diff --git a/lib/compiler_rt/memcpy.zig b/lib/compiler_rt/memcpy.zig
index 97188ac65f..d67a6e766a 100644
--- a/lib/compiler_rt/memcpy.zig
+++ b/lib/compiler_rt/memcpy.zig
@@ -1,4 +1,5 @@
 const std = @import("std");
+const assert = std.debug.assert;
 const common = @import("./common.zig");
 const builtin = @import("builtin");
 
@@ -17,6 +18,19 @@ comptime {
     }
 }
 
+const Element = if (std.simd.suggestVectorLength(u8)) |vec_size|
+    @Type(.{ .vector = .{
+        .child = u8,
+        .len = vec_size,
+    } })
+else
+    usize;
+
+comptime {
+    assert(@sizeOf(Element) >= @alignOf(Element));
+    assert(std.math.isPowerOfTwo(@sizeOf(Element)));
+}
+
 fn memcpySmall(noalias dest: ?[*]u8, noalias src: ?[*]const u8, len: usize) callconv(.C) ?[*]u8 {
     @setRuntimeSafety(builtin.is_test);
 
@@ -27,6 +41,173 @@ fn memcpySmall(noalias dest: ?[*]u8, noalias src: ?[*]const u8, len: usize) call
     return dest;
 }
 
