[Bug libstdc++/86590] Codegen is poor when passing std::string by value with _GLIBCXX_EXTERN_TEMPLATE undefined

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86590

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-07-20
     Ever confirmed|0                           |1

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we could try to somehow pattern-match this
"is-this-string-compile-time-constant" test or add a
__builtin_cstring_constant_p () ... after all we
end up evaluating this to false in the end, using __builtin_strlen but
then passing it the constant and folding it anyway.

  <bb 3> [local count: 10037315480]:
  # __s_63 = PHI <"Hello world"(2), __s_66(5)>
  _64 = *__s_63;
  _65 = __builtin_constant_p (_64);
  if (_65 == 0)
    goto <bb 6>; [5.50%]
  else
    goto <bb 4>; [94.50%]

  <bb 4> [local count: 9485263123]:
  if (_64 == 0)
    goto <bb 6>; [5.50%]
  else
    goto <bb 5>; [94.50%]

  <bb 5> [local count: 8963573645]:
  __s_66 = __s_63 + 1;
  goto <bb 3>; [100.00%]

  <bb 6> [local count: 1073741826]:
  # _67 = PHI <_65(3), _65(4)>
  if (_67 != 0)
    goto <bb 7>; [50.00%]
  else
    goto <bb 9>; [50.00%]

  <bb 7> [local count: 4880644630]:
  # __i_68 = PHI <0(6), __i_71(8)>
  _69 = "Hello world" + __i_68;
  _70 = *_69;
  if (_70 != 0)
    goto <bb 8>; [89.00%]
  else
    goto <bb 10>; [11.00%]

  <bb 8> [local count: 4343773717]:
  __i_71 = __i_68 + 1;
  goto <bb 7>; [100.00%]

  <bb 9> [local count: 536870913]:
  _72 = 11;

  <bb 10> [local count: 1073741817]:
  # _73 = PHI <__i_68(7), _72(9)>

so we have

  if (complicated way to tell the string is constant)
    {
      yay, compute it inline with a loop!
    }
  else
    lenght = 11;

stupid.  Very stupid.