replace compiler_rt memcpy with a better version

Author: Rexicon226

lib/compiler_rt.zig

@@ -233,7 +233,6 @@ comptime {
 
         _ = @import("compiler_rt/memcpy.zig");
         _ = @import("compiler_rt/memset.zig");
-        _ = @import("compiler_rt/memmove.zig");
         _ = @import("compiler_rt/memcmp.zig");
         _ = @import("compiler_rt/bcmp.zig");
         _ = @import("compiler_rt/ssp.zig");

lib/compiler_rt/memcpy.zig

@@ -5,24 +5,141 @@ const builtin = @import("builtin");
 comptime {
     if (builtin.object_format != .c) {
         @export(&memcpy, .{ .name = "memcpy", .linkage = common.linkage, .visibility = common.visibility });
+        @export(&memcpy, .{ .name = "memmove", .linkage = common.linkage, .visibility = common.visibility });
     }
 }
 
-pub fn memcpy(noalias dest: ?[*]u8, noalias src: ?[*]const u8, len: usize) callconv(.C) ?[*]u8 {
-    @setRuntimeSafety(false);
-
-    if (len != 0) {
-        var d = dest.?;
-        var s = src.?;
-        var n = len;
-        while (true) {
-            d[0] = s[0];
-            n -= 1;
-            if (n == 0) break;
-            d += 1;
-            s += 1;
+// a port of https://github.com/facebook/folly/blob/1c8bc50e88804e2a7361a57cd9b551dd10f6c5fd/folly/memcpy.S
+pub fn memcpy(maybe_dest: ?[*]u8, maybe_src: ?[*]const u8, len: usize) callconv(.C) ?[*]u8 {
+    if (len == 0) {
+        @branchHint(.unlikely);
+        return maybe_dest;
+    }
+
+    const dest = maybe_dest.?;
+    const src = maybe_src.?;
+
+    if (len < 8) {
+        @branchHint(.unlikely);
+        if (len == 1) {
+            @branchHint(.unlikely);
+            dest[0] = src[0];
+        } else if (len >= 4) {
+            @branchHint(.unlikely);
+            blockCopy(dest, src, 4, len);
+        } else {
+            blockCopy(dest, src, 2, len);
         }
+        return dest;
     }
 
+    if (len > 32) {
+        @branchHint(.unlikely);
+        if (len > 256) {
+            @branchHint(.unlikely);
+            copyMove(dest, src, len);
+            return dest;
+        }
+        copyLong(dest, src, len);
+        return dest;
+    }
+
+    if (len > 16) {
+        @branchHint(.unlikely);
+        blockCopy(dest, src, 16, len);
+        return dest;
+    }
+
+    blockCopy(dest, src, 8, len);
+
     return dest;
 }
+
+inline fn blockCopy(dest: [*]u8, src: [*]const u8, block_size: comptime_int, len: usize) void {
+    const first = @as(*align(1) const @Vector(block_size, u8), src[0..block_size]).*;
+    const second = @as(*align(1) const @Vector(block_size, u8), src[len - block_size ..][0..block_size]).*;
+    dest[0..block_size].* = first;
+    dest[len - block_size ..][0..block_size].* = second;
+}
+
+inline fn copyLong(dest: [*]u8, src: [*]const u8, len: usize) void {
+    var array: [8]@Vector(32, u8) = undefined;
+
+    inline for (.{ 64, 128, 192, 256 }, 0..) |N, i| {
+        array[i * 2] = src[(N / 2) - 32 ..][0..32].*;
+        array[(i * 2) + 1] = src[len - N / 2 ..][0..32].*;
+
+        if (len <= N) {
+            @branchHint(.unlikely);
+            for (0..i + 1) |j| {
+                dest[j * 32 ..][0..32].* = array[j * 2];
+                dest[len - ((j * 32) + 32) ..][0..32].* = array[(j * 2) + 1];
+            }
+            return;
+        }
+    }
+}
+
+inline fn copyMove(dest: [*]u8, src: [*]const u8, len: usize) void {
+    if (@intFromPtr(src) >= @intFromPtr(dest)) {
+        @branchHint(.unlikely);
+        copyForward(dest, src, len);
+    } else if (@intFromPtr(src) + len > @intFromPtr(dest)) {
+        @branchHint(.unlikely);
+        overlapBwd(dest, src, len);
+    } else {
+        copyForward(dest, src, len);
+    }
+}
+
+inline fn copyForward(dest: [*]u8, src: [*]const u8, len: usize) void {
+    const tail: @Vector(32, u8) = src[len - 32 ..][0..32].*;
+
+    const N: usize = len & ~@as(usize, 127);
+    var i: usize = 0;
+
+    while (i < N) : (i += 128) {
+        dest[i..][0..32].* = src[i..][0..32].*;
+        dest[i + 32 ..][0..32].* = src[i + 32 ..][0..32].*;
+        dest[i + 64 ..][0..32].* = src[i + 64 ..][0..32].*;
+        dest[i + 96 ..][0..32].* = src[i + 96 ..][0..32].*;
+    }
+
+    if (len - i <= 32) {
+        @branchHint(.unlikely);
+        dest[len - 32 ..][0..32].* = tail;
+    } else {
+        copyLong(dest[i..], src[i..], len - i);
+    }
+}
+
+inline fn overlapBwd(dest: [*]u8, src: [*]const u8, len: usize) void {
+    var array: [5]@Vector(32, u8) = undefined;
+    array[0] = src[len - 32 ..][0..32].*;
+    inline for (1..5) |i| array[i] = src[(i - 1) << 5 ..][0..32].*;
+
+    const end: usize = (@intFromPtr(dest) + len - 32) & 31;
+    const range = len - end;
+    var s = src + range;
+    var d = dest + range;
+
+    while (@intFromPtr(s) > @intFromPtr(src + 128)) {
+        // zig fmt: off
+        const first  = @as(*align(1) const @Vector(32, u8), @ptrCast(s - 32)).*;
+        const second = @as(*align(1) const @Vector(32, u8), @ptrCast(s - 64)).*;
+        const third  = @as(*align(1) const @Vector(32, u8), @ptrCast(s - 96)).*;
+        const fourth = @as(*align(1) const @Vector(32, u8), @ptrCast(s - 128)).*;
+
+        @as(*align(32) @Vector(32, u8), @alignCast(@ptrCast(d - 32))).*  = first;
+        @as(*align(32) @Vector(32, u8), @alignCast(@ptrCast(d - 64))).*  = second;
+        @as(*align(32) @Vector(32, u8), @alignCast(@ptrCast(d - 96))).*  = third;
+        @as(*align(32) @Vector(32, u8), @alignCast(@ptrCast(d - 128))).* = fourth;
+        // zig fmt: on
+
+        s -= 128;
+        d -= 128;
+    }
+
+    inline for (array[1..], 0..) |vec, i| dest[i * 32 ..][0..32].* = vec;
+    dest[len - 32 ..][0..32].* = array[0];
+}