[Bug middle-end/105533] UBSAN: gcc/expmed.cc:3272:26: runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long int'

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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 <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:c655c8d8d845b36c59babb2413ce7aa3584dbeda

commit r14-9354-gc655c8d8d845b36c59babb2413ce7aa3584dbeda
Author: Jakub Jelinek <jakub@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  <jakub@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.