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From: "ktietz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/59429] Missed optimization opportunity in qsort-style comparison functions
Date: Fri, 04 Apr 2014 11:37:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59429

Kai Tietz <ktietz at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-04-04
                 CC|                            |ktietz at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #4 from Kai Tietz <ktietz at gcc dot gnu.org> ---
This issue seems to be another problem caused by missing unification of
expressions.

For given example we get for 'comGE' the following gimple:

comGE (int a, int b)
{
  int iftmp.8_1;
  int _2;
  _Bool _6;
  int _7;

  <bb 2>:
  if (a_3(D) <= b_4(D))
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  _6 = a_3(D) != b_4(D);
  _2 = (int) _6;
  _7 = -_2;

  <bb 4>:
  # iftmp.8_1 = PHI <_7(3), 1(2)>
  return iftmp.8_1;

}

As we see we don't optimize out the negate case.
For expression:
  (a_3(D) > b_4(D) ? 1 : -((int) (a_3(D) != b_4(D))))
we should transform it instead to:
  (a_3(D) < b_4(D) ? -1 : (int) (a_3(D) != b_4(D)))

The reference to gold's --icf option looks for me wrong here.  Such
optimizations - as done here for gold - can't be done by gcc.  At least not for
the given example provided.  In other TUs there always might users of direct
function-address in comparison.  So to find and reduce identical code-blocks is
just a linker feature (with some danger IMHO).

So back to this issue.  I would think it is more a problem to be solved on
gimple, and not that much a RTL issue in first hand.