diff --git a/lib/std/heap.zig b/lib/std/heap.zig
index d1874c5b00..10e4cec608 100644
--- a/lib/std/heap.zig
+++ b/lib/std/heap.zig
@@ -9,11 +9,12 @@ const Allocator = std.mem.Allocator;
 const windows = std.os.windows;
 
 pub const ArenaAllocator = @import("heap/arena_allocator.zig").ArenaAllocator;
-pub const WasmAllocator = @import("heap/WasmAllocator.zig");
-pub const PageAllocator = @import("heap/PageAllocator.zig");
-pub const ThreadSafeAllocator = @import("heap/ThreadSafeAllocator.zig");
-pub const SbrkAllocator = @import("heap/sbrk_allocator.zig").SbrkAllocator;
+pub const SmpAllocator = @import("heap/SmpAllocator.zig");
 pub const FixedBufferAllocator = @import("heap/FixedBufferAllocator.zig");
+pub const PageAllocator = @import("heap/PageAllocator.zig");
+pub const SbrkAllocator = @import("heap/sbrk_allocator.zig").SbrkAllocator;
+pub const ThreadSafeAllocator = @import("heap/ThreadSafeAllocator.zig");
+pub const WasmAllocator = @import("heap/WasmAllocator.zig");
 
 pub const DebugAllocatorConfig = @import("heap/debug_allocator.zig").Config;
 pub const DebugAllocator = @import("heap/debug_allocator.zig").DebugAllocator;
@@ -358,6 +359,11 @@ else if (builtin.target.isWasm()) .{
     .vtable = &PageAllocator.vtable,
 };
 
+pub const smp_allocator: Allocator = .{
+    .ptr = undefined,
+    .vtable = &SmpAllocator.vtable,
+};
+
 /// This allocator is fast, small, and specific to WebAssembly. In the future,
 /// this will be the implementation automatically selected by
 /// `GeneralPurposeAllocator` when compiling in `ReleaseSmall` mode for wasm32
@@ -978,4 +984,5 @@ test {
     if (builtin.target.isWasm()) {
         _ = WasmAllocator;
     }
+    if (!builtin.single_threaded) _ = smp_allocator;
 }
diff --git a/lib/std/heap/PageAllocator.zig b/lib/std/heap/PageAllocator.zig
index 433e0f1218..a1eae13efc 100644
--- a/lib/std/heap/PageAllocator.zig
+++ b/lib/std/heap/PageAllocator.zig
@@ -16,11 +16,7 @@ pub const vtable: Allocator.VTable = .{
     .free = free,
 };
 
-fn alloc(context: *anyopaque, n: usize, alignment: mem.Alignment, ra: usize) ?[*]u8 {
-    _ = context;
-    _ = ra;
-    assert(n > 0);
-
+pub fn map(n: usize, alignment: mem.Alignment) ?[*]u8 {
     const page_size = std.heap.pageSize();
     if (n >= maxInt(usize) - page_size) return null;
     const alignment_bytes = alignment.toByteUnits();
@@ -101,6 +97,13 @@ fn alloc(context: *anyopaque, n: usize, alignment: mem.Alignment, ra: usize) ?[*
     return result_ptr;
 }
 
+fn alloc(context: *anyopaque, n: usize, alignment: mem.Alignment, ra: usize) ?[*]u8 {
+    _ = context;
+    _ = ra;
+    assert(n > 0);
+    return map(n, alignment);
+}
+
 fn resize(
     context: *anyopaque,
     memory: []u8,
@@ -114,7 +117,7 @@ fn resize(
     return realloc(memory, new_len, false) != null;
 }
 
-pub fn remap(
+fn remap(
     context: *anyopaque,
     memory: []u8,
     alignment: mem.Alignment,
@@ -127,21 +130,24 @@ pub fn remap(
     return realloc(memory, new_len, true);
 }
 
-fn free(context: *anyopaque, slice: []u8, alignment: mem.Alignment, return_address: usize) void {
+fn free(context: *anyopaque, memory: []u8, alignment: mem.Alignment, return_address: usize) void {
     _ = context;
     _ = alignment;
     _ = return_address;
+    return unmap(@alignCast(memory));
+}
 
