[Bug tree-optimization/104165] [12 Regression] -Warray-bounds for unreachable code inlined from std::sort()

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104165

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |https://bugzilla.redhat.com
                   |                            |/show_bug.cgi?id=2051783
           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=2051783:

#include <algorithm>

bool cond;
int foo;

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

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

The warning is issued for the synthesized call to std::__insertion_sort.isra()
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. 
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 defined in
system headers that's inlined into user code.