: []u8, log2_buf_align: u8, return_address: usize) void {
-    _ = context;
-    _ = log2_buf_align;
-    _ = return_address;
-
-    if (native_os == .windows) {
-        windows.VirtualFree(slice.ptr, 0, windows.MEM_RELEASE);
-    } else {
-        const buf_aligned_len = mem.alignForward(usize, slice.len, std.heap.pageSize());
-        posix.munmap(@alignCast(slice.ptr[0..buf_aligned_len]));
+    if (new_size_aligned < page_aligned_len) {
+        const ptr = memory.ptr + new_size_aligned;
+        // TODO: if the next_mmap_addr_hint is within the unmapped range, update it
+        posix.munmap(@alignCast(ptr[0 .. page_aligned_len - new_size_aligned]));
+        return memory.ptr;
     }
+
+    return null;
 }
diff --git a/lib/std/heap/arena_allocator.zig b/lib/std/heap/arena_allocator.zig
index 3cff6b439f..15d9044479 100644
--- a/lib/std/heap/arena_allocator.zig
+++ b/lib/std/heap/arena_allocator.zig
@@ -29,12 +29,14 @@ pub const ArenaAllocator = struct {
             .vtable = &.{
                 .alloc = alloc,
                 .resize = resize,
+                .remap = remap,
                 .free = free,
             },
         };
     }
 
     const BufNode = std.SinglyLinkedList(usize).Node;
+    const BufNode_alignment: mem.Alignment = .fromByteUnits(@alignOf(BufNode));
 
     pub fn init(child_allocator: Allocator) ArenaAllocator {
         return (State{}).promote(child_allocator);
@@ -47,9 +49,8 @@ pub const ArenaAllocator = struct {
         while (it) |node| {
             // this has to occur before the free because the free frees node
             const next_it = node.next;
-            const align_bits = std.math.log2_int(usize, @alignOf(BufNode));
             const alloc_buf = @as([*]u8, @ptrCast(node))[0..node.data];
-            self.child_allocator.rawFree(alloc_buf, align_bits, @returnAddress());
+            self.child_allocator.rawFree(alloc_buf, BufNode_alignment, @returnAddress());
             it = next_it;
         }
     }
@@ -120,7 +121,6 @@ pub const ArenaAllocator = struct {
             return true;
         }
         const total_size = requested_capacity + @sizeOf(BufNode);
-        const align_bits = std.math.log2_int(usize, @alignOf(BufNode));
         // Free all nodes except for the last one
         var it = self.state.buffer_list.first;
         const maybe_first_node = while (it) |node| {
@@ -129,7 +129,7 @@ pub const ArenaAllocator = struct {
             if (next_it == null)
                 break node;
             const alloc_buf = @as([*]u8, @ptrCast(node))[0..node.data];
-            self.child_allocator.rawFree(alloc_buf, align_bits, @returnAddress());
+            self.child_allocator.rawFree(alloc_buf, BufNode_alignment, @returnAddress());
             it = next_it;
         } else null;
         std.debug.assert(maybe_first_node == null or maybe_first_node.?.next == null);
@@ -141,16 +141,16 @@ pub const ArenaAllocator = struct {
             if (first_node.data == total_size)
                 return true;
             const first_alloc_buf = @as([*]u8, @ptrCast(first_node))[0..first_node.data];
-            if (self.child_allocator.rawResize(first_alloc_buf, align_bits, total_size, @returnAddress())) {
+            if (self.child_allocator.rawResize(first_alloc_buf, BufNode_alignment, total_size, @returnAddress())) {
                 // successful resize
                 first_node.data = total_size;
             } else {
                 // manual realloc
-                const new_ptr = self.child_allocator.rawAlloc(total_size, align_bits, @returnAddress()) orelse {
+                const new_ptr = self.child_allocator.rawAlloc(total_size, BufNode_alignment, @returnAddress()) orelse {
                     // we failed to preheat the arena properly, signal this to the user.
                     return false;
                 };
-                self.child_allocator.rawFree(first_alloc_buf, align_bits, @returnAddress());
+                self.child_allocator.rawFree(first_alloc_buf, BufNode_alignment, @returnAddress());
                 const node: *BufNode = @ptrCast(@alignCast(new_ptr));
                 node.* = .{ .data = total_size };
                 self.state.buffer_list.first = node;
@@ -163,8 +163,7 @@ pub const ArenaAllocator = struct {
         const actual_min_size = minimum_size + (@sizeOf(BufNode) + 16);
         const big_enough_len = prev_len + actual_min_size;
         const len = big_enough_len + big_enough_len / 2;
-        const log2_align = comptime std.math.log2_int(usize, @alignOf(BufNode));
-        const ptr = self.child_allocator.rawAlloc(len, log2_align, @returnAddress()) orelse
+        const ptr = self.child_allocator.rawAlloc(len, BufNode_alignment, @returnAddress()) orelse
             return null;
         const buf_node: *BufNode = @ptrCast(@alignCast(ptr));
         buf_node.* = .{ .data = len };
@@ -173,11 +172,11 @@ pub const ArenaAllocator = struct {
         return buf_node;
     }
 
-    fn alloc(ctx: *anyopaque, n: usize, log2_ptr_align: u8, ra: usize) ?[*]u8 {
+    fn alloc(ctx: *anyopaque, n: usize, alignment: mem.Alignment, ra: usize) ?[*]u8 {
         const self: *ArenaAllocator = @ptrCast(@alignCast(ctx));
         _ = ra;
 
