Mirrors/zig
1
0
Fork 0
mirror of https://codeberg.org/ziglang/zig.git synced 2026-04-27 19:09:47 +03:00
Code Issues Packages Projects Releases Wiki Activity

std.mem: delete illegal code in findSentinel()

Browse Source 
I understand the temptation to exploit page size knowledge to make this function
faster, but I don't think this code can ever be compatible with any reasonable
set of pointer provenance/aliasing rules. At the very least, I don't believe it
is compatible with LLVM's.

closes https://github.com/ziglang/zig/issues/23184
This commit is contained in:
Alex Rønne Petersen
2026-03-19 18:53:12 +01:00
committed by Andrew Kelley
parent 029719cf47
commit c7d69f0160
1 changed files with 0 additions and 57 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/std/mem.zig
-57
View File
@@ -1126,63 +1126,6 @@ pub const indexOfSentinel = findSentinel;
/// Linear search through memory until the sentinel is found.
pub fn findSentinel(comptime T: type, comptime sentinel: T, p: [*:sentinel]const T) usize {
var i: usize = 0;
if (use_vectors_for_comparison and
!std.debug.inValgrind() and // https://github.com/ziglang/zig/issues/17717
!@inComptime() and
(@typeInfo(T) == .int or @typeInfo(T) == .float) and std.math.isPowerOfTwo(@bitSizeOf(T)))
{
switch (@import("builtin").cpu.arch) {
// The below branch assumes that reading past the end of the buffer is valid, as long
// as we don't read into a new page. This should be the case for most architectures
// which use paged memory, however should be confirmed before adding a new arch below.
.aarch64, .x86, .x86_64 => if (std.simd.suggestVectorLength(T)) |block_len| {
const page_size = std.heap.page_size_min;
const block_size = @sizeOf(T) * block_len;
const Block = @Vector(block_len, T);
const mask: Block = @splat(sentinel);
comptime assert(std.heap.page_size_min % @sizeOf(Block) == 0);
assert(page_size % @sizeOf(Block) == 0);
// First block may be unaligned
const start_addr = @intFromPtr(&p[i]);
const offset_in_page = start_addr & (page_size - 1);
if (offset_in_page <= page_size - @sizeOf(Block)) {
// Will not read past the end of a page, full block.
const block: Block = p[i..][0..block_len].*;
const matches = block == mask;
if (@reduce(.Or, matches)) {
return i + std.simd.firstTrue(matches).?;
}
i += @divExact(std.mem.alignForward(usize, start_addr, block_size) - start_addr, @sizeOf(T));
} else {
@branchHint(.unlikely);
// Would read over a page boundary. Per-byte at a time until aligned or found.
// 0.39% chance this branch is taken for 4K pages at 16b block length.
//
// An alternate strategy is to do read a full block (the last in the page) and
// mask the entries before the pointer.
while ((@intFromPtr(&p[i]) & (block_size - 1)) != 0) : (i += 1) {
if (p[i] == sentinel) return i;
}
}
std.debug.assertAligned(&p[i], .fromByteUnits(block_size));
while (true) {
const block: Block = p[i..][0..block_len].*;
const matches = block == mask;
if (@reduce(.Or, matches)) {
return i + std.simd.firstTrue(matches).?;
}
i += block_len;
}
},
else => {},
}
}
while (p[i] != sentinel) {
i += 1;
}