[Bug tree-optimization/67947] 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=67947

            Bug ID: 67947
           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 20151012 (experimental) [trunk revision 228712] (GCC) 
$ 
$ gcc-trunk -O2 small.c; ./a.out
0
0
0
$ gcc-5.2 -O3 small.c; ./a.out
0
0
0
$ 
$ gcc-trunk -O3 small.c
$ ./a.out
$ 


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


int printf (const char *, ...);

int a;

int
main (int argc, char* argv[])
{
  int j, k, b = 0;
  if (argc == 0) 
    b = 1;
  for (j = 0; j < 3; j++)
    for (k = 0; k < 1; k++)
      {
        printf ("%d\n", 0);
        if (b)
          for (k = -1; a;)
            ;
      }
  return 0; 
}