-        const ptr_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_ptr_align));
+        const ptr_align = alignment.toByteUnits();
         var cur_node = if (self.state.buffer_list.first) |first_node|
             first_node
         else
@@ -197,8 +196,7 @@ pub const ArenaAllocator = struct {
             }
 
             const bigger_buf_size = @sizeOf(BufNode) + new_end_index;
-            const log2_align = comptime std.math.log2_int(usize, @alignOf(BufNode));
-            if (self.child_allocator.rawResize(cur_alloc_buf, log2_align, bigger_buf_size, @returnAddress())) {
+            if (self.child_allocator.rawResize(cur_alloc_buf, BufNode_alignment, bigger_buf_size, @returnAddress())) {
                 cur_node.data = bigger_buf_size;
             } else {
                 // Allocate a new node if that's not possible
@@ -207,9 +205,9 @@ pub const ArenaAllocator = struct {
         }
     }
 
-    fn resize(ctx: *anyopaque, buf: []u8, log2_buf_align: u8, new_len: usize, ret_addr: usize) bool {
+    fn resize(ctx: *anyopaque, buf: []u8, alignment: mem.Alignment, new_len: usize, ret_addr: usize) bool {
         const self: *ArenaAllocator = @ptrCast(@alignCast(ctx));
-        _ = log2_buf_align;
+        _ = alignment;
         _ = ret_addr;
 
         const cur_node = self.state.buffer_list.first orelse return false;
@@ -231,8 +229,18 @@ pub const ArenaAllocator = struct {
         }
     }
 
-    fn free(ctx: *anyopaque, buf: []u8, log2_buf_align: u8, ret_addr: usize) void {
-        _ = log2_buf_align;
+    fn remap(
+        context: *anyopaque,
+        memory: []u8,
+        alignment: mem.Alignment,
+        new_len: usize,
+        return_address: usize,
+    ) ?[*]u8 {
+        return if (resize(context, memory, alignment, new_len, return_address)) memory.ptr else null;
+    }
+
+    fn free(ctx: *anyopaque, buf: []u8, alignment: mem.Alignment, ret_addr: usize) void {
+        _ = alignment;
         _ = ret_addr;
 
         const self: *ArenaAllocator = @ptrCast(@alignCast(ctx));
diff --git a/lib/std/heap/general_purpose_allocator.zig b/lib/std/heap/general_purpose_allocator.zig
index d572536860..d1e4cda833 100644
--- a/lib/std/heap/general_purpose_allocator.zig
+++ b/lib/std/heap/general_purpose_allocator.zig
@@ -226,7 +226,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             requested_size: if (config.enable_memory_limit) usize else void,
             stack_addresses: [trace_n][stack_n]usize,
             freed: if (config.retain_metadata) bool else void,
-            log2_ptr_align: if (config.never_unmap and config.retain_metadata) u8 else void,
+            alignment: if (config.never_unmap and config.retain_metadata) mem.Alignment else void,
 
             const trace_n = if (config.retain_metadata) traces_per_slot else 1;
 
@@ -281,11 +281,11 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                 return sizes[0..slot_count];
             }
 
-            fn log2PtrAligns(bucket: *BucketHeader, size_class: usize) []u8 {
+            fn log2PtrAligns(bucket: *BucketHeader, size_class: usize) []mem.Alignment {
                 if (!config.safety) @compileError("requested size is only stored when safety is enabled");
                 const aligns_ptr = @as([*]u8, @ptrCast(bucket)) + bucketAlignsStart(size_class);
                 const slot_count = @divExact(page_size, size_class);
-                return aligns_ptr[0..slot_count];
+                return @ptrCast(aligns_ptr[0..slot_count]);
             }
 
             fn stackTracePtr(
@@ -326,6 +326,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                 .vtable = &.{
                     .alloc = alloc,
                     .resize = resize,
+                    .remap = remap,
                     .free = free,
                 },
             };
@@ -455,7 +456,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                     var it = self.large_allocations.iterator();
                     while (it.next()) |large| {
                         if (large.value_ptr.freed) {
-                            self.backing_allocator.rawFree(large.value_ptr.bytes, large.value_ptr.log2_ptr_align, @returnAddress());
+                            self.backing_allocator.rawFree(large.value_ptr.bytes, large.value_ptr.alignment, @returnAddress());
                         }
                     }
                 }
@@ -583,10 +584,11 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
         fn resizeLarge(
             self: *Self,
             old_mem: []u8,
-            log2_old_align: u8,
+            alignment: mem.Alignment,
             new_size: usize,
             ret_addr: usize,
-        ) bool {
+            may_move: bool,
+        ) ?[*]u8 {
             const entry = self.large_allocations.getEntry(@intFromPtr(old_mem.ptr)) orelse {
                 if (config.safety) {
                     @panic("Invalid free");
@@ -628,30 +630,37 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             if (config.enable_memory_limit) {
                 const new_req_bytes = prev_req_bytes + new_size - entry.value_ptr.requested_size;
                 if (new_req_bytes > prev_req_bytes and new_req_bytes > self.requested_memory_limit) {
-                    return false;
+                    return null;
                 }
                 self.total_requested_bytes = new_req_bytes;
             }
 
