[Bug rtl-optimization/64041] 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=64041

            Bug ID: 64041
           Summary: wrong code at -O2 and -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk and 4.9.x miscompile the following code on x86_64-linux
at -O2 and -O3 in both 32-bit and 64-bit modes. 

This is a regression from 4.8.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/5.0.0/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 5.0.0 20141122 (experimental) [trunk revision 217971] (GCC) 

$ gcc-trunk -Os small.c; a.out
0
$ gcc-4.8 -O2 small.c; a.out
0
$ 
$ gcc-trunk -O2 small.c; a.out
$ gcc-4.9 -O2 small.c; a.out
$ 

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

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

int a, b = 1, d;

union U1
{
  unsigned int f0;
  int f1;
};

union U2
{
  int f2;
  int f3;
} c;

int
fn1 (int p)
{
  int t = p && a || p && a && p; 
  return t ? t : a;
}

unsigned
fn2 (union U1 p1, union U2 p2)
{
  if (p1.f1 <= 0)
    {
      for (; p2.f2;)
    c.f2 = 0;
      p2.f2 = fn1 (d);
    }
  return p2.f3;
}

int
main ()
{
  if (b)
    {
      union U1 f = { 0xFFFFFFFFU }; 
      fn2 (f, c);
    }
  printf ("%d\n", 0);
  return 0; 
}