+pub fn unmap(memory: []align(page_size_min) u8) void {
     if (native_os == .windows) {
-        windows.VirtualFree(slice.ptr, 0, windows.MEM_RELEASE);
+        windows.VirtualFree(memory.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]));
+        const page_aligned_len = mem.alignForward(usize, memory.len, std.heap.pageSize());
+        posix.munmap(memory.ptr[0..page_aligned_len]);
     }
 }
 
-fn realloc(uncasted_memory: []u8, new_len: usize, may_move: bool) ?[*]u8 {
-    const memory: []align(std.heap.page_size_min) u8 = @alignCast(uncasted_memory);
+pub fn realloc(uncasted_memory: []u8, new_len: usize, may_move: bool) ?[*]u8 {
+    const memory: []align(page_size_min) u8 = @alignCast(uncasted_memory);
     const page_size = std.heap.pageSize();
     const new_size_aligned = mem.alignForward(usize, new_len, page_size);
 
diff --git a/lib/std/heap/SmpAllocator.zig b/lib/std/heap/SmpAllocator.zig
new file mode 100644
index 0000000000..6fd68c19b2
--- /dev/null
+++ b/lib/std/heap/SmpAllocator.zig
@@ -0,0 +1,288 @@
+//! An allocator that is designed for ReleaseFast optimization mode, with
+//! multi-threading enabled.
+//!
+//! This allocator is a singleton; it uses global state and only one should be
+//! instantiated for the entire process.
+//!
+//! ## Basic Design
+//!
+//! Avoid locking the global mutex as much as possible.
+//!
+//! Each thread gets a separate freelist, however, the data must be recoverable
+//! when the thread exits. We do not directly learn when a thread exits, so
+//! occasionally, one thread must attempt to reclaim another thread's
+//! resources.
+//!
+//! Above a certain size, those allocations are memory mapped directly, with no
+//! storage of allocation metadata. This works because the implementation
+//! refuses resizes that would move an allocation from small category to large
+//! category or vice versa.
+//!
+//! Each allocator operation checks the thread identifier from a threadlocal
+//! variable to find out which metadata in the global state to access, and
+//! attempts to grab its lock. This will usually succeed without contention,
+//! unless another thread has been assigned the same id. In the case of such
+//! contention, the thread moves on to the next thread metadata slot and
+//! repeats the process of attempting to obtain the lock.
+//!
+//! By limiting the thread-local metadata array to the same number as the CPU
+//! count, ensures that as threads are created and destroyed, they cycle
+//! through the full set of freelists.
+
+const builtin = @import("builtin");
+const native_os = builtin.os.tag;
+
+const std = @import("../std.zig");
+const assert = std.debug.assert;
+const mem = std.mem;
+const math = std.math;
+const Allocator = std.mem.Allocator;
+const SmpAllocator = @This();
+const PageAllocator = std.heap.PageAllocator;
+
+/// Protects the state in this struct (global state), except for `threads`
+/// which each have their own mutex.
+mutex: std.Thread.Mutex,
+next_thread_index: u32,
+cpu_count: u32,
+threads: [max_thread_count]Thread,
+
+var global: SmpAllocator = .{
+    .mutex = .{},
+    .next_thread_index = 0,
+    .threads = @splat(.{}),
+    .cpu_count = 0,
+};
+threadlocal var thread_id: Thread.Id = .none;
+
+const max_thread_count = 128;
+const slab_len: usize = @max(std.heap.page_size_max, switch (builtin.os.tag) {
+    .windows => 64 * 1024, // Makes `std.heap.PageAllocator` take the happy path.
+    .wasi => 64 * 1024, // Max alignment supported by `std.heap.WasmAllocator`.
+    else => 256 * 1024, // Avoids too many active mappings when `page_size_max` is low.
+});
+/// Because of storing free list pointers, the minimum size class is 3.
+const min_class = math.log2(math.ceilPowerOfTwoAssert(usize, 1 + @sizeOf(usize)));
+const size_class_count = math.log2(slab_len) - min_class;
+
+const Thread = struct {
