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Subject: [Bug tree-optimization/99873] [11 Regression] GCC no longer makes as
 much use of ST3
Date: Wed, 07 Apr 2021 14:22:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99873

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Richard Sandiford <rsandifo@gcc.gnu.o=
rg>:

https://gcc.gnu.org/g:5c5b31975e62b4c52d76dc5efd9dc717a361c710

commit r11-8029-g5c5b31975e62b4c52d76dc5efd9dc717a361c710
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Wed Apr 7 15:21:55 2021 +0100

    vect: Don't split store groups if we have IFN_STORE_LANES [PR99873]

    As noted in the PR, we were no longer using ST3 for the testcase and
    instead stored each lane individually.  This is because we'd split
    the store group during SLP and couldn't recover when SLP failed.

    However, we can also get better code with ST3 and ST4 even if SLP would
    have succeeded, such as for vect-complex-5.c.  I'm not sure exactly
    where the cut-off point is, but it seems reasonable to allow the split
    if either of the new groups would operate on full vectors *within*
    rather than across scalar loop iterations.

    E.g. on a Cortex-A57, pr99873_3.c performs better using ST4 while
    pr99873_2.c performs better with SLP.

    Another factor is that SLP can handle smaller iteration counts than
    IFN_STORE_LANES can, but we don't have the infrastructure to choose
    reliably based on that.

    gcc/
            PR tree-optimization/99873
            * tree-vect-slp.c (vect_slp_prefer_store_lanes_p): New function.
            (vect_build_slp_instance): Don't split store groups that could
            use IFN_STORE_LANES.

    gcc/testsuite/
            * gcc.dg/vect/slp-21.c: Only expect 2 of the loops to use SLP
            if IFN_STORE_LANES is available.
            * gcc.dg/vect/vect-complex-5.c: Expect no loops to use SLP if
            IFN_STORE_LANES is available.
            * gcc.target/aarch64/pr99873_1.c: New test.
            * gcc.target/aarch64/pr99873_2.c: Likewise.
            * gcc.target/aarch64/pr99873_3.c: Likewise.
            * gcc.target/aarch64/sve/pr99873_1.c: Likewise.
            * gcc.target/aarch64/sve/pr99873_2.c: Likewise.
            * gcc.target/aarch64/sve/pr99873_3.c: Likewise.=