[Bug tree-optimization/68624] New: wrong code at -O2 and -O3 on x86_64-linux-gnu (in 64-bit mode)

[su at cs dot ucdavis.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68624

            Bug ID: 68624
           Summary: wrong code at -O2 and -O3 on x86_64-linux-gnu (in
                    64-bit mode)
           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-gnu at -O2
and -O3 in the 64-bit mode (but not in the 32-bit mode). 

This is a regression from 5.2.x. 

Perhaps to do with VRP as -fno-tree-vrp seems to fix the miscompilation. 


$ 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 20151130 (experimental) [trunk revision 231056] (GCC) 
$ 
$ gcc-trunk -m64 -Os small.c; ./a.out
$ gcc-trunk -m32 -O2 small.c; ./a.out
$ gcc-trunk -m64 -O2 -fno-tree-vrp small.c; ./a.out
$ gcc-5.2 -m64 -O2 small.c; ./a.out
$ 
$ gcc-trunk -m64 -O2 small.c
$ ./a.out
Aborted (core dumped)
$ 


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


int b, c, d, e = 1, f, g, h, j;

static int
fn1 ()
{
  int a = c;
  if (h)
    return 9;
  g = (c || b) % e;
  if ((g || f) && b)
    return 9;
  e = d;
  for (c = 0; c > -4; c--)
    ;
  if (d)
    c--;
  j = c;
  return d;
}

int
main ()
{
  fn1 ();

  if (c != -4) 
    __builtin_abort (); 

  return 0;
}