[Bug tree-optimization/66272] 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=66272

            Bug ID: 66272
           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 at -O3 in
both 32-bit and 64-bit modes.  

It also affects gcc 5.1.x and 4.9.2 (but not 4.9.0 and I didn't check 4.9.1). 


$ 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 20150524 (experimental) [trunk revision 223630] (GCC) 
$ 
$ gcc-trunk -O2 small.c; ./a.out
$ gcc-4.9.0 -O3 small.c; ./a.out
$ 
$ gcc-trunk -O3 small.c  
$ ./a.out
Aborted (core dumped)
$ gcc-4.9.2 -O3 small.c
$ ./a.out
Aborted (core dumped)
$ gcc-5.1.0 -O3 small.c
$ ./a.out
Aborted (core dumped)
$ 


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


struct S
{
  int f0;
  int f1;
} a[2], c;

int b;

int
main ()
{
  struct S d = { 0, 1 };

  for (b = 0; b < 2; b++)
    {
      a[b] = d;
      c = d = a[0];
    }

  if (c.f1 != 1)
    __builtin_abort ();

  return 0;
}