[Bug tree-optimization/121921] `e - (e - b)` for pointers is not optimized to just b

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121921

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Andrew Pinski <pinskia@gcc.gnu.org>:

https://gcc.gnu.org/g:d3e963d1ccc3500654bec4e2ac94360e864ed343

commit r16-4261-gd3e963d1ccc3500654bec4e2ac94360e864ed343
Author: Andrew Pinski <andrew.pinski@oss.qualcomm.com>
Date:   Sun Oct 5 09:08:15 2025 -0700

    match: Extend `-(a ptrdiff b)` pattern by allowing a nop conversion between
the neg and ptrdiff [PR121921]

    When I tried to fix this before I didn't realize there was already a
pattern for
    `-(a ptrdiff b) -> (b ptrdiff a)`, I had added a complex pattern to match
`ptr0 - (ptr0 - ptr1)`.
    But with there being a pattern for `-(a ptrdiff b)`, we just need to extend
the pattern
    to support a nop conversion inbetween the negative and the ptrdiff.
    Also the check for TYPE_OVERFLOW_UNDEFINED was wrong, in the case of `-(a -
b) -> (b - a)`, the check
    is !TYPE_OVERFLOW_SANITIZED so this pattern should use the same check.

    Bootstrapped and tested on x86_64-linux-gnu.

    Changes since v1:
    * v2: Use the old type of the pointer_diff rather than ssizetype.

            PR tree-optimization/121921

    gcc/ChangeLog:

            * match.pd (`-(a ptrdiff b)`): Extend for a nop_convert
            between the neg and ptrdiff.

    gcc/testsuite/ChangeLog:

            * gcc.dg/pr121921-1.c: New test.

    Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>