+    /// Avoid false sharing.
+    _: void align(std.atomic.cache_line) = {},
+
+    /// Protects the state in this struct (per-thread state).
+    ///
+    /// Threads lock this before accessing their own state in order
+    /// to support freelist reclamation.
+    mutex: std.Thread.Mutex = .{},
+
+    next_addrs: [size_class_count]usize = @splat(0),
+    /// For each size class, points to the freed pointer.
+    frees: [size_class_count]usize = @splat(0),
+
+    /// Index into `SmpAllocator.threads`.
+    const Id = enum(usize) {
+        none = 0,
+        first = 1,
+        _,
+
+        fn fromIndex(index: usize) Id {
+            return @enumFromInt(index + 1);
+        }
+
+        fn toIndex(id: Id) usize {
+            return @intFromEnum(id) - 1;
+        }
+    };
+
+    fn lock() *Thread {
+        const id = thread_id;
+        if (id != .none) {
+            var index = id.toIndex();
+            {
+                const t = &global.threads[index];
+                if (t.mutex.tryLock()) return t;
+            }
+            const cpu_count = global.cpu_count;
+            assert(cpu_count != 0);
+            while (true) {
+                index = (index + 1) % cpu_count;
+                const t = &global.threads[index];
+                if (t.mutex.tryLock()) {
+                    thread_id = .fromIndex(index);
+                    return t;
+                }
+            }
+        }
+        while (true) {
+            const thread_index = i: {
+                global.mutex.lock();
+                defer global.mutex.unlock();
+                const cpu_count = c: {
+                    const cpu_count = global.cpu_count;
+                    if (cpu_count == 0) {
+                        const n: u32 = @intCast(@max(std.Thread.getCpuCount() catch max_thread_count, max_thread_count));
+                        global.cpu_count = n;
+                        break :c n;
+                    }
+                    break :c cpu_count;
+                };
+                const thread_index = global.next_thread_index;
+                global.next_thread_index = @intCast((thread_index + 1) % cpu_count);
+                break :i thread_index;
+            };
+            const t = &global.threads[thread_index];
+            if (t.mutex.tryLock()) {
+                thread_id = .fromIndex(thread_index);
+                return t;
+            }
+        }
+    }
+
+    fn unlock(t: *Thread) void {
+        t.mutex.unlock();
+    }
+};
+
+pub const vtable: Allocator.VTable = .{
+    .alloc = alloc,
+    .resize = resize,
+    .remap = remap,
+    .free = free,
+};
+
+comptime {
+    assert(!builtin.single_threaded); // you're holding it wrong
+}
+
+fn alloc(context: *anyopaque, len: usize, alignment: mem.Alignment, ra: usize) ?[*]u8 {
+    _ = context;
+    _ = ra;
+    const class = sizeClassIndex(len, alignment);
+    if (class >= size_class_count) {
+        @branchHint(.unlikely);
+        return PageAllocator.map(len, alignment);
+    }
+
+    const t = Thread.lock();
+    defer t.unlock();
+
+    const slot_size = slotSize(class);
+
+    const top_free_ptr = t.frees[class];
+    if (top_free_ptr != 0) {
+        const node: *usize = @ptrFromInt(top_free_ptr + (slot_size - @sizeOf(usize)));
+        t.frees[class] = node.*;
+        return @ptrFromInt(top_free_ptr);
+    }
+
+    const next_addr = t.next_addrs[class];
+    if (next_addr % slab_len == 0) {
+        const slab = PageAllocator.map(slab_len, .fromByteUnits(std.heap.pageSize())) orelse return null;
+        t.next_addrs[class] = @intFromPtr(slab) + slot_size;
+        return slab;
+    }
+
+    t.next_addrs[class] = next_addr + slot_size;
+    return @ptrFromInt(next_addr);
+}
+
+fn resize(context: *anyopaque, memory: []u8, alignment: mem.Alignment, new_len: usize, ra: usize) bool {
+    _ = context;
