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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/105219] [12 Regression] SVE: Wrong code with -O3
 -msve-vector-bits=128 -mtune=thunderx
Date: Mon, 11 Apr 2022 15:56:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105219

Tamar Christina <tnfchris at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tnfchris at gcc dot gnu.org
   Last reconfirmed|                            |2022-4-11

--- Comment #1 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
Smaller reproducer getting rid of the loop nest and simplify the inner
condition.

int a;=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20
                               char b[60];
short d[19];
long long f;

__attribute__ ((noinline, noipa))=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20
                               void e(int g, int h, short k[19]) {
    for (signed j =3D 1; j < h + 14; j++) {
      int i =3D 0;
      b[j] =3D 1;
      i =3D 2;
      b[i * 14 + j] =3D 1;
      a =3D g ? k[j] : 0;
    }
}

__attribute__ ((optimize("O0")))
int main() {
  e(9, 1, d);
  for (long l =3D 0; l < 6; ++l)
    for (long m =3D 0; m < 4; ++m)
      f ^=3D b[l + m * 4];
  if (f)
    __builtin_abort ();
}

and the -mtune=3Dthunderx doesn't seem needed. I can reproduce with just -O3
-march=3Darmv8.2-a+sve -msve-vector-bits=3D128.

the write to `a` itself is significant not for the result but to block the =
loop
from being optimized to a memset.=