https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128384
Bug ID: 128384
Summary: Unnecessary alignment versioning for early-exit
comparison of fixed-size arrays
Product: gcc
Version: 17.0
URL: https://godbolt.org/z/a6dd9ro8v
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: adamant.pwn at gmail dot com
Target Milestone: ---
Host: x86_64-linux-gnu
Target: x86_64-linux-gnu
Build: x86_64-linux-gnu
Created attachment 66072
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=66072&action=edit
GCC 17 snapshot: compiler version and configuration
GCC 17.0.0 20261102 (7eb405c3d119), x86_64-linux-gnu:
extern "C" bool equal(const char (&a)[64], const char (&b)[64]) {
for (int i = 0; i < 64; ++i)
if (a[i] != b[i]) return false;
return true;
}
g++ -O3 -march=x86-64-v3 -S repro.cpp
GCC checks whether the pointers have matching alignment modulo 32, peels to
alignment, and uses vmovdqa. Different alignments select a fully scalar loop.
Replacing the body with `return __builtin_memcmp(a, b, 64) == 0;` gives
unaligned vector loads without these guards. libstdc++'s std::equal and
std::array::operator== use this memcmp path. Could GCC also use the array
extent to establish that the loop's vector loads are safe?
Two relevant checks:
- ref_within_array_bound rejects these accesses as potentially flexible-sized
(confirmed under GDB with GCC 16.2.1):
https://github.com/gcc-mirror/gcc/blob/7eb405c3d119d4fc84b31f21d6499069fbfedad8/gcc/tree-if-conv.cc#L790-L797
- Even with known bounds, vect_enhance_data_refs_alignment only relaxes the
speculative-read alignment requirement for nonconstant vector widths, excluding
AVX2:
https://github.com/gcc-mirror/gcc/blob/7eb405c3d119d4fc84b31f21d6499069fbfedad8/gcc/tree-vect-data-refs.cc#L2331-L2352
Original std::array example: https://godbolt.org/z/a6dd9ro8v
Related: PR115130 and PR126366. PR120352 fixed the scalar recheck after a
vector mismatch; alignment handling remains.
Reduced and checked with AI assistance.
