[Bug rtl-optimization/67124] New: wrong code at -O1, -O2 and -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67124

            Bug ID: 67124
           Summary: wrong code at -O1, -O2 and -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-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
-O1, -O2 and -O3 in the 64-bit mode (but not in the 32-bit mode). 

This is a regression from 5.1.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 20150804 (experimental) [trunk revision 226592] (GCC) 
$ 
$ gcc-trunk -O0 small.c; ./a.out
$ gcc-5.1 -O1 small.c; ./a.out
$ 
$ gcc-trunk -O1 small.c
$ ./a.out
Aborted (core dumped)
$ 


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


struct S1
{
  int:11;
  char f1;
  short f2;
  int f3;
  int f4;
} a[2] = {{0, 0, 0, 0}, {1, 0, 1, 0}}, *b = &a[0];

int
main ()
{
  struct S1 c = a[1];
  c.f2 = 0;
  *b = c;
  for (;;)
    if (c.f1)
      break;

  if (a[0].f3 != 1) 
    __builtin_abort (); 

  return 0;
}