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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98854] [11 Regression] cray benchmark is
 about 15% slower since r11-4428-g4a369d199bf2f34e
Date: Wed, 27 Jan 2021 14:22:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98854

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Little bit convoluted testcase:

double a[1024];

int bar();
void foo (int n)
{
  double x =3D 0, y =3D 0;
  int i =3D 1023;
  do
    {
      x +=3D a[i] + a[i+1];
      y +=3D a[i] / a[i+1];
      if (bar ())
        break;
    }
  while (--i);
  a[0] =3D x;
  a[1] =3D y;
}

where we end up with the {x, y} vector CTOR inside the loop (and even
spill/reload it because of the call).  We have a PHI node-only feed
for the vectorized store:

t.c:16:8: note: Vectorizing SLP tree:
t.c:16:8: note: node 0x3b21ee0 (max_nunits=3D2, refcnt=3D1)
t.c:16:8: note: op template: a[0] =3D x_22;
t.c:16:8: note:         stmt 0 a[0] =3D x_22;
t.c:16:8: note:         stmt 1 a[1] =3D y_21;
t.c:16:8: note:         children 0x3b21f68
t.c:16:8: note: node 0x3b21f68 (max_nunits=3D2, refcnt=3D1)
t.c:16:8: note: op template: x_22 =3D PHI <x_26(9), x_25(10)>
t.c:16:8: note:         stmt 0 x_22 =3D PHI <x_26(9), x_25(10)>
t.c:16:8: note:         stmt 1 y_21 =3D PHI <y_24(9), y_23(10)>
t.c:16:8: note:         children 0x3b21ff0 0x3b22210
t.c:16:8: note: node 0x3b21ff0 (max_nunits=3D2, refcnt=3D1)
t.c:16:8: note: op template: x_26 =3D PHI <x_14(3)>
t.c:16:8: note:         stmt 0 x_26 =3D PHI <x_14(3)>
t.c:16:8: note:         stmt 1 y_24 =3D PHI <y_15(3)>
t.c:16:8: note:         children 0x3b22320
t.c:16:8: note: node (external) 0x3b22320 (max_nunits=3D1, refcnt=3D1)
t.c:16:8: note:         { x_14, y_15 }
t.c:16:8: note: node 0x3b22210 (max_nunits=3D2, refcnt=3D1)
t.c:16:8: note: op template: x_25 =3D PHI <x_14(4)>
t.c:16:8: note:         stmt 0 x_25 =3D PHI <x_14(4)>
t.c:16:8: note:         stmt 1 y_23 =3D PHI <y_15(4)>
t.c:16:8: note:         children 0x3b223a8
t.c:16:8: note: node (external) 0x3b223a8 (max_nunits=3D1, refcnt=3D1)
t.c:16:8: note:         { x_14, y_15 }

fixing this issue fixes the slowdown.  Testing a patch.=