[Bug tree-optimization/78170] New: wrong code at -O2 and -O3 on x86_64-linux-gnu

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78170

            Bug ID: 78170
           Summary: wrong code at -O2 and -O3 on x86_64-linux-gnu
           Product: gcc
           Version: unknown
            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: ---

This is a regression from 6.2.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 20161031 (experimental) [trunk revision 241694] (GCC) 
$ 
$ gcc-trunk -Os small.c; ./a.out
0
$ gcc-6.2 -O2 small.c; ./a.out
0
$ 
$ gcc-trunk -O2 small.c; ./a.out
-1
$ 


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


int printf (const char *, ...);

int a, b, d;

struct S0
{
  int f0; 
  int f1; 
  int f2; 
  int f3; 
  int f4;
  int f5:15;
  int f6:17;
  int f7:2;
  int f8:30;
} c;

void fn1 ()
{
  d = b = 1;
  for (; b; b = a)
    {
      struct S0 e = { 0, 0, 0, 0, 0, 0, 1, 0, 1 };
      c = e;
      c.f6 = -1; 
    }
}

int main ()
{
  fn1 ();
  printf ("%d\n", c.f7);
  return 0; 
}