[Bug tree-optimization/92155] New: strlen(a) not folded after memset(a, 0, sizeof a)

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92155

            Bug ID: 92155
           Summary: strlen(a) not folded after memset(a, 0, sizeof a)
           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: ---

Now that GCC unrolls memset-like loops with small numbers of iterations
(pr91975) and transforms some of them into MEM_REF, the strlen pass can also
determine the lengths of zeroed-out arrays to be zero.  This can be seen in
function f below.

But GCC doesn't yet transform memset calls into the equivalent MEM_REFs, and
the strlen pass for some reason can't figure out that the length of an array
that's been zeroed-out by memset is also zero.  This missed optimization can be
seen in function g below.

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout z.c
extern char a4[4];
extern char b4[4];

void f (void)
{
  for (int i = 0; i != sizeof a4; ++i)
    a4[i] = 0;
  for (int i = 0; i != sizeof b4; ++i)
    b4[i] = 0;

  if (__builtin_strlen (a4) != __builtin_strlen (b4))
    __builtin_abort ();
}

void g (void)
{
  __builtin_memset (a4, 0, sizeof a4);
  __builtin_memset (b4, 0, sizeof b4);

  if (__builtin_strlen (a4) != __builtin_strlen (b4))
    __builtin_abort ();
}

;; Function f (f, funcdef_no=0, decl_uid=1932, cgraph_uid=1, symbol_order=0)

f ()
{
  <bb 2> [local count: 214748369]:
  MEM <unsigned int> [(char *)&a4] = 0;
  MEM <unsigned int> [(char *)&b4] = 0;
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1943, cgraph_uid=2, symbol_order=1)

g ()
{
  long unsigned int _1;
  long unsigned int _2;

  <bb 2> [local count: 1073741824]:
  __builtin_memset (&a4, 0, 4);
  __builtin_memset (&b4, 0, 4);
  _1 = __builtin_strlen (&a4);
  _2 = __builtin_strlen (&b4);
  if (_1 != _2)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

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

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

}