-            if (!self.backing_allocator.rawResize(old_mem, log2_old_align, new_size, ret_addr)) {
+            const opt_resized_ptr = if (may_move)
+                self.backing_allocator.rawRemap(old_mem, alignment, new_size, ret_addr)
+            else if (self.backing_allocator.rawResize(old_mem, alignment, new_size, ret_addr))
+                old_mem.ptr
+            else
+                null;
+
+            const resized_ptr = opt_resized_ptr orelse {
                 if (config.enable_memory_limit) {
                     self.total_requested_bytes = prev_req_bytes;
                 }
-                return false;
-            }
+                return null;
+            };
 
             if (config.enable_memory_limit) {
                 entry.value_ptr.requested_size = new_size;
             }
 
             if (config.verbose_log) {
-                log.info("large resize {d} bytes at {*} to {d}", .{
-                    old_mem.len, old_mem.ptr, new_size,
+                log.info("large resize {d} bytes at {*} to {d} at {*}", .{
+                    old_mem.len, old_mem.ptr, new_size, resized_ptr,
                 });
             }
-            entry.value_ptr.bytes = old_mem.ptr[0..new_size];
+            entry.value_ptr.bytes = resized_ptr[0..new_size];
             entry.value_ptr.captureStackTrace(ret_addr, .alloc);
-            return true;
+            return resized_ptr;
         }
 
         /// This function assumes the object is in the large object storage regardless
@@ -659,7 +668,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
         fn freeLarge(
             self: *Self,
             old_mem: []u8,
-            log2_old_align: u8,
+            alignment: mem.Alignment,
             ret_addr: usize,
         ) void {
             const entry = self.large_allocations.getEntry(@intFromPtr(old_mem.ptr)) orelse {
@@ -695,7 +704,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             }
 
             if (!config.never_unmap) {
-                self.backing_allocator.rawFree(old_mem, log2_old_align, ret_addr);
+                self.backing_allocator.rawFree(old_mem, alignment, ret_addr);
             }
 
             if (config.enable_memory_limit) {
@@ -719,22 +728,42 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
         }
 
         fn resize(
-            ctx: *anyopaque,
+            context: *anyopaque,
+            memory: []u8,
+            alignment: mem.Alignment,
+            new_len: usize,
+            return_address: usize,
+        ) bool {
+            return realloc(context, memory, alignment, new_len, return_address, false) != null;
+        }
+
+        fn remap(
+            context: *anyopaque,
+            memory: []u8,
+            alignment: mem.Alignment,
+            new_len: usize,
+            return_address: usize,
+        ) ?[*]u8 {
+            return realloc(context, memory, alignment, new_len, return_address, true);
+        }
+
+        fn realloc(
+            context: *anyopaque,
             old_mem: []u8,
-            log2_old_align_u8: u8,
-            new_size: usize,
+            alignment: mem.Alignment,
+            new_len: usize,
             ret_addr: usize,
-        ) bool {
-            const self: *Self = @ptrCast(@alignCast(ctx));
-            const log2_old_align = @as(Allocator.Log2Align, @intCast(log2_old_align_u8));
+            may_move: bool,
+        ) ?[*]u8 {
+            const self: *Self = @ptrCast(@alignCast(context));
             self.mutex.lock();
             defer self.mutex.unlock();
 
             assert(old_mem.len != 0);
 
-            const aligned_size = @max(old_mem.len, @as(usize, 1) << log2_old_align);
+            const aligned_size = @max(old_mem.len, alignment.toByteUnits());
             if (aligned_size > largest_bucket_object_size) {
-                return self.resizeLarge(old_mem, log2_old_align, new_size, ret_addr);
+                return self.resizeLarge(old_mem, alignment, new_len, ret_addr, may_move);
             }
             const size_class_hint = math.ceilPowerOfTwoAssert(usize, aligned_size);
 
@@ -758,7 +787,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                         }
                     }
                 }
-                return self.resizeLarge(old_mem, log2_old_align, new_size, ret_addr);
+                return self.resizeLarge(old_mem, alignment, new_len, ret_addr, may_move);
             };
             const byte_offset = @intFromPtr(old_mem.ptr) - @intFromPtr(bucket.page);
             const slot_index = @as(SlotIndex, @intCast(byte_offset / size_class));
@@ -779,8 +808,8 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             if (config.safety) {
                 const requested_size = bucket.requestedSizes(size_class)[slot_index];
                 if (requested_size == 0) @panic("Invalid free");
-                const log2_ptr_align = bucket.log2PtrAligns(size_class)[slot_index];
-                if (old_mem.len != requested_size or log2_old_align != log2_ptr_align) {
+                const slot_alignment = bucket.log2PtrAligns(size_class)[slot_index];
+                if (old_mem.len != requested_size or alignment != slot_alignment) {
                     var addresses: [stack_n]usize = [1]usize{0} ** stack_n;
                     var free_stack_trace = StackTrace{
                         .instruction_addresses = &addresses,
@@ -795,10 +824,10 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                             free_stack_trace,
                         });
                     }
-                    if (log2_old_align != log2_ptr_align) {
+                    if (alignment != slot_alignment) {
                         log.err("Allocation alignment {d} does not match resize alignment {d}. Allocation: {} Resize: {}", .{
-                            @as(usize, 1) << @as(math.Log2Int(usize), @intCast(log2_ptr_align)),
-                            @as(usize, 1) << @as(math.Log2Int(usize), @intCast(log2_old_align)),
+                            slot_alignment.toByteUnits(),
+                            alignment.toByteUnits(),
                             bucketStackTrace(bucket, size_class, slot_index, .alloc),
                             free_stack_trace,
                         });
@@ -807,52 +836,51 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             }
             const prev_req_bytes = self.total_requested_bytes;
             if (config.enable_memory_limit) {
-                const new_req_bytes = prev_req_bytes + new_size - old_mem.len;
+                const new_req_bytes = prev_req_bytes + new_len - old_mem.len;
                 if (new_req_bytes > prev_req_bytes and new_req_bytes > self.requested_memory_limit) {
-                    return false;
+                    return null;
                 }
                 self.total_requested_bytes = new_req_bytes;
             }
 
