[Bug target/98438] Rather bad optimization of midpoint implementation for __int128 (and other types)

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98438

--- Comment #2 from Gabriel Ravier <gabravier at gmail dot com> ---
Directly based off the emitted GIMPLE, here's a self-contained example that
emits the same assembly I was observing before :

using _Tp = int;
using _Up = unsigned;
_Tp f(_Tp __a, _Tp __b)
{
  _Up __sign_bit_12;
  _Up __diff_11;
  unsigned int __b_0_1;
  unsigned int __a_1_2;
  bool _3;
  unsigned int _4;
  unsigned int _5;
  unsigned int _7;
  unsigned int _8;
  unsigned int _13;
  _Tp _14;
  unsigned int _17;

  __b_0_1 = (unsigned int) __b;
  __a_1_2 = (unsigned int) __a;
  __diff_11 = __b_0_1 - __a_1_2;
  _3 = __b < __a;
  __sign_bit_12 = (_Up) _3;
  _4 = __diff_11 >> 1;
  _5 = __sign_bit_12 << 31;
  _13 = _4 + _5;
  _7 = __diff_11 & __sign_bit_12;
  _17 = __a_1_2 + _7;
  _8 = _13 + _17;
  _14 = (_Tp) _8;
  return _14;
}