[Bug tree-optimization/90663] New: [7/8/9 Regression] strcmp (&a[i], a + i) not folded for arrays and constant index

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90663

            Bug ID: 90663
           Summary: [7/8/9 Regression] strcmp (&a[i], a + i) not folded
                    for arrays and constant index
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The two functions below are more or less equivalent and should result in
optimal code (a no-op) but starting with GCC 4.7 g() does not.  Prior to GCC
4.7 both were optimized into a return statement.  Clang also emits optimally
efficient code here.

$ cat a.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout a.c
extern char a[];

void f (const char *s)
{
  if (__builtin_strcmp (&s[2], s + 2))   // folded
    __builtin_abort ();
}

void g (void)
{
  if (__builtin_strcmp (&a[2], a + 2))   // not folded
    __builtin_abort ();
}

;; Function f (f, funcdef_no=0, decl_uid=1907, cgraph_uid=1, symbol_order=0)

f (const char * s)
{
  <bb 2> [local count: 1073741824]:
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1910, cgraph_uid=2, symbol_order=1)

g ()
{
  int _1;

  <bb 2> [local count: 1073741824]:
  _1 = __builtin_strcmp (&a[2], &MEM[(void *)&a + 2B]);
  if (_1 != 0)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

  <bb 3> [count: 0]:
  __builtin_abort ();

  <bb 4> [local count: 1073741824]:
  return;

}