[Bug tree-optimization/61787] New: wrong code at -Os on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61787

            Bug ID: 61787
           Summary: wrong code at -Os on x86_64-linux-gnu
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk miscompiles the following code on x86_64-linux at (only)
-Os in both 32-bit and 64-bit modes.

This is a regression from 4.9.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/4.10.0/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 4.10.0 20140711 (experimental) [trunk revision 212447] (GCC) 
$ 
$ gcc-trunk -O1 small.c; a.out
$ gcc-trunk -O2 small.c; a.out
$ gcc-4.9 -Os small.c; a.out
$ 
$ gcc-trunk -Os small.c; a.out
Aborted (core dumped)
$ 


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


unsigned int a = 1;

int
fn1 ()
{
  char b;
  for (;;)
    {
      for (b = 0; b < 5; b++)
    if (!a - (unsigned int) b)
      return 0;
      a = 0;
    }
}

int
main ()
{
  fn1 ();

  if (a != 1) 
    __builtin_abort (); 

  return 0;
}