[Bug tree-optimization/86919] New: strlen of a multidimensional array element with non-constant offset not folded

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86919

            Bug ID: 86919
           Summary: strlen of a multidimensional array element with
                    non-constant offset not folded
           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: ---

GCC 9 folds the first of the strlen() calls below where the first index into
the array is a literal constant into an efficient expression but it misses the
same opportunity in the second call where the index is a variable with a known
value.  (GCC 8folds neither.)

$ cat f.c && gcc -O2 -S -Wall -Wextra -Wnull-dereference
-fdump-tree-optimized=/dev/stdout f.c
const char b[][5] = { "123", "1234" };

int f (int j)
{
  return __builtin_strlen (&b[0][j]);   // folded to 3 - i
}

int g (int j)
{
  int i = 0;
  return __builtin_strlen (&b[i][j]);   // not folded
}

;; Function f (f, funcdef_no=0, decl_uid=1906, cgraph_uid=1, symbol_order=1)

Removing basic block 5
f (int j)
{
  unsigned int j.1_1;
  unsigned int _2;
  int iftmp.0_3;
  int iftmp.0_5;

  <bb 2> [local count: 1073741825]:
  if (j_4(D) <= 3)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870912]:
  j.1_1 = (unsigned int) j_4(D);
  _2 = 3 - j.1_1;
  iftmp.0_5 = (int) _2;

  <bb 4> [local count: 1073741825]:
  # iftmp.0_3 = PHI <iftmp.0_5(3), 0(2)>
  return iftmp.0_3;

}



;; Function g (g, funcdef_no=1, decl_uid=1909, cgraph_uid=2, symbol_order=2)

g (int j)
{
  const char * _1;
  long unsigned int _2;
  int _5;
  sizetype _6;

  <bb 2> [local count: 1073741825]:
  _6 = (sizetype) j_3(D);
  _1 = &b + _6;
  _2 = __builtin_strlen (_1);
  _5 = (int) _2;
  return _5;

}