[Bug tree-optimization/86364] New: strnlen before strlen of same argument not folded

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86364

            Bug ID: 86364
           Summary: strnlen before strlen of same argument not folded
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

When a strlen() call is followed by strnlen() with the same argument of unknown
length GCC folds the latter call into a MIN_EXPR involving the bound and the
length computed by the strlen() call.

But when the order of the two calls is reversed, GCC doesn't perform the same
optimization, even though in principle it could by reordering the two calls and
starting with strlen() instead.

$ cat c.c && gcc -O2 -S -fdump-tree-optimized=/dev/stdout c.c
char a[] = "12345";

__attribute__ ((noipa))
int f1 (void)
{
  int n0 = __builtin_strlen (a);       // call (unavoidably) emitted
  int n1 = __builtin_strnlen (a, 1);   // folded into a MIN_EXPR

  return n0 + n1;
}

__attribute__ ((noipa))
int f2 (void)
{
  int n0 = __builtin_strnlen (a, 1);   // not folded
  int n1 = __builtin_strlen (a);       // call emitted

  return n0 + n1;
}

;; Function f1 (f1, funcdef_no=0, decl_uid=1899, cgraph_uid=1, symbol_order=1)

__attribute__((noipa, noinline, noclone, no_icf))
f1 ()
{
  int n1;
  int n0;
  long unsigned int _1;
  long unsigned int _2;
  int _6;

  <bb 2> [local count: 1073741825]:
  _1 = __builtin_strlen (&a);
  n0_4 = (int) _1;
  _2 = MIN_EXPR <_1, 1>;
  n1_5 = (int) _2;
  _6 = n0_4 + n1_5;
  return _6;

}



;; Function f2 (f2, funcdef_no=1, decl_uid=1904, cgraph_uid=2, symbol_order=2)

__attribute__((noipa, noinline, noclone, no_icf))
f2 ()
{
  int n1;
  int n0;
  long unsigned int _1;
  long unsigned int _2;
  int _6;

  <bb 2> [local count: 1073741825]:
  _1 = __builtin_strnlen (&a, 1);
  n0_4 = (int) _1;
  _2 = __builtin_strlen (&a);
  n1_5 = (int) _2;
  _6 = n0_4 + n1_5;
  return _6;

}