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memset should match the C ABI

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The custom memset accepted its fill value as u8 even though C callers
pass int. Accept c_int and explicitly truncate it to the low byte, which
is what other implementations of this do.
This commit is contained in:
Riccardo Mazzarini
2026-07-26 14:31:49 -07:00
committed by Mitchell Hashimoto
parent 4c725242b7
commit 20c3eae04d
1 changed files with 25 additions and 9 deletions
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34
src/quirks_memset.zig
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@@ -101,13 +101,18 @@ const zva_enabled = builtin.cpu.arch == .aarch64 and
/// plain vector loop measures faster. Empirically mesaured.
const zva_threshold = 16384;
/// Matches compiler_rt's memset signature (lib/compiler_rt.zig).
fn memset(dest: ?[*]u8, c: u8, len: usize) callconv(.c) ?[*]u8 {
/// Matches the C memset ABI.
fn memset(dest: ?[*]u8, c: c_int, len: usize) callconv(.c) ?[*]u8 {
@setRuntimeSafety(false);
if (len == 0) return dest;
const d = dest.?;
// Only the low byte of `c` is used. The C ABI allows a "int"
// as the c parameter but this behavior of just grabbing the
// low byte seems consistent across implementations.
const byte: u8 = @truncate(@as(c_uint, @bitCast(c)));
// Large path: full-width vector stores.
if (len >= vec_bytes) {
// Very large zero fills: zero whole cachelines with `dc zva`
@@ -115,7 +120,7 @@ fn memset(dest: ?[*]u8, c: u8, len: usize) callconv(.c) ?[*]u8 {
// 64-byte block size; anything else falls through to the
// vector loop.
if (comptime zva_enabled) {
if (len >= zva_threshold and c == 0) zva: {
if (len >= zva_threshold and byte == 0) zva: {
if (zvaSize() != 64) break :zva;
const splat64: @Vector(64, u8) = @splat(0);
@@ -148,7 +153,7 @@ fn memset(dest: ?[*]u8, c: u8, len: usize) callconv(.c) ?[*]u8 {
}
}
const splat: @Vector(vec_bytes, u8) = @splat(c);
const splat: @Vector(vec_bytes, u8) = @splat(byte);
// Fill [0, N) where N is len rounded down to a multiple of
// vec_bytes.
@@ -188,7 +193,7 @@ fn memset(dest: ?[*]u8, c: u8, len: usize) callconv(.c) ?[*]u8 {
// pair of 64-byte stores covers len <= 128. Only emitted
// when vec_bytes > 64 (e.g. AVX-512), otherwise the
// vector loop above already handled these lengths.
const splat64: @Vector(64, u8) = @splat(c);
const splat64: @Vector(64, u8) = @splat(byte);
d[0..64].* = splat64;
d[len - 64 ..][0..64].* = splat64;
return dest;
@@ -200,7 +205,7 @@ fn memset(dest: ?[*]u8, c: u8, len: usize) callconv(.c) ?[*]u8 {
// second pair extends the covered prefix to [0..32] and the
// covered suffix to [len-32..len], which meet or overlap in
// the middle.
const splat16: @Vector(16, u8) = @splat(c);
const splat16: @Vector(16, u8) = @splat(byte);
d[0..16].* = splat16;
d[len - 16 ..][0..16].* = splat16;
if (len > 32) {
@@ -211,14 +216,14 @@ fn memset(dest: ?[*]u8, c: u8, len: usize) callconv(.c) ?[*]u8 {
}
if (len >= 8) {
// 8 <= len <= 15: one 8-byte pair (covers len <= 16).
const splat8: @Vector(8, u8) = @splat(c);
const splat8: @Vector(8, u8) = @splat(byte);
d[0..8].* = splat8;
d[len - 8 ..][0..8].* = splat8;
return dest;
}
if (len >= 4) {
// 4 <= len <= 7: one 4-byte pair (covers len <= 8).
const splat4: @Vector(4, u8) = @splat(c);
const splat4: @Vector(4, u8) = @splat(byte);
d[0..4].* = splat4;
d[len - 4 ..][0..4].* = splat4;
return dest;
@@ -231,7 +236,7 @@ fn memset(dest: ?[*]u8, c: u8, len: usize) callconv(.c) ?[*]u8 {
// statically known.
var i: usize = 0;
while (i < len) : (i += 1) {
d[i] = c;
d[i] = byte;
asm volatile ("" ::: .{ .memory = true });
}
return dest;
@@ -286,6 +291,17 @@ test memset {
try testing.expectEqual(0xAA, one[0]);
}
test "memset truncates C int fill value" {
const testing = std.testing;
var buf: [4]u8 = undefined;
_ = memset(&buf, -1, buf.len);
for (buf) |b| try testing.expectEqual(0xFF, b);
_ = memset(&buf, 0x1234, buf.len);
for (buf) |b| try testing.expectEqual(0x34, b);
}
test "memset large zero fills (dc zva path)" {
const testing = std.testing;
const alloc = testing.allocator;