[Bug target/121910] RISC-V: dynamic lmul choosing wrong vector mode

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121910

--- Comment #9 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jeff Law <l...@gcc.gnu.org>:

https://gcc.gnu.org/g:642504b41cee46cf06aaa391386108563739a80c

commit r16-3954-g642504b41cee46cf06aaa391386108563739a80c
Author: Zhongyao Chen <chenzhongyao....@gmail.com>
Date:   Thu Sep 18 09:31:18 2025 -0600

    RISC-V: Correct lmul estimation

    The vectorizer's compute_estimated_lmul function could previously
    return a bad value when the estimated lmul was larger than RVV_M8.
    This is corrected to return RVV_M8, preventing a register spill.

    The patch includes a new regression test for PR target/121910, based
    on the x264 mc_chroma function. The test uses scan-tree-dump to
    confirm that the compiler chooses the expected vector mode (RVVM1QI)
    at -O3, verifying the fix.

            PR target/121910
    gcc/ChangeLog:

            * config/riscv/riscv-vector-costs.cc (compute_estimated_lmul):
            Return RVV_M8 when estimated lmul is too large.

    gcc/testsuite/ChangeLog:

            * gcc.target/riscv/rvv/autovec/pr121910.c: New file.