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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/101207] [12 Regression]
 gcc.dg/vect/vect-strided-*-mult.c fails after SLP improvements
Date: Mon, 28 Jun 2021 07:26:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101207

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ah, so what happens is that we elide the load permutation that feeds the
plus reduction originally but then we vectorize the live operands of the
minus reduction as BIT_FIELD_REFs ending up extracting the wrong lanes.

Testcase for x86_64:

/* { dg-additional-options "-ftree-slp-vectorize -ffast-math" } */

double a[2];
double x, y;

void __attribute__((noipa)) foo ()
{
  x =3D a[1] - a[0];
  y =3D a[0] + a[1];
}

int main()
{
  a[0] =3D 0.;
  a[1] =3D 1.;
  foo ();
  if (x !=3D 1. || y !=3D 1.)
    __builtin_abort ();
  return 0;
}=