[Bug tree-optimization/83899] New: missing strlen optimization for constant string plus offset

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83899

            Bug ID: 83899
           Summary: missing strlen optimization for constant string plus
                    offset
           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: ---

Thanks to r255790, GCC 8 is able to fold the conditional in the first function
based on the size of the array, but it cannot perform the similar optimization
for constant strings.

$ cat z.c && gcc -O1 -S -fdump-tree-optimized=/dev/stdout z.c
extern char a[4];

void f (int i)
{
  const char *s = a;

  __SIZE_TYPE__ n = __builtin_strlen (s);

  if (n >= 4)   // folded to false (GCC 8)
    __builtin_abort ();
}

void g (int i)
{
  const char* const s = "123" + i;

  __SIZE_TYPE__ n = __builtin_strlen (s);

  if (n >= 4)   // not folded
    __builtin_abort ();
}

;; Function f (f, funcdef_no=0, decl_uid=1951, cgraph_uid=0, symbol_order=0)

f (int i)
{
  <bb 2> [local count: 1073741825]:
  return;

}



;; Function g (g, funcdef_no=1, decl_uid=1956, cgraph_uid=1, symbol_order=1)

g (int i)
{
  long unsigned int n;
  const char * const s;
  sizetype _1;

  <bb 2> [local count: 1073741825]:
  _1 = (sizetype) i_2(D);
  s_3 = "123" + _1;
  n_5 = __builtin_strlen (s_3);
  if (n_5 > 3)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [99.96%]

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

  <bb 4> [local count: 1073312327]:
  return;

}