[Bug tree-optimization/60183] New: [4.7/4.8/4.9 Regression] phiprop creates invalid code

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60183

            Bug ID: 60183
           Summary: [4.7/4.8/4.9 Regression] phiprop creates invalid code
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org

On:

unsigned long c[32] = { 1 };

static void
foo (unsigned long *x, unsigned long *y)
{
  int i;
  unsigned long w = x[0];
  for (i = 0; i < 8; i++)
    {
      w ^= *y++;
      w += *y++;
      w ^= *y++;
      w += *y++;
    }
  x[1] = w;
}

__attribute__((noinline, noclone)) void
bar (unsigned long *x)
{
  foo (x, c);
}

int
main ()
{
  unsigned long a[2] = { 0, -1UL };
  asm volatile ("" : : "r" (c) : "memory");
  c[0] = 0;
  bar (a);
  if (a[1] != 0)
    __builtin_abort ();
  return 0;
}

at -O1 or higher phiprop causes invalid code to be generated, where the loop
body reads the next *y value into a SSA_NAME and in loop preheader it reads
c[0] into a SSA_NAME which is then used in a PHI on the loop header and the
result of the PHI is used instead of the first *y read.  In this particular
case, I don't even see any advantages of doing that, but more importantly it
can read one past the end of the array.  With -O{1,2,3} -fsanitize=address
this fails loudly, otherwise if you are unlucky enough and the variable is at
the end of some mmapped area, you could get a crash as well.