[Bug tree-optimization/68083] New: wrong code at -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68083

            Bug ID: 68083
           Summary: wrong code at -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu
  Target Milestone: ---

The current gcc trunk miscompiles the following code on x86_64-linux-gnu at -O3
in both 32-bit and 64-bit modes. 

This is a regression from 5.2.x.


$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --prefix=/usr/local/gcc-trunk
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
gcc version 6.0.0 20151023 (experimental) [trunk revision 229251] (GCC) 
$ 
$ gcc-trunk -O2 small.c; ./a.out
$ gcc-5.2 -O3 small.c; ./a.out
$ 
$ gcc-trunk -O3 small.c
$ ./a.out
Aborted (core dumped)
$ 


------------------------------------


int a = 2, b = 1, c = 1;

int
fn1 ()
{
  int d;
  for (; a; a--)
    {
      for (d = 0; d < 4; d++)
        {
          int k;
          if (c < 1)
            if (k)
              c = 0;
          if (b)
            continue;
          return 0;
        }
      b = !1;
    }
  return 0;
}

int
main ()
{
  fn1 ();

  if (a != 1) 
    __builtin_abort (); 

  return 0;
}