[Bug tree-optimization/66186] 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=66186

            Bug ID: 66186
           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 (as well as 4.9.x and 5.1.x) miscompiles the following
code on x86_64-linux at -O3 in both 32-bit and 64-bit modes. 

This is a regression from 4.8.x. 


$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/usr/local/gcc-trunk/libexec/gcc/x86_64-unknown-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-unknown-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 20150517 (experimental) [trunk revision 223265] (GCC) 
$ 
$ gcc-trunk -O2 small.c; ./a.out
$ gcc-4.8.4 -O3 small.c; ./a.out
$ 
$ gcc-trunk -O3 small.c
$ ./a.out
Segmentation fault (core dumped)
$ gcc-5.1 -O3 small.c 
$ ./a.out
Segmentation fault (core dumped)
$ gcc-4.9.2 -O3 small.c
$ ./a.out
Segmentation fault (core dumped)
$ 


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


int a;

int
main ()
{
  int b = -1, d, e = 0, f[2] = { 0 };
  unsigned short c = b;
  for (; e < 3; e++)
    for (d = 0; d < 2; d++)
      if (a < 0)
        for (d = 0; d < 2; d++)
          if (f[c])
            break;
  return 0;
}