diff --git a/crates/compiler/builtins/bitcode/src/libc.zig b/crates/compiler/builtins/bitcode/src/libc.zig
index 83cd14b48b..6dd555578b 100644
--- a/crates/compiler/builtins/bitcode/src/libc.zig
+++ b/crates/compiler/builtins/bitcode/src/libc.zig
@@ -21,26 +21,13 @@ pub var memcpy_target: Memcpy = switch (arch) {
 
 pub fn memcpy(noalias dest: [*]u8, noalias src: [*]const u8, len: usize) callconv(.C) [*]u8 {
     switch (arch) {
+        // x86_64 has a special optimized memcpy that can use avx2.
         .x86_64 => {
             return memcpy_target(dest, src, len);
         },
-        .i386 => {
-            @memcpy(dest, src, len);
-            return dest;
+        else => {
+            return musl.memcpy(dest, src, len);
         },
-        .aarch64 => {
-            @memcpy(dest, src, len);
-            return dest;
-        },
-        .arm => {
-            @memcpy(dest, src, len);
-            return dest;
-        },
-        .wasm32 => {
-            @memcpy(dest, src, len);
-            return dest;
-        },
-        else => @compileError("Unsupported architecture for memcpy"),
     }
 }
 
diff --git a/crates/compiler/builtins/bitcode/src/libc/musl.zig b/crates/compiler/builtins/bitcode/src/libc/musl.zig
index ac02d730d8..8bff515806 100644
--- a/crates/compiler/builtins/bitcode/src/libc/musl.zig
+++ b/crates/compiler/builtins/bitcode/src/libc/musl.zig
@@ -1 +1 @@
-pub const memcpy = @import("musl/memcpy.zig").musl_memcpy;
+pub const memcpy = @import("musl/memcpy.zig").memcpy;
diff --git a/crates/compiler/builtins/bitcode/src/libc/musl/memcpy-i386.S b/crates/compiler/builtins/bitcode/src/libc/musl/memcpy-i386.S
new file mode 100644
index 0000000000..881537731b
--- /dev/null
+++ b/crates/compiler/builtins/bitcode/src/libc/musl/memcpy-i386.S
@@ -0,0 +1,30 @@
+.global musl_memcpy
+# Windows does not support the type directive.
+# .type memcpy,@function
+musl_memcpy:
+	push %esi
+	push %edi
+	mov 12(%esp),%edi
+	mov 16(%esp),%esi
+	mov 20(%esp),%ecx
+	mov %edi,%eax
+	cmp $4,%ecx
+	jc 1f
+	test $3,%edi
+	jz 1f
+2:	movsb
+	dec %ecx
+	test $3,%edi
+	jnz 2b
+1:	mov %ecx,%edx
+	shr $2,%ecx
+	rep
+	movsl
+	and $3,%edx
+	jz 1f
+2:	movsb
+	dec %edx
+	jnz 2b
+1:	pop %edi
+	pop %esi
+	ret
\ No newline at end of file
diff --git a/crates/compiler/builtins/bitcode/src/libc/musl/memcpy-x86_64.S b/crates/compiler/builtins/bitcode/src/libc/musl/memcpy-x86_64.S
index db2573ae2d..ddb8f2fef5 100644
--- a/crates/compiler/builtins/bitcode/src/libc/musl/memcpy-x86_64.S
+++ b/crates/compiler/builtins/bitcode/src/libc/musl/memcpy-x86_64.S
@@ -1,6 +1,6 @@
 .global musl_memcpy
 # Windows does not support the type directive.
-# .type memcpy,@function
+# .type musl_memcpy,@function
 musl_memcpy:
 	mov %rdi,%rax
 	cmp $8,%rdx
diff --git a/crates/compiler/builtins/bitcode/src/libc/musl/memcpy.zig b/crates/compiler/builtins/bitcode/src/libc/musl/memcpy.zig
index 941622a5a8..7224355c37 100644
--- a/crates/compiler/builtins/bitcode/src/libc/musl/memcpy.zig
+++ b/crates/compiler/builtins/bitcode/src/libc/musl/memcpy.zig
@@ -6,8 +6,194 @@ comptime {
         .x86_64 => {
             asm (@embedFile("memcpy-x86_64.S"));
         },
-        else => unreachable,
+        .i386 => {
+            asm (@embedFile("memcpy-i386.S"));
+        },
+        // TODO: add assembly implementations for other platforms.
+        else => {},
     }
 }
 