-            const new_aligned_size = @max(new_size, @as(usize, 1) << log2_old_align);
+            const new_aligned_size = @max(new_len, alignment.toByteUnits());
             const new_size_class = math.ceilPowerOfTwoAssert(usize, new_aligned_size);
             if (new_size_class <= size_class) {
-                if (old_mem.len > new_size) {
-                    @memset(old_mem[new_size..], undefined);
+                if (old_mem.len > new_len) {
+                    @memset(old_mem[new_len..], undefined);
                 }
                 if (config.verbose_log) {
                     log.info("small resize {d} bytes at {*} to {d}", .{
-                        old_mem.len, old_mem.ptr, new_size,
+                        old_mem.len, old_mem.ptr, new_len,
                     });
                 }
                 if (config.safety) {
-                    bucket.requestedSizes(size_class)[slot_index] = @intCast(new_size);
+                    bucket.requestedSizes(size_class)[slot_index] = @intCast(new_len);
                 }
-                return true;
+                return old_mem.ptr;
             }
 
             if (config.enable_memory_limit) {
                 self.total_requested_bytes = prev_req_bytes;
             }
-            return false;
+            return null;
         }
 
         fn free(
             ctx: *anyopaque,
             old_mem: []u8,
-            log2_old_align_u8: u8,
+            alignment: mem.Alignment,
             ret_addr: usize,
         ) void {
             const self: *Self = @ptrCast(@alignCast(ctx));
-            const log2_old_align = @as(Allocator.Log2Align, @intCast(log2_old_align_u8));
             self.mutex.lock();
             defer self.mutex.unlock();
 
             assert(old_mem.len != 0);
 
-            const aligned_size = @max(old_mem.len, @as(usize, 1) << log2_old_align);
+            const aligned_size = @max(old_mem.len, alignment.toByteUnits());
             if (aligned_size > largest_bucket_object_size) {
-                self.freeLarge(old_mem, log2_old_align, ret_addr);
+                self.freeLarge(old_mem, alignment, ret_addr);
                 return;
             }
             const size_class_hint = math.ceilPowerOfTwoAssert(usize, aligned_size);
@@ -877,7 +905,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                         }
                     }
                 }
-                self.freeLarge(old_mem, log2_old_align, ret_addr);
+                self.freeLarge(old_mem, alignment, ret_addr);
                 return;
             };
             const byte_offset = @intFromPtr(old_mem.ptr) - @intFromPtr(bucket.page);
@@ -900,8 +928,8 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             if (config.safety) {
                 const requested_size = bucket.requestedSizes(size_class)[slot_index];
                 if (requested_size == 0) @panic("Invalid free");
-                const log2_ptr_align = bucket.log2PtrAligns(size_class)[slot_index];
-                if (old_mem.len != requested_size or log2_old_align != log2_ptr_align) {
+                const slot_alignment = bucket.log2PtrAligns(size_class)[slot_index];
+                if (old_mem.len != requested_size or alignment != slot_alignment) {
                     var addresses: [stack_n]usize = [1]usize{0} ** stack_n;
                     var free_stack_trace = StackTrace{
                         .instruction_addresses = &addresses,
@@ -916,10 +944,10 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                             free_stack_trace,
                         });
                     }
-                    if (log2_old_align != log2_ptr_align) {
+                    if (alignment != slot_alignment) {
                         log.err("Allocation alignment {d} does not match free alignment {d}. Allocation: {} Free: {}", .{
-                            @as(usize, 1) << @as(math.Log2Int(usize), @intCast(log2_ptr_align)),
-                            @as(usize, 1) << @as(math.Log2Int(usize), @intCast(log2_old_align)),
+                            slot_alignment.toByteUnits(),
+                            alignment.toByteUnits(),
                             bucketStackTrace(bucket, size_class, slot_index, .alloc),
                             free_stack_trace,
                         });
@@ -981,24 +1009,24 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             return true;
         }
 
-        fn alloc(ctx: *anyopaque, len: usize, log2_ptr_align: u8, ret_addr: usize) ?[*]u8 {
+        fn alloc(ctx: *anyopaque, len: usize, alignment: mem.Alignment, ret_addr: usize) ?[*]u8 {
             const self: *Self = @ptrCast(@alignCast(ctx));
             self.mutex.lock();
             defer self.mutex.unlock();
             if (!self.isAllocationAllowed(len)) return null;
-            return allocInner(self, len, @as(Allocator.Log2Align, @intCast(log2_ptr_align)), ret_addr) catch return null;
+            return allocInner(self, len, alignment, ret_addr) catch return null;
         }
 
         fn allocInner(
             self: *Self,
             len: usize,
-            log2_ptr_align: Allocator.Log2Align,
+            alignment: mem.Alignment,
             ret_addr: usize,
         ) Allocator.Error![*]u8 {
-            const new_aligned_size = @max(len, @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_ptr_align)));
+            const new_aligned_size = @max(len, alignment.toByteUnits());
             if (new_aligned_size > largest_bucket_object_size) {
                 try self.large_allocations.ensureUnusedCapacity(self.backing_allocator, 1);
-                const ptr = self.backing_allocator.rawAlloc(len, log2_ptr_align, ret_addr) orelse
+                const ptr = self.backing_allocator.rawAlloc(len, alignment, ret_addr) orelse
                     return error.OutOfMemory;
                 const slice = ptr[0..len];
 
@@ -1016,7 +1044,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                 if (config.retain_metadata) {
                     gop.value_ptr.freed = false;
                     if (config.never_unmap) {
-                        gop.value_ptr.log2_ptr_align = log2_ptr_align;
+                        gop.value_ptr.alignment = alignment;
                     }
                 }
 
