https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105533
--- Comment #12 from GCC 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:c655c8d8d845b36c59babb2413ce7aa3584dbeda commit r14-9354-gc655c8d8d845b36c59babb2413ce7aa3584dbeda Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Mar 7 10:02:00 2024 +0100 expand: Fix UB in choose_mult_variant [PR105533] As documented in the function comment, choose_mult_variant attempts to compute costs of 3 different cases, val, -val and val - 1. The -val case is actually only done if val fits into host int, so there should be no overflow, but the val - 1 case is done unconditionally. val is shwi (but inside of synth_mult already uhwi), so when val is HOST_WIDE_INT_MIN, val - 1 invokes UB. The following patch fixes that by using val - HOST_WIDE_INT_1U, but I'm not really convinced it would DTRT for > 64-bit modes, so I've guarded it as well. Though, arch would need to have really strange costs that something that could be expressed as x << 63 would be better expressed as (x * 0x7fffffffffffffff) + 1 In the long term, I think we should just rewrite choose_mult_variant/synth_mult etc. to work on wide_int. 2024-03-07 Jakub Jelinek <ja...@redhat.com> PR middle-end/105533 * expmed.cc (choose_mult_variant): Only try the val - 1 variant if val is not HOST_WIDE_INT_MIN or if mode has exactly HOST_BITS_PER_WIDE_INT precision. Avoid triggering UB while computing val - 1. * gcc.dg/pr105533.c: New test.