-fn memcpyFast(opt_dest: ?[*]u8, opt_src: ?[*]const u8, len: usize) callconv(.C) ?[*]u8 {
-    return @call(.always_inline, @import("memmove.zig").memmove, .{ opt_dest, opt_src, len });
+fn memcpyFast(noalias dest: ?[*]u8, noalias src: ?[*]const u8, len: usize) callconv(.C) ?[*]u8 {
+    @setRuntimeSafety(builtin.is_test);
+
+    const small_limit = 2 * @sizeOf(Element);
+
+    if (copySmallLength(small_limit, dest.?, src.?, len)) return dest;
+
+    copyForwards(dest.?, src.?, len);
+
+    return dest;
+}
+
+inline fn copySmallLength(
+    comptime small_limit: comptime_int,
+    dest: [*]u8,
+    src: [*]const u8,
+    len: usize,
+) bool {
+    if (len < 16) {
+        copyLessThan16(dest, src, len);
+        return true;
+    }
+
+    if (comptime 2 < (std.math.log2(small_limit) + 1) / 2) {
+        if (copy16ToSmallLimit(small_limit, dest, src, len)) return true;
+    }
+
+    return false;
+}
+
+inline fn copyLessThan16(
+    dest: [*]u8,
+    src: [*]const u8,
+    len: usize,
+) void {
+    @setRuntimeSafety(builtin.is_test);
+    if (len < 4) {
+        if (len == 0) return;
+        dest[0] = src[0];
+        dest[len / 2] = src[len / 2];
+        dest[len - 1] = src[len - 1];
+        return;
+    }
+    copyRange4(4, dest, src, len);
+}
+
+inline fn copy16ToSmallLimit(
+    comptime small_limit: comptime_int,
+    dest: [*]u8,
+    src: [*]const u8,
+    len: usize,
+) bool {
+    @setRuntimeSafety(builtin.is_test);
+    inline for (2..(std.math.log2(small_limit) + 1) / 2 + 1) |p| {
+        const limit = 1 << (2 * p);
+        if (len < limit) {
+            copyRange4(limit / 4, dest, src, len);
+            return true;
+        }
+    }
+    return false;
+}
+
+inline fn copyForwards(
+    noalias dest: [*]u8,
+    noalias src: [*]const u8,
+    len: usize,
+) void {
+    @setRuntimeSafety(builtin.is_test);
+    assert(len >= 2 * @sizeOf(Element));
+
+    dest[0..@sizeOf(Element)].* = src[0..@sizeOf(Element)].*;
+    const alignment_offset = @alignOf(Element) - @intFromPtr(src) % @alignOf(Element);
+    const n = len - alignment_offset;
+    const d = dest + alignment_offset;
+    const s = src + alignment_offset;
+
+    copyBlocksAlignedSource(@ptrCast(d), @alignCast(@ptrCast(s)), n);
+
+    // copy last `@sizeOf(Element)` bytes unconditionally, since block copy
+    // methods only copy a multiple of `@sizeOf(Element)` bytes.
+    const offset = len - @sizeOf(Element);
+    dest[offset..][0..@sizeOf(Element)].* = src[offset..][0..@sizeOf(Element)].*;
+}
+
+inline fn copyBlocksAlignedSource(
+    noalias dest: [*]align(1) Element,
+    noalias src: [*]const Element,
+    max_bytes: usize,
+) void {
+    copyBlocks(dest, src, max_bytes);
+}
+
+/// Copies the largest multiple of `@sizeOf(T)` bytes from `src` to `dest`,
+/// that is less than `max_bytes` where `T` is the child type of `src` and
+/// `dest`.
+inline fn copyBlocks(
+    noalias dest: anytype,
+    noalias src: anytype,
+    max_bytes: usize,
+) void {
+    @setRuntimeSafety(builtin.is_test);
+
+    const T = @typeInfo(@TypeOf(dest)).pointer.child;
+    comptime assert(T == @typeInfo(@TypeOf(src)).pointer.child);
+
+    const loop_count = max_bytes / @sizeOf(T);
+
+    for (dest[0..loop_count], src[0..loop_count]) |*d, s| {
+        d.* = s;
+    }
+}
+
+/// copy `len` bytes from `src` to `dest`; `len` must be in the range
+/// `[copy_len, 4 * copy_len)`.
+inline fn copyRange4(
+    comptime copy_len: comptime_int,
+    noalias dest: [*]u8,
+    noalias src: [*]const u8,
+    len: usize,
+) void {
+    @setRuntimeSafety(builtin.is_test);
+    comptime assert(std.math.isPowerOfTwo(copy_len));
+    assert(len >= copy_len);
+    assert(len < 4 * copy_len);
+
+    const a = len & (copy_len * 2);
+    const b = a / 2;
+
+    const last = len - copy_len;
+    const pen = last - b;
+
+    dest[0..copy_len].* = src[0..copy_len].*;
+    dest[b..][0..copy_len].* = src[b..][0..copy_len].*;
+    dest[pen..][0..copy_len].* = src[pen..][0..copy_len].*;
+    dest[last..][0..copy_len].* = src[last..][0..copy_len].*;
+}
+
+test {
+    const S = struct {
+        fn testFunc(comptime copy_func: anytype) !void {
+            const max_len = 1024;
+            var buffer: [max_len + @alignOf(Element) - 1]u8 align(@alignOf(Element)) = undefined;
+            for (&buffer, 0..) |*b, i| {
+                b.* = @intCast(i % 97);
+            }
+            var dest: [max_len + @alignOf(Element) - 1]u8 align(@alignOf(Element)) = undefined;
+
+            for (0..max_len) |copy_len| {
+                for (0..@alignOf(Element)) |s_offset| {
+                    for (0..@alignOf(Element)) |d_offset| {
+                        @memset(&dest, 0xff);
+                        const s = buffer[s_offset..][0..copy_len];
+                        const d = dest[d_offset..][0..copy_len];
+                        _ = copy_func(@ptrCast(d.ptr), @ptrCast(s.ptr), s.len);
+                        std.testing.expectEqualSlices(u8, s, d) catch |e| {
+                            std.debug.print("error encountered for length={d}, s_offset={d}, d_offset={d}\n", .{
+                                copy_len, s_offset, d_offset,
+                            });
+                            return e;
+                        };
+                    }
+                }
+            }
+        }
+    };
+
+    try S.testFunc(memcpySmall);
+    try S.testFunc(memcpyFast);
 }