@@ -1030,7 +1058,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             const slot = try self.allocSlot(new_size_class, ret_addr);
             if (config.safety) {
                 slot.bucket.requestedSizes(new_size_class)[slot.slot_index] = @intCast(len);
-                slot.bucket.log2PtrAligns(new_size_class)[slot.slot_index] = log2_ptr_align;
+                slot.bucket.log2PtrAligns(new_size_class)[slot.slot_index] = alignment;
             }
             if (config.verbose_log) {
                 log.info("small alloc {d} bytes at {*}", .{ len, slot.ptr });
@@ -1150,7 +1178,7 @@ test "realloc" {
 }
 
 test "shrink" {
-    var gpa = GeneralPurposeAllocator(test_config){};
+    var gpa: GeneralPurposeAllocator(test_config) = .{};
     defer std.testing.expect(gpa.deinit() == .ok) catch @panic("leak");
     const allocator = gpa.allocator();
 
@@ -1214,7 +1242,7 @@ test "realloc small object to large object" {
 }
 
 test "shrink large object to large object" {
-    var gpa = GeneralPurposeAllocator(test_config){};
+    var gpa: GeneralPurposeAllocator(test_config) = .{};
     defer std.testing.expect(gpa.deinit() == .ok) catch @panic("leak");
     const allocator = gpa.allocator();
 
diff --git a/lib/std/mem/Allocator.zig b/lib/std/mem/Allocator.zig
index 277758501b..7c0e59906c 100644
--- a/lib/std/mem/Allocator.zig
+++ b/lib/std/mem/Allocator.zig
@@ -6,19 +6,21 @@ const math = std.math;
 const mem = std.mem;
 const Allocator = @This();
 const builtin = @import("builtin");
+const Alignment = std.mem.Alignment;
 
 pub const Error = error{OutOfMemory};
 pub const Log2Align = math.Log2Int(usize);
 
 /// The type erased pointer to the allocator implementation.
-/// Any comparison of this field may result in illegal behavior, since it may be set to
-/// `undefined` in cases where the allocator implementation does not have any associated
-/// state.
+///
+/// Any comparison of this field may result in illegal behavior, since it may
+/// be set to `undefined` in cases where the allocator implementation does not
+/// have any associated state.
 ptr: *anyopaque,
 vtable: *const VTable,
 
 pub const VTable = struct {
-    /// Allocate exactly `len` bytes aligned to `1 << ptr_align`, or return `null`
+    /// Allocate exactly `len` bytes aligned to `alignment`, or return `null`
     /// indicating the allocation failed.
     ///
     /// `ret_addr` is optionally provided as the first return address of the
@@ -27,12 +29,14 @@ pub const VTable = struct {
     ///
     /// The returned slice of memory must have been `@memset` to `undefined`
     /// by the allocator implementation.
-    alloc: *const fn (ctx: *anyopaque, len: usize, ptr_align: u8, ret_addr: usize) ?[*]u8,
+    alloc: *const fn (*anyopaque, len: usize, alignment: Alignment, ret_addr: usize) ?[*]u8,
 
-    /// Attempt to expand or shrink memory in place. `buf.len` must equal the
-    /// length requested from the most recent successful call to `alloc` or
-    /// `resize`. `buf_align` must equal the same value that was passed as the
-    /// `ptr_align` parameter to the original `alloc` call.
+    /// Attempt to expand or shrink memory in place.
+    ///
+    /// `memory.len` must equal the length requested from the most recent
+    /// successful call to `alloc` or `resize`. `alignment` must equal the same
+    /// value that was passed as the `alignment` parameter to the original
+    /// `alloc` call.
     ///
     /// A result of `true` indicates the resize was successful and the
     /// allocation now has the same address but a size of `new_len`. `false`
@@ -44,72 +48,114 @@ pub const VTable = struct {
     /// `ret_addr` is optionally provided as the first return address of the
     /// allocation call stack. If the value is `0` it means no return address
     /// has been provided.
-    resize: *const fn (ctx: *anyopaque, buf: []u8, buf_align: u8, new_len: usize, ret_addr: usize) bool,
+    resize: *const fn (*anyopaque, memory: []u8, alignment: Alignment, new_len: usize, ret_addr: usize) bool,
 
-    /// Free and invalidate a buffer.
+    /// Attempt to expand or shrink memory, allowing relocation.
+    ///
+    /// `memory.len` must equal the length requested from the most recent
+    /// successful call to `alloc` or `resize`. `alignment` must equal the same
+    /// value that was passed as the `alignment` parameter to the original
+    /// `alloc` call.
+    ///
+    /// A non-`null` return value indicates the resize was successful. The
+    /// allocation may have same address, or may have been relocated. In either
+    /// case, the allocation now has size of `new_len`. A `null` return value
+    /// indicates that the resize would be equivalent to allocating new memory,
+    /// copying the bytes from the old memory, and then freeing the old memory.
+    /// In such case, it is more efficient for the caller to perform the copy.
     ///
-    /// `buf.len` must equal the most recent length returned by `alloc` or
+    /// `new_len` must be greater than zero.
+    ///
+    /// `ret_addr` is optionally provided as the first return address of the
+    /// allocation call stack. If the value is `0` it means no return address
+    /// has been provided.
+    remap: *const fn (*anyopaque, memory: []u8, alignment: Alignment, new_len: usize, ret_addr: usize) ?[*]u8,
+
+    /// Free and invalidate a region of memory.
+    ///
+    /// `memory.len` must equal the most recent length returned by `alloc` or
     /// given to a successful `resize` call.
     ///
-    /// `buf_align` must equal the same value that was passed as the
-    /// `ptr_align` parameter to the original `alloc` call.
+    /// `alignment` must equal the same value that was passed as the
+    /// `alignment` parameter to the original `alloc` call.
     ///
     /// `ret_addr` is optionally provided as the first return address of the
     /// allocation call stack. If the value is `0` it means no return address
     /// has been provided.
-    free: *const fn (ctx: *anyopaque, buf: []u8, buf_align: u8, ret_addr: usize) void,
+    free: *const fn (*anyopaque, memory: []u8, alignment: Alignment, ret_addr: usize) void,
 };
 
