[Bug tree-optimization/95803] New: Failure to optimize strlen in certain situations properly, instead leading to weird code

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95803

            Bug ID: 95803
           Summary: Failure to optimize strlen in certain situations
                    properly, instead leading to weird code
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

bool f(int i)
{
    if (i < 4)
        i = 4;

    const char *s = &"abc"[i];

    return strlen(s) > 3;
}

This can be optimized to a simple `return true` or `return false` (considering
UB is invoked here). LLVM does this transformation, but GCC does not. Obviously
this code is pretty weird and may not be very realistic itself, but this
optimization would probably be effective for other code, and currently it
results in rather weird code generation :

f(int):
  mov ecx, 4
  cmp edi, 4
  mov eax, 3
  mov edx, ecx
  cmovge edx, edi
  movsx rdx, edx
  sub rax, rdx
  cmp rdx, 3
  mov edx, 0
  cmova rax, rdx
  cmp rax, 3
  seta al
  ret

accompanied by very weird final tree optimized code :

;; Function f (_Z1fi, funcdef_no=287, decl_uid=9588, cgraph_uid=216,
symbol_order=215)

f (int i)
{
  const char * s;
  long unsigned int _1;
  int _3;
  bool _6;
  sizetype _7;

  <bb 2> [local count: 1073741824]:
  # DEBUG BEGIN_STMT
  _3 = MAX_EXPR <i_2(D), 4>;
  # DEBUG i => _3
  # DEBUG BEGIN_STMT
  _7 = (sizetype) _3;
  s_4 = "abc" + _7;
  # DEBUG s => s_4
  # DEBUG BEGIN_STMT
  _1 = __builtin_strlen (s_4);
  _6 = _1 > 3;
  return _6;
}

Whereas this code :

bool f(int i)
{
    if (i < 4)
        i = 4;

    return strlen(&"abc"[i]) > 3;
}

Optimizes to this :

;; Function f (_Z1fi, funcdef_no=287, decl_uid=9588, cgraph_uid=216,
symbol_order=215)

f (int i)
{
  <bb 2> [local count: 1073741824]:
  # DEBUG BEGIN_STMT
  # DEBUG i => NULL
  # DEBUG BEGIN_STMT
  return 0;

}