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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/104165] [12 Regression] -Warray-bounds for
 unreachable code inlined from std::sort()
Date: Tue, 08 Feb 2022 21:07:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104165

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |https://bugzilla.redhat.com
                   |                            |/show_bug.cgi?id=3D2051783
           Keywords|                            |missed-optimization

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
See also another similar test case reduced in
https://bugzilla.redhat.com/show_bug.cgi?id=3D2051783:

#include <algorithm>

bool cond;
int foo;

int func (void)
{
  int a[3], qa =3D 0;
  for(int i =3D 0; i < 3; i++)
    if (cond)
      a[qa++] =3D foo;

  std::sort (a, a + qa);
  return 0;
}

The warning is issued for the synthesized call to std::__insertion_sort.isr=
a()
in basic block 9 (vrp1 output):

  <bb 9> [local count: 71766220]:
  std::__insertion_sort.isra (&a, &MEM <int[3]> [(void *)&a + 64B]);   <<<
-Warray-bounds
  goto <bb 14>; [100.00%]

The block is unreachable but GCC is unable to determine that from the loop.=
=20
Adding an annotation just after the loop asserting that avoids the warning =
and
also improves the emitted object code.

The same warning triggers in GCC 11 with -Wsystem-headers.  The difference =
in
GCC 12 is that -Wsystem-headers no longer disables warnings for code define=
d in
system headers that's inlined into user code.=