 pub fn noResize(
     self: *anyopaque,
-    buf: []u8,
-    log2_buf_align: u8,
+    memory: []u8,
+    alignment: Alignment,
     new_len: usize,
     ret_addr: usize,
 ) bool {
     _ = self;
-    _ = buf;
-    _ = log2_buf_align;
+    _ = memory;
+    _ = alignment;
     _ = new_len;
     _ = ret_addr;
     return false;
 }
 
+pub fn noRemap(
+    self: *anyopaque,
+    memory: []u8,
+    alignment: Alignment,
+    new_len: usize,
+    ret_addr: usize,
+) ?[*]u8 {
+    _ = self;
+    _ = memory;
+    _ = alignment;
+    _ = new_len;
+    _ = ret_addr;
+    return null;
+}
+
 pub fn noFree(
     self: *anyopaque,
-    buf: []u8,
-    log2_buf_align: u8,
+    memory: []u8,
+    alignment: Alignment,
     ret_addr: usize,
 ) void {
     _ = self;
-    _ = buf;
-    _ = log2_buf_align;
+    _ = memory;
+    _ = alignment;
     _ = ret_addr;
 }
 
 /// This function is not intended to be called except from within the
 /// implementation of an Allocator
-pub inline fn rawAlloc(self: Allocator, len: usize, ptr_align: u8, ret_addr: usize) ?[*]u8 {
-    return self.vtable.alloc(self.ptr, len, ptr_align, ret_addr);
+pub inline fn rawAlloc(a: Allocator, len: usize, alignment: Alignment, ret_addr: usize) ?[*]u8 {
+    return a.vtable.alloc(a.ptr, len, alignment, ret_addr);
 }
 
 /// This function is not intended to be called except from within the
-/// implementation of an Allocator
-pub inline fn rawResize(self: Allocator, buf: []u8, log2_buf_align: u8, new_len: usize, ret_addr: usize) bool {
-    return self.vtable.resize(self.ptr, buf, log2_buf_align, new_len, ret_addr);
+/// implementation of an Allocator.
+pub inline fn rawResize(a: Allocator, memory: []u8, alignment: Alignment, new_len: usize, ret_addr: usize) bool {
+    return a.vtable.resize(a.ptr, memory, alignment, new_len, ret_addr);
+}
+
+/// This function is not intended to be called except from within the
+/// implementation of an Allocator.
+pub inline fn rawRemap(a: Allocator, memory: []u8, alignment: Alignment, new_len: usize, ret_addr: usize) ?[*]u8 {
+    return a.vtable.remap(a.ptr, memory, alignment, new_len, ret_addr);
 }
 
 /// This function is not intended to be called except from within the
 /// implementation of an Allocator
-pub inline fn rawFree(self: Allocator, buf: []u8, log2_buf_align: u8, ret_addr: usize) void {
-    return self.vtable.free(self.ptr, buf, log2_buf_align, ret_addr);
+pub inline fn rawFree(a: Allocator, memory: []u8, alignment: Alignment, ret_addr: usize) void {
+    return a.vtable.free(a.ptr, memory, alignment, ret_addr);
 }
 
 /// Returns a pointer to undefined memory.
 /// Call `destroy` with the result to free the memory.
-pub fn create(self: Allocator, comptime T: type) Error!*T {
+pub fn create(a: Allocator, comptime T: type) Error!*T {
     if (@sizeOf(T) == 0) return @as(*T, @ptrFromInt(math.maxInt(usize)));
-    const ptr: *T = @ptrCast(try self.allocBytesWithAlignment(@alignOf(T), @sizeOf(T), @returnAddress()));
+    const ptr: *T = @ptrCast(try a.allocBytesWithAlignment(@alignOf(T), @sizeOf(T), @returnAddress()));
     return ptr;
 }
 
@@ -121,7 +167,7 @@ pub fn destroy(self: Allocator, ptr: anytype) void {
     const T = info.child;
     if (@sizeOf(T) == 0) return;
     const non_const_ptr = @as([*]u8, @ptrCast(@constCast(ptr)));
-    self.rawFree(non_const_ptr[0..@sizeOf(T)], log2a(info.alignment), @returnAddress());
+    self.rawFree(non_const_ptr[0..@sizeOf(T)], .fromByteUnits(info.alignment), @returnAddress());
 }
 
 /// Allocates an array of `n` items of type `T` and sets all the
@@ -224,36 +270,88 @@ fn allocBytesWithAlignment(self: Allocator, comptime alignment: u29, byte_count:
         return @as([*]align(alignment) u8, @ptrFromInt(ptr));
     }
 
-    const byte_ptr = self.rawAlloc(byte_count, log2a(alignment), return_address) orelse return Error.OutOfMemory;
+    const byte_ptr = self.rawAlloc(byte_count, .fromByteUnits(alignment), return_address) orelse return Error.OutOfMemory;
     // TODO: https://github.com/ziglang/zig/issues/4298
     @memset(byte_ptr[0..byte_count], undefined);
     return @alignCast(byte_ptr);
 }
 
