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Subject: [Bug middle-end/40106] [4.4/4.5 Regression] Time increase for the Polyhedron test air.f90 due to bad optimization
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From: "rguenth at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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------- Comment #33 from rguenth at gcc dot gnu dot org  2009-10-18 13:22 -------
It looks like basic-block frequencies are completely off.  The BB in question
is

  # BLOCK 7 freq:3