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indexOfSentinel: fix ub

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This commit is contained in:
Jacob Young 2024-07-15 00:24:50 -04:00
parent 14caccb477
commit e32454796c
1 changed files with 6 additions and 5 deletions
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11
lib/std/mem.zig
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@ -1050,15 +1050,16 @@ pub fn indexOfSentinel(comptime T: type, comptime sentinel: T, p: [*:sentinel]co
// 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 block_size = @sizeOf(T) * block_len;
const Block = @Vector(block_len, T);
const mask: Block = @splat(sentinel);
comptime std.debug.assert(std.mem.page_size % @sizeOf(Block) == 0);
comptime std.debug.assert(std.mem.page_size % block_size == 0);
// First block may be unaligned
const start_addr = @intFromPtr(&p[i]);
const offset_in_page = start_addr & (std.mem.page_size - 1);
if (offset_in_page <= std.mem.page_size - @sizeOf(Block)) {
if (offset_in_page <= std.mem.page_size - block_size) {
// Will not read past the end of a page, full block.
const block: Block = p[i..][0..block_len].*;
const matches = block == mask;
@ -1066,19 +1067,19 @@ pub fn indexOfSentinel(comptime T: type, comptime sentinel: T, p: [*:sentinel]co
return i + std.simd.firstTrue(matches).?;
}
i += (std.mem.alignForward(usize, start_addr, @alignOf(Block)) - start_addr) / @sizeOf(T);
i += @divExact(std.mem.alignForward(usize, start_addr, block_size) - start_addr, @sizeOf(T));
} else {
// 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]) & (@alignOf(Block) - 1)) != 0) : (i += 1) {
while ((@intFromPtr(&p[i]) & (block_size - 1)) != 0) : (i += 1) {
if (p[i] == sentinel) return i;
}
}
std.debug.assert(std.mem.isAligned(@intFromPtr(&p[i]), @alignOf(Block)));
std.debug.assert(std.mem.isAligned(@intFromPtr(&p[i]), block_size));
while (true) {
const block: *const Block = @ptrCast(@alignCast(p[i..][0..block_len]));
const matches = block.* == mask;