+pub const memcpy =
+    switch (arch) {
+    .x86_64, .i386 => musl_memcpy,
+    else => fallback_memcpy,
+};
+
 pub extern fn musl_memcpy(noalias dest: [*]u8, noalias src: [*]const u8, len: usize) callconv(.C) [*]u8;
+
+// Note: this is written to only support little endian targets.
+// To support big endian, `<<` and `>>` wold need to be swapped.
+pub fn fallback_memcpy(noalias dest: [*]u8, noalias src: [*]const u8, len: usize) callconv(.C) [*]u8 {
+    var d = dest;
+    var s = src;
+    var n = len;
+    while (@ptrToInt(s) % 4 != 0 and n != 0) : (n -= 1) {
+        d[0] = s[0];
+        d += 1;
+        s += 1;
+    }
+
+    if (@ptrToInt(d) % 4 == 0) {
+        var d4 = @alignCast(4, d);
+        var s4 = @alignCast(4, s);
+        while (n >= 16) : (n -= 16) {
+            var d_u32 = @ptrCast([*]u32, d4);
+            var s_u32 = @ptrCast([*]const u32, s4);
+            d_u32[0] = s_u32[0];
+            d_u32[1] = s_u32[1];
+            d_u32[2] = s_u32[2];
+            d_u32[3] = s_u32[3];
+
+            d4 += 16;
+            s4 += 16;
+        }
+        if (n & 8 != 0) {
+            var d_u32 = @ptrCast([*]u32, d4);
+            var s_u32 = @ptrCast([*]const u32, s4);
+            d_u32[0] = s_u32[0];
+            d_u32[1] = s_u32[1];
+
+            d4 += 8;
+            s4 += 8;
+        }
+        if (n & 4 != 0) {
+            var d_u32 = @ptrCast([*]u32, d4);
+            var s_u32 = @ptrCast([*]const u32, s4);
+            d_u32[0] = s_u32[0];
+
+            d4 += 4;
+            s4 += 4;
+        }
+        d = d4;
+        s = s4;
+        if (n & 2 != 0) {
+            d[0] = s[0];
+            d += 1;
+            s += 1;
+            d[0] = s[0];
+            d += 1;
+            s += 1;
+        }
+        if (n & 1 != 0) {
+            d[0] = s[0];
+        }
+        return dest;
+    }
+    if (n >= 32) {
+        switch (@ptrToInt(d) % 4) {
+            1 => {
+                var w = @ptrCast([*]const u32, @alignCast(4, s))[0];
+                d[0] = s[0];
+                d += 1;
+                s += 1;
+                d[0] = s[0];
+                d += 1;
+                s += 1;
+                d[0] = s[0];
+                d += 1;
+                s += 1;
+                n -= 3;
+                while (n >= 17) : (n -= 16) {
+                    var d_u32 = @ptrCast([*]u32, @alignCast(4, d));
+                    var s_u32 = @ptrCast([*]const u32, @alignCast(4, s + 1));
+                    var x = s_u32[0];
+                    d_u32[0] = (w >> 24) | (x << 8);
+                    w = s_u32[1];
+                    d_u32[1] = (x >> 24) | (w << 8);
+                    x = s_u32[2];
+                    d_u32[2] = (w >> 24) | (x << 8);
+                    w = s_u32[3];
+                    d_u32[3] = (x >> 24) | (w << 8);
+
+                    d += 16;
+                    s += 16;
+                }
+            },
+            2 => {
+                var w = @ptrCast([*]const u32, @alignCast(4, s))[0];
+                d[0] = s[0];
+                d += 1;
+                s += 1;
+                d[0] = s[0];
+                d += 1;
+                s += 1;
+                n -= 2;
+                while (n >= 18) : (n -= 16) {
+                    var d_u32 = @ptrCast([*]u32, @alignCast(4, d));
+                    var s_u32 = @ptrCast([*]const u32, @alignCast(4, s + 2));
+                    var x = s_u32[0];
+                    d_u32[0] = (w >> 16) | (x << 16);
+                    w = s_u32[1];
+                    d_u32[1] = (x >> 16) | (w << 16);
+                    x = s_u32[2];
+                    d_u32[2] = (w >> 16) | (x << 16);
+                    w = s_u32[3];
+                    d_u32[3] = (x >> 16) | (w << 16);
+
+                    d += 16;
+                    s += 16;
+                }
+            },
+            3 => {
+                var w = @ptrCast([*]const u32, @alignCast(4, s))[0];
+                d[0] = s[0];
+                d += 1;
+                s += 1;
+                n -= 1;
+                while (n >= 19) : (n -= 16) {
+                    var d_u32 = @ptrCast([*]u32, @alignCast(4, d));
+                    var s_u32 = @ptrCast([*]const u32, @alignCast(4, s + 3));
+                    var x = s_u32[0];
+                    d_u32[0] = (w >> 8) | (x << 24);
+                    w = s_u32[1];
+                    d_u32[1] = (x >> 8) | (w << 24);
+                    x = s_u32[2];
+                    d_u32[2] = (w >> 8) | (x << 24);
+                    w = s_u32[3];
+                    d_u32[3] = (x >> 8) | (w << 24);
+
+                    d += 16;
+                    s += 16;
+                }
+            },
+            else => unreachable,
+        }
+    }
+    if (n & 16 != 0) {
+        comptime var i = 0;
+        inline while (i < 16) : (i += 1) {
+            d[0] = s[0];
+            d += 1;
+            s += 1;
+        }
+    }
+    if (n & 8 != 0) {
+        comptime var i = 0;
+        inline while (i < 8) : (i += 1) {
+            d[0] = s[0];
+            d += 1;
+            s += 1;
+        }
+    }
+    if (n & 4 != 0) {
+        comptime var i = 0;
+        inline while (i < 4) : (i += 1) {
+            d[0] = s[0];
+            d += 1;
+            s += 1;
+        }
+    }
+    if (n & 2 != 0) {
+        d[0] = s[0];
+        d += 1;
+        s += 1;
+        d[0] = s[0];
+        d += 1;
+        s += 1;
+    }
+    if (n & 1 != 0) {
+        d[0] = s[0];
+    }
+    return dest;
+}