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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/99881] New: Regression compare -O2 -ftree-vectorize with
 -O2 on SKX/CLX
Date: Fri, 02 Apr 2021 03:49:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99881

            Bug ID: 99881
           Summary: Regression compare -O2 -ftree-vectorize with -O2 on
                    SKX/CLX
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: crazylht at gmail dot com
  Target Milestone: ---

testcase is extracted from 557.xz_r

void
foo (int* __restrict a, int n, int c)
{
    a[0] =3D n;
    a[1] =3D c;
}

gcc -O2 -ftree-vectorize -fvect-cost-model=3Dvery-cheap

foo(int*, int, int):
        movd    xmm0, esi
        movd    xmm1, edx
        punpckldq       xmm0, xmm1
        movq    QWORD PTR [rdi], xmm0
        ret

without vectorization

foo(int*, int, int):
        mov     DWORD PTR [rdi], esi
        mov     DWORD PTR [rdi+4], edx
        ret

cost model:
scalar: 2 times scalar_store costs 24,
vector: 1 times unaligned_store costs 12, vec_contruct 8

I know that the current strategy of the cost model is to enable vectorizati=
on
as much as possible, but for the case above, it hurts performance. Because =
the
throughput of punpckldq is 1 on SKX/CLX, which becomes a bottleneck (znver2=
 is
ok). with -march=3DSKX, the second vmovd and unpck will be replaced by vpin=
sr,
and it regression more since vpinsr has throught 2 on CLX/SKX.

So i'm thinking to add extra cost for 2-element vec_construct to prevent the
above vectorization, at the same time, try not to affect other vectorization
situations.=