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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113676] [12 Regression] Miscompilation
 tree-vrp __builtin_unreachable
Date: Wed, 31 Jan 2024 09:40:32 +0000
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X-Bugzilla-Keywords: wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113676

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |amacleod at redhat dot com,
                   |                            |jakub at gcc dot gnu.org
           Keywords|needs-bisection             |

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Bisection with -O2 -ftree-vrp
#include <vector>

unsigned
bit_ceil (unsigned x)
{
  if (x <=3D 1)
    return 1U;
  int w =3D 32 - __builtin_clz (x - 1);
  return 1U << w;
}

int
main (int argc, char **)
{
  unsigned rounded_n =3D bit_ceil ((unsigned) (argc + 1));
  auto a =3D std::vector<int> (2UL * rounded_n);
  for (long unsigned int i =3D rounded_n; i-- > 1;)
    {
      if (!(0 < i && i < rounded_n))
        __builtin_unreachable();
      a[i] =3D 0;
    }
}
shows this started with r12-155-gd8e1f1d24179690fd9c0f63c27b12e030010d9ea
and went away with r13-3596-ge7310e24b1c0ca67b1bb507c1330b2bf39e59e32
so nothing really backportable.=