[Bug rtl-optimization/66922] New: wrong code for bit-field struct at -O1 and above on x86_64-linux-gnu

["su at cs dot ucdavis.edu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66922

            Bug ID: 66922
           Summary: wrong code for bit-field struct at -O1 and above 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 mis-compiles the following code on x86_64-linux-gnu at
-O1 and above 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-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 20150717 (experimental) [trunk revision 225918] (GCC) 
$ 
$ gcc-trunk -m64 -O0 small.c; ./a.out
$ gcc-5.1 -m64 -O1 small.c; ./a.out
$ gcc-trunk -m32 -O1 small.c; ./a.out
$ 
$ gcc-trunk -m64 -O1 small.c
$ ./a.out
Aborted (core dumped)
$ 


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


struct S 
{
  int:31;
  int:2;
  int f0:16;
  int f1;
  int f2;
};

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

  if (a.f0 != 1)
    __builtin_abort(); 

  return 0;
}