[Bug rtl-optimization/71423] New: wrong code at -Os and above on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71423

            Bug ID: 71423
           Summary: wrong code at -Os and above on x86_64-linux-gnu
           Product: gcc
           Version: 7.0
            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 -Os
and above in both 32-bit and 64-bit modes.

This is a regression from 5.3.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/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-source-trunk/configure --enable-languages=c,c++,lto
--prefix=/usr/local/gcc-trunk --disable-bootstrap
Thread model: posix
gcc version 7.0.0 20160604 (experimental) [trunk revision 237092] (GCC) 
$ 
$ gcc-trunk -O1 small.c
$ ./a.out
$ 
$ gcc-5.3 -Os small.c
$ ./a.out
$ 
$ gcc-6.1 -Os small.c
$ ./a.out
Aborted (core dumped)
$ 
$ gcc-trunk -Os small.c
$ ./a.out
Aborted (core dumped)
$ 


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


struct S1
{
  int f1:1;
};

volatile struct S1 b = { 0 };

int
main ()
{
  char c = b.f1;
  b.f1 = 1; 

  if (b.f1 > -1 || c)
    __builtin_abort (); 

  return 0; 
}