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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/97832] AoSoA complex caxpy-like loops:
 AVX2+FMA -Ofast 7 times slower than -O3
Date: Wed, 18 Nov 2020 08:53:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97832

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
So for example we'd like to vectorize with SLP when reassociation is permit=
ted
(thus with -Ofast for example):

double a[1024], b[1024], c[1024];

void foo()
{
  for (int i =3D 0; i < 256; ++i)
    {
      a[2*i] =3D 1. - a[2*i] + b[2*i];
      a[2*i+1] =3D a[2*i+1] + b[2*i+1] + 1.;
    }
}

it again works when written as follows and with -fno-tree-reassoc

double a[1024], b[1024], c[1024];

void foo()
{
  for (int i =3D 0; i < 256; ++i)
    {
      a[2*i] =3D 1. - a[2*i] + b[2*i];
      a[2*i+1] =3D 1 + a[2*i+1] + b[2*i+1];
    }
}=