[Bug tree-optimization/58984] New: wrong code at -Os and above on x86_64-linux-gnu in 64-bit mode

[su at cs dot ucdavis.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58984

            Bug ID: 58984
           Summary: wrong code at -Os and above on x86_64-linux-gnu in
                    64-bit mode
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk and 4.8.x miscompile the following code on x86_64-linux
at -Os and above in 64-bit mode. 

This is a regression from 4.7.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/4.9.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-trunk/configure
--enable-languages=c,c++,objc,obj-c++,fortran,lto --disable-werror
--enable-checking=release --with-gmp=/usr/local/gcc-trunk
--with-mpfr=/usr/local/gcc-trunk --with-mpc=/usr/local/gcc-trunk
--with-cloog=/usr/local/gcc-trunk --prefix=/usr/local/gcc-trunk
Thread model: posix
gcc version 4.9.0 20131103 (experimental) [trunk revision 204333] (GCC) 
$ 
$ gcc-trunk -O1 small.c; a.out
1
$ gcc-trunk -Os small.c; a.out
0
$ gcc-4.8.2 -Os small.c; a.out
0
$ gcc-4.7.3 -Os small.c; a.out
1
$ 

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

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

struct S
{
  int f0:8;
  int:6;
  int f1:5;
};

int a, *c = &a, e, n, b;

static int
foo (struct S p)
{
  const unsigned short *f[36];
  for (; e < 2; e++)
    {
      const unsigned short **i = &f[0];
      *c ^= 1;
      if (p.f1)
    {
      *i = 0;
      return b;
    }
    }
  return 0;
}

int
main ()
{
  struct S o = { 1, 1 };
  foo (o);
  n || o.f0;
  printf ("%d\n", a);
  return 0;
}