[Bug lto/64685] New: wrong code by LTO on x86_64-linux-gnu

["su at cs dot ucdavis.edu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64685

            Bug ID: 64685
           Summary: wrong code by LTO on x86_64-linux-gnu
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The current gcc trunk miscompiles the following code when using LTO on
x86_64-linux-gnu in both 32-bit and 64-bit modes. 

This is a regression from 4.9.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 20150119 (experimental) [trunk revision 219832] (GCC) 

$ 
$ gcc-trunk -O0 -c fn1.c
$ gcc-trunk -O1 -c main.c
$ gcc-trunk -O1 fn1.o main.o
$ ./a.out
$ 
$ gcc-4.9 -flto -O0 -c fn1.c
$ gcc-4.9 -flto -O1 -c main.c
$ gcc-4.9 -flto -O1 fn1.o main.o
$ ./a.out
$ 
$ 
$ gcc-trunk -flto -O0 -c fn1.c
$ gcc-trunk -flto -O1 -c main.c
$ gcc-trunk -flto -O1 fn1.o main.o
$ ./a.out
Aborted (core dumped)
$ 
$ cat fn1.c
extern int b;

void
fn1 (void)
{
  b = 0;
}
$ cat main.c
extern void fn1 (void); 

int a[2], b;

static void
foo (int p)
{
  b = 1 ^ a[(b ^ 1) & 1];
  b = 1 ^ a[b & 1];
  if (p)
    __builtin_abort ();
}

int
main ()
{
  foo (0);
  b = 0;
  foo (0);

  if (b != 1)
    __builtin_abort ();

  return 0;
}
$