https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103435
--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:567d5f3d62fba2a23a9e975f7e7c7b61bb67cf24 commit r12-5557-g567d5f3d62fba2a23a9e975f7e7c7b61bb67cf24 Author: Jakub Jelinek <ja...@redhat.com> Date: Sat Nov 27 13:00:55 2021 +0100 bswap: Fix UB in find_bswap_or_nop_finalize [PR103435] On gcc.c-torture/execute/pr103376.c in the following code we trigger UB in the compiler. n->range is 8 because it is 64-bit load and rsize is 0 because it is a bswap sequence with load and known to be 0: /* Find real size of result (highest non-zero byte). */ if (n->base_addr) for (tmpn = n->n, rsize = 0; tmpn; tmpn >>= BITS_PER_MARKER, rsize++); else rsize = n->range; The shifts then shift uint64_t by 64 bits. For this case mask is 0 and we want both *cmpxchg and *cmpnop as 0, the operation can be done as both nop and bswap and callers will prefer nop. 2021-11-27 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/103435 * gimple-ssa-store-merging.c (find_bswap_or_nop_finalize): Avoid UB if n->range - rsize == 8, just clear both *cmpnop and *cmpxchg in that case.
