[Bug tree-optimization/86572] New: unsafe strlen folding of const arguments with non-const offset

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86572

            Bug ID: 86572
           Summary: unsafe strlen folding of const arguments with
                    non-const offset
           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: ---

This came up in a conversation of a change in this area (bug 86532).  The
following test case has undefined behavior so anything can happen, but the
result in GCC is unnecessarily and arbitrarily "hostile" in that the value
computed by the strlen expression is excessive large, so large in fact as to be
meaningless (no object can be as large as 18446744073709551615 or SIZE_MAX
bytes).

$ cat c.c && gcc -Wall -fdump-tree-gimple=/dev/stdout c.c && ./a.out 
const char a[] = "123";

__attribute__ ((noipa))
__SIZE_TYPE__ f (int i)
{
  return __builtin_strlen (a + i);
}

int main (void)
{
  __SIZE_TYPE__ n = f (4);

  __builtin_printf ("%zu\n", n);
}
__attribute__((noipa, noinline, noclone, no_icf))
f (int i)
{
  long unsigned int D.1964;

  _1 = (ssizetype) i;
  _2 = 3 - _1;                       // strlen folded into this
  D.1964 = (long unsigned int) _2;
  return D.1964;
}


main ()
{
  int D.1966;

  {
    long unsigned int n;

    n = f (4);
    __builtin_printf ("%zu\n", n);
  }
  D.1966 = 0;
  return D.1966;
}


18446744073709551615