[Bug tree-optimization/81385] New: missing optimization involving strlen of arrays of known size

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81385

            Bug ID: 81385
           Summary: missing optimization involving strlen of arrays of
                    known size
           Product: gcc
           Version: 8.0
            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: ---

A valid argument to the strlen() function that is an array of known size (or
that points to one) must contain a string whose length is less than the array
size.  It is possible to rely on this guarantee to emit more efficient code
than without it.  The test case below shows that GCC doesn't take advantage of
this optimization opportunity.

$ cat b.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout b.c
typedef __SIZE_TYPE__ size_t;

size_t strlen (const char*);

char a[7];

void f (void)
{
  unsigned n = strlen (a);
  if (n >= sizeof a)      // can never hold
    __builtin_abort ();   // can be eliminated
}

struct S { char a[7]; int i; };

void g (const struct S *s)
{
  unsigned n = strlen (s->a);
  if (n >= sizeof s->a)   // can never hold
    __builtin_abort ();   // can be eliminated
}

;; Function f (f, funcdef_no=0, decl_uid=1819, cgraph_uid=0, symbol_order=1)

f ()
{
  unsigned int n;
  long unsigned int _1;

  <bb 2> [100.00%] [count: INV]:
  _1 = strlen (&a);
  n_3 = (unsigned int) _1;
  if (n_3 > 6)
    goto <bb 3>; [0.04%] [count: 0]
  else
    goto <bb 4>; [99.96%] [count: INV]

  <bb 3> [0.04%] [count: 0]:
  __builtin_abort ();

  <bb 4> [99.96%] [count: INV]:
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1826, cgraph_uid=1, symbol_order=2)

g (const struct S * s)
{
  unsigned int n;
  const char[7] * _1;
  long unsigned int _2;

  <bb 2> [100.00%] [count: INV]:
  _1 = &s_3(D)->a;
  _2 = strlen (_1);
  n_5 = (unsigned int) _2;
  if (n_5 > 6)
    goto <bb 3>; [0.04%] [count: 0]
  else
    goto <bb 4>; [99.96%] [count: INV]

  <bb 3> [0.04%] [count: 0]:
  __builtin_abort ();

  <bb 4> [99.96%] [count: INV]:
  return;

}