[Bug tree-optimization/124545] VN nary lookup trick for converted plus/minus lacks inverse trick

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124545

--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Philipp Tomsich <ptomsich@gcc.gnu.org>:

https://gcc.gnu.org/g:8395fa7c79eecf102b276ffee37436e75a522978

commit r17-2078-g8395fa7c79eecf102b276ffee37436e75a522978
Author: Philipp Tomsich <philipp.tomsich@vrull.eu>
Date:   Wed Jul 1 12:19:30 2026 +0200

    tree-optimization/124545 - VN: add inverse widening lookup for PLUS/MINUS

    visit_nary_op canonicalises (T)(A + C) into (T)A + (T)C for its VN
    lookup, but not the reverse -- so whether VN discovers (T)A + C ==
    (T)(A + C) depends on which form it sees first.  Add a match.pd rule
    that rewrites (T)A +- CST into (T)(A +- CST') using the op! qualifier,
    so the fold only fires when the narrow expression already has a value
    number -- i.e. only inside VN via mprts_hook.

    Restrict to TYPE_OVERFLOW_UNDEFINED inner types: for unsigned inner the
    narrow op wraps mod 2^prec (defined) while the widened outer op does
    not, changing the observed value (bitfld-5.c is the concrete miscompile
    when the guard is loosened).

    Use wi::min_precision (CST, SIGNED) rather than int_fits_type_p for the
    fits-check, so sign-encoded small negatives (e.g. -1 as sizetype's
    0xFFFF...FFFF) qualify.

            PR tree-optimization/124545

    gcc/ChangeLog:

            * match.pd: Add (T)A +- CST -> (T)(A +- CST') for widening
            conversions from a signed inner type with undefined overflow.

    gcc/testsuite/ChangeLog:

            * gcc.dg/pr124545.c: New test.
            * gcc.dg/pr124545-2.c: New test.