[Bug tree-optimization/87445] New: missing null test optimization for a pointer member

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87445

            Bug ID: 87445
           Summary: missing null test optimization for a pointer member
           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: ---

Because f() is declared nonnull, GCC successfully eliminates the unnecessary
null pointer test in function g below.  However, it fails to perform the same
simplification in function h().

I noticed this after adding attribute nonnull to a few GCC functions in tree.h
didn't have the effect on the emitted object code that I expected.  For
example, in something as simple as the following, GCC doesn't eliminate the
unnecessary test:

    extern HOST_WIDE_INT tree_to_shwi (const_tree) ATTRIBUTE_NONNULL (1);

    HOST_WIDE_INT foobar (tree t)
    {
      HOST_WIDE_INT x = tree_to_shwi (TYPE_SIZE (t));
      if (!TYPE_SIZE (t))
        __builtin_abort ();
      return x;
    }

$ cat t.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout t.c
__attribute__ ((nonnull)) int f (void*);

struct S { void *p; };

int g (void *p)
{
  int i = f (p);
  if (!p)                 // folded to false
    __builtin_abort ();   // eliminated
  return i;
}

int h (struct S *p)
{
  int i = f (p->p);
  if (!p->p)              // not folded but could be
    __builtin_abort ();   // not eliminated
  return i;
}


;; Function g (g, funcdef_no=0, decl_uid=1910, cgraph_uid=1, symbol_order=0)

g (void * p)
{
  int i;

  <bb 2> [local count: 1073741824]:
  i_4 = f (p_2(D)); [tail call]
  return i_4;

}



;; Function h (h, funcdef_no=1, decl_uid=1914, cgraph_uid=2, symbol_order=1)

h (struct S * p)
{
  int i;
  void * _1;
  void * _2;

  <bb 2> [local count: 1073741824]:
  _1 = p_4(D)->p;
  i_6 = f (_1);
  _2 = p_4(D)->p;
  if (_2 == 0B)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [99.96%]

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

  <bb 4> [local count: 1073312328]:
  return i_6;

}