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Subject: [Bug tree-optimization/101895] [11/12 Regression] SLP Vectorizer
 change pushes VEC_PERM_EXPR into bad location spoiling further optimization
 opportunities
Date: Tue, 15 Mar 2022 09:06:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101895

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:49fb0af9bf8f16907980d383c2bbc85e185ec2e0

commit r12-7653-g49fb0af9bf8f16907980d383c2bbc85e185ec2e0
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue Mar 15 09:05:28 2022 +0000

    PR tree-optimization/101895: Fold VEC_PERM to help recognize FMA.

    This patch resolves PR tree-optimization/101895 a missed optimization
    regression, by adding a costant folding simplification to match.pd to
    simplify the transform "mult; vec_perm; plus" into "vec_perm; mult; plu=
s"
    with the aim that keeping the multiplication and addition next to each
    other allows them to be recognized as fused-multiply-add on suitable
    targets.  This transformation requires a tweak to match.pd's
    vec_same_elem_p predicate to handle CONSTRUCTOR_EXPRs using the same
    SSA_NAME_DEF_STMT idiom used for constructors elsewhere in match.pd.

    The net effect is that the following code example:

    void foo(float * __restrict__ a, float b, float *c) {
      a[0] =3D c[0]*b + a[0];
      a[1] =3D c[2]*b + a[1];
      a[2] =3D c[1]*b + a[2];
      a[3] =3D c[3]*b + a[3];
    }

    when compiled on x86_64-pc-linux-gnu with -O2 -march=3Dcascadelake
    currently generates:

            vbroadcastss    %xmm0, %xmm0
            vmulps  (%rsi), %xmm0, %xmm0
            vpermilps       $216, %xmm0, %xmm0
            vaddps  (%rdi), %xmm0, %xmm0
            vmovups %xmm0, (%rdi)
            ret

    but with this patch now generates the improved:

            vpermilps       $216, (%rsi), %xmm1
            vbroadcastss    %xmm0, %xmm0
            vfmadd213ps     (%rdi), %xmm0, %xmm1
            vmovups %xmm1, (%rdi)
            ret

    2022-03-15  Roger Sayle  <roger@nextmovesoftware.com>
                Marc Glisse  <marc.glisse@inria.fr>
                Richard Biener  <rguenther@suse.de>

    gcc/ChangeLog
            PR tree-optimization/101895
            * match.pd (vec_same_elem_p): Handle CONSTRUCTOR_EXPR def.
            (plus (vec_perm (mult ...) ...) ...): New reordering
simplification.

    gcc/testsuite/ChangeLog
            PR tree-optimization/101895
            * gcc.target/i386/pr101895.c: New test case.=