+    _ = ra;
+    const class = sizeClassIndex(memory.len, alignment);
+    const new_class = sizeClassIndex(new_len, alignment);
+    if (class >= size_class_count) {
+        if (new_class < size_class_count) return false;
+        return PageAllocator.realloc(memory, new_len, false) != null;
+    }
+    return new_class == class;
+}
+
+fn remap(context: *anyopaque, memory: []u8, alignment: mem.Alignment, new_len: usize, ra: usize) ?[*]u8 {
+    _ = context;
+    _ = ra;
+    const class = sizeClassIndex(memory.len, alignment);
+    const new_class = sizeClassIndex(new_len, alignment);
+    if (class >= size_class_count) {
+        if (new_class < size_class_count) return null;
+        return PageAllocator.realloc(memory, new_len, true);
+    }
+    return if (new_class == class) memory.ptr else null;
+}
+
+fn free(context: *anyopaque, memory: []u8, alignment: mem.Alignment, ra: usize) void {
+    _ = context;
+    _ = ra;
+    const class = sizeClassIndex(memory.len, alignment);
+    if (class >= size_class_count) {
+        @branchHint(.unlikely);
+        return PageAllocator.unmap(@alignCast(memory));
+    }
+
+    const t = Thread.lock();
+    defer t.unlock();
+
+    const slot_size = slotSize(class);
+    const addr = @intFromPtr(memory.ptr);
+    const node: *usize = @ptrFromInt(addr + (slot_size - @sizeOf(usize)));
+    node.* = t.frees[class];
+    t.frees[class] = addr;
+}
+
+fn sizeClassIndex(len: usize, alignment: mem.Alignment) usize {
+    return @max(
+        @bitSizeOf(usize) - @clz(len - 1),
+        @intFromEnum(alignment),
+        min_class,
+    );
+}
+
+fn slotSize(class: usize) usize {
+    const Log2USize = std.math.Log2Int(usize);
+    return @as(usize, 1) << @as(Log2USize, @intCast(class));
+}
+
+test "large alloc, resize, remap, free" {
+    const gpa = std.heap.smp_allocator;
+
+    const ptr1 = try gpa.alloc(u64, 42768);
+    const ptr2 = try gpa.alloc(u64, 52768);
+    gpa.free(ptr1);
+    const ptr3 = try gpa.alloc(u64, 62768);
+    gpa.free(ptr3);
+    gpa.free(ptr2);
+}
+
+test "small allocations - free in same order" {
+    const gpa = std.heap.smp_allocator;
+
+    var list = std.ArrayList(*u64).init(std.testing.allocator);
+    defer list.deinit();
+
+    var i: usize = 0;
+    while (i < 513) : (i += 1) {
+        const ptr = try gpa.create(u64);
+        try list.append(ptr);
+    }
+
+    for (list.items) |ptr| {
+        gpa.destroy(ptr);
+    }
+}
+
+test "small allocations - free in reverse order" {
+    const gpa = std.heap.smp_allocator;
+
+    var list = std.ArrayList(*u64).init(std.testing.allocator);
+    defer list.deinit();
+
+    var i: usize = 0;
+    while (i < 513) : (i += 1) {
+        const ptr = try gpa.create(u64);
+        try list.append(ptr);
+    }
+
+    while (list.popOrNull()) |ptr| {
+        gpa.destroy(ptr);
+    }
+}
diff --git a/lib/std/heap/WasmAllocator.zig b/lib/std/heap/WasmAllocator.zig
index e30ac5ab01..0a9003f245 100644
--- a/lib/std/heap/WasmAllocator.zig
+++ b/lib/std/heap/WasmAllocator.zig
@@ -1,5 +1,3 @@
-//! This is intended to be merged into GeneralPurposeAllocator at some point.
-
 const std = @import("../std.zig");
 const builtin = @import("builtin");
 const Allocator = std.mem.Allocator;
diff --git a/lib/std/heap/debug_allocator.zig b/lib/std/heap/debug_allocator.zig
index 8abf6133bf..296014aa3f 100644
--- a/lib/std/heap/debug_allocator.zig
+++ b/lib/std/heap/debug_allocator.zig
@@ -851,8 +851,6 @@ pub fn DebugAllocator(comptime config: Config) type {
             self.mutex.lock();
             defer self.mutex.unlock();
 
-            assert(old_memory.len != 0);
-
             const size_class_index: usize = @max(@bitSizeOf(usize) - @clz(old_memory.len - 1), @intFromEnum(alignment));
             if (size_class_index >= self.buckets.len) {
                 @branchHint(.unlikely);