[Bug target/126454] [16/17 Regression] RISCV: cm.popretz pass deletes "li a0,0" and gives wrong return value

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126454

--- Comment #12 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-16 branch has been updated by Jeff Law <[email protected]>:

https://gcc.gnu.org/g:3cdb74f43678a97b01f1de0cfe9eb218d73460f7

commit r16-9534-g3cdb74f43678a97b01f1de0cfe9eb218d73460f7
Author: Kito Cheng <[email protected]>
Date:   Tue Aug 4 10:43:37 2026 +0800

    RISC-V: Don't let popretz combine across a call [PR target/126454]

    The popretz pass scans backwards from cm.popret for the "li a0, 0" that
    feeds the (use a0), but nothing stopped the scan at a call: the argument
    registers of a call live in CALL_INSN_FUNCTION_USAGE, which
    reg_referenced_p does not look at, and the a0 set by a call_value hides
    in a PARALLEL, which the bare SET test did not match.  A "li a0, 0" that
    sets up the first argument of a call was therefore deleted and cm.popret
    became cm.popretz, so the callee got garbage in a0 and the caller
    returned 0 instead of the result of the callee.

    Use insn level helpers instead, find_reg_fusage for the uses and
    reg_set_p for the definitions, and ask them about the word_mode a0
    rather than about the a0 of the (use a0), whose mode covers a0 and a1
    for a DImode return value on rv32.

    gcc/ChangeLog:

            PR target/126454
            * config/riscv/riscv-opt-popretz.cc
(pass_combine_popretz::execute):
            Stop the backward scan at any use or definition of a0, including
            those hidden in CALL_INSN_FUNCTION_USAGE or in a PARALLEL.

    gcc/testsuite/ChangeLog:

            PR target/126454
            * gcc.target/riscv/pr126454.c: New test.

    (cherry picked from commit c80ac3cb559f6ea2be6be3d0458c1615ff27c9a0)