-/// Requests to modify the size of an allocation. It is guaranteed to not move
-/// the pointer, however the allocator implementation may refuse the resize
-/// request by returning `false`.
-pub fn resize(self: Allocator, old_mem: anytype, new_n: usize) bool {
-    const Slice = @typeInfo(@TypeOf(old_mem)).pointer;
+/// Request to modify the size of an allocation.
+///
+/// It is guaranteed to not move the pointer, however the allocator
+/// implementation may refuse the resize request by returning `false`.
+///
+/// `allocation` may be an empty slice, in which case a new allocation is made.
+///
+/// `new_len` may be zero, in which case the allocation is freed.
+pub fn resize(self: Allocator, allocation: anytype, new_len: usize) bool {
+    const Slice = @typeInfo(@TypeOf(allocation)).pointer;
     const T = Slice.child;
-    if (new_n == 0) {
-        self.free(old_mem);
+    const alignment = Slice.alignment;
+    if (new_len == 0) {
+        self.free(allocation);
         return true;
     }
-    if (old_mem.len == 0) {
+    if (allocation.len == 0) {
         return false;
     }
-    const old_byte_slice = mem.sliceAsBytes(old_mem);
+    const old_memory = mem.sliceAsBytes(allocation);
+    // I would like to use saturating multiplication here, but LLVM cannot lower it
+    // on WebAssembly: https://github.com/ziglang/zig/issues/9660
+    //const new_len_bytes = new_len *| @sizeOf(T);
+    const new_len_bytes = math.mul(usize, @sizeOf(T), new_len) catch return false;
+    return self.rawResize(old_memory, .fromByteUnits(alignment), new_len_bytes, @returnAddress());
+}
+
+/// Request to modify the size of an allocation, allowing relocation.
+///
+/// A non-`null` return value indicates the resize was successful. The
+/// allocation may have same address, or may have been relocated. In either
+/// case, the allocation now has size of `new_len`. A `null` return value
+/// indicates that the resize would be equivalent to allocating new memory,
+/// copying the bytes from the old memory, and then freeing the old memory.
+/// In such case, it is more efficient for the caller to perform those
+/// operations.
+///
+/// `allocation` may be an empty slice, in which case a new allocation is made.
+///
+/// `new_len` may be zero, in which case the allocation is freed.
+pub fn remap(self: Allocator, allocation: anytype, new_len: usize) t: {
+    const Slice = @typeInfo(@TypeOf(allocation)).pointer;
+    break :t ?[]align(Slice.alignment) Slice.child;
+} {
+    const Slice = @typeInfo(@TypeOf(allocation)).pointer;
+    const T = Slice.child;
+    const alignment = Slice.alignment;
+    if (new_len == 0) {
+        self.free(allocation);
+        return allocation[0..0];
+    }
+    if (allocation.len == 0) {
+        return null;
+    }
+    const old_memory = mem.sliceAsBytes(allocation);
     // I would like to use saturating multiplication here, but LLVM cannot lower it
     // on WebAssembly: https://github.com/ziglang/zig/issues/9660
-    //const new_byte_count = new_n *| @sizeOf(T);
-    const new_byte_count = math.mul(usize, @sizeOf(T), new_n) catch return false;
-    return self.rawResize(old_byte_slice, log2a(Slice.alignment), new_byte_count, @returnAddress());
+    //const new_len_bytes = new_len *| @sizeOf(T);
+    const new_len_bytes = math.mul(usize, @sizeOf(T), new_len) catch return null;
+    const new_ptr = self.rawRemap(old_memory, .fromByteUnits(alignment), new_len_bytes, @returnAddress()) orelse return null;
+    const new_memory: []align(alignment) u8 = @alignCast(new_ptr[0..new_len_bytes]);
+    return mem.bytesAsSlice(T, new_memory);
 }
 
 /// This function requests a new byte size for an existing allocation, which
 /// can be larger, smaller, or the same size as the old memory allocation.
+///
 /// If `new_n` is 0, this is the same as `free` and it always succeeds.
+///
+/// `old_mem` may have length zero, which makes a new allocation.
+///
+/// This function only fails on out-of-memory conditions, unlike:
+/// * `remap` which returns `null` when the `Allocator` implementation cannot
+///   do the realloc more efficiently than the caller
+/// * `resize` which returns `false` when the `Allocator` implementation cannot
+///   change the size without relocating the allocation.
 pub fn realloc(self: Allocator, old_mem: anytype, new_n: usize) t: {
     const Slice = @typeInfo(@TypeOf(old_mem)).pointer;
     break :t Error![]align(Slice.alignment) Slice.child;
@@ -284,18 +382,18 @@ pub fn reallocAdvanced(
     const old_byte_slice = mem.sliceAsBytes(old_mem);
     const byte_count = math.mul(usize, @sizeOf(T), new_n) catch return Error.OutOfMemory;
     // Note: can't set shrunk memory to undefined as memory shouldn't be modified on realloc failure
-    if (self.rawResize(old_byte_slice, log2a(Slice.alignment), byte_count, return_address)) {
-        const new_bytes: []align(Slice.alignment) u8 = @alignCast(old_byte_slice.ptr[0..byte_count]);
+    if (self.rawRemap(old_byte_slice, .fromByteUnits(Slice.alignment), byte_count, return_address)) |p| {
+        const new_bytes: []align(Slice.alignment) u8 = @alignCast(p[0..byte_count]);
         return mem.bytesAsSlice(T, new_bytes);
     }
 
-    const new_mem = self.rawAlloc(byte_count, log2a(Slice.alignment), return_address) orelse
+    const new_mem = self.rawAlloc(byte_count, .fromByteUnits(Slice.alignment), return_address) orelse
         return error.OutOfMemory;
     const copy_len = @min(byte_count, old_byte_slice.len);
     @memcpy(new_mem[0..copy_len], old_byte_slice[0..copy_len]);
     // TODO https://github.com/ziglang/zig/issues/4298
     @memset(old_byte_slice, undefined);
-    self.rawFree(old_byte_slice, log2a(Slice.alignment), return_address);
+    self.rawFree(old_byte_slice, .fromByteUnits(Slice.alignment), return_address);
 
     const new_bytes: []align(Slice.alignment) u8 = @alignCast(new_mem[0..byte_count]);
     return mem.bytesAsSlice(T, new_bytes);
@@ -312,7 +410,7 @@ pub fn free(self: Allocator, memory: anytype) void {
     const non_const_ptr = @constCast(bytes.ptr);
     // TODO: https://github.com/ziglang/zig/issues/4298
     @memset(non_const_ptr[0..bytes_len], undefined);
-    self.rawFree(non_const_ptr[0..bytes_len], log2a(Slice.alignment), @returnAddress());
+    self.rawFree(non_const_ptr[0..bytes_len], .fromByteUnits(Slice.alignment), @returnAddress());
 }
 
 /// Copies `m` to newly allocated memory. Caller owns the memory.
@@ -329,17 +427,3 @@ pub fn dupeZ(allocator: Allocator, comptime T: type, m: []const T) Error![:0]T {
     new_buf[m.len] = 0;
     return new_buf[0..m.len :0];
 }
-
-/// TODO replace callsites with `@log2` after this proposal is implemented:
-/// https://github.com/ziglang/zig/issues/13642
-inline fn log2a(x: anytype) switch (@typeInfo(@TypeOf(x))) {
-    .int => math.Log2Int(@TypeOf(x)),
-    .comptime_int => comptime_int,
-    else => @compileError("int please"),
-} {
-    switch (@typeInfo(@TypeOf(x))) {
-        .int => return math.log2_int(@TypeOf(x), x),
-        .comptime_int => return math.log2(x),
-        else => @compileError("bad"),
-    }
-}
diff --git a/lib/std/testing/failing_allocator.zig b/lib/std/testing/failing_allocator.zig
index 3a83f313bf..e8b70eaf21 100644
--- a/lib/std/testing/failing_allocator.zig
+++ b/lib/std/testing/failing_allocator.zig
@@ -62,6 +62,7 @@ pub const FailingAllocator = struct {
             .vtable = &.{
                 .alloc = alloc,
                 .resize = resize,
+                .remap = remap,
                 .free = free,
             },
         };
@@ -70,7 +71,7 @@ pub const FailingAllocator = struct {
     fn alloc(
         ctx: *anyopaque,
         len: usize,
-        log2_ptr_align: u8,
+        alignment: mem.Alignment,
         return_address: usize,
     ) ?[*]u8 {
         const self: *FailingAllocator = @ptrCast(@alignCast(ctx));
@@ -86,7 +87,7 @@ pub const FailingAllocator = struct {
             }
             return null;
         }
-        const result = self.internal_allocator.rawAlloc(len, log2_ptr_align, return_address) orelse
+        const result = self.internal_allocator.rawAlloc(len, alignment, return_address) orelse
             return null;
         self.allocated_bytes += len;
         self.allocations += 1;
@@ -96,33 +97,52 @@ pub const FailingAllocator = struct {
 
     fn resize(
         ctx: *anyopaque,
-        old_mem: []u8,
-        log2_old_align: u8,
+        memory: []u8,
+        alignment: mem.Alignment,
         new_len: usize,
         ra: usize,
     ) bool {
         const self: *FailingAllocator = @ptrCast(@alignCast(ctx));
         if (self.resize_index == self.resize_fail_index)
             return false;
-        if (!self.internal_allocator.rawResize(old_mem, log2_old_align, new_len, ra))
+        if (!self.internal_allocator.rawResize(memory, alignment, new_len, ra))
             return false;
-        if (new_len < old_mem.len) {
-            self.freed_bytes += old_mem.len - new_len;
+        if (new_len < memory.len) {
+            self.freed_bytes += memory.len - new_len;
         } else {
-            self.allocated_bytes += new_len - old_mem.len;
+            self.allocated_bytes += new_len - memory.len;
         }
         self.resize_index += 1;
         return true;
     }
 
+    fn remap(
+        ctx: *anyopaque,
+        memory: []u8,
+        alignment: mem.Alignment,
+        new_len: usize,
+        ra: usize,
+    ) ?[*]u8 {
+        const self: *FailingAllocator = @ptrCast(@alignCast(ctx));
+        if (self.resize_index == self.resize_fail_index) return null;
+        const new_ptr = self.internal_allocator.rawRemap(memory, alignment, new_len, ra) orelse return null;
+        if (new_len < memory.len) {
+            self.freed_bytes += memory.len - new_len;
+        } else {
+            self.allocated_bytes += new_len - memory.len;
+        }
+        self.resize_index += 1;
+        return new_ptr;
+    }
+
     fn free(
         ctx: *anyopaque,
         old_mem: []u8,
-        log2_old_align: u8,
+        alignment: mem.Alignment,
         ra: usize,
     ) void {
         const self: *FailingAllocator = @ptrCast(@alignCast(ctx));
-        self.internal_allocator.rawFree(old_mem, log2_old_align, ra);
+        self.internal_allocator.rawFree(old_mem, alignment, ra);
         self.deallocations += 1;
         self.freed_bytes += old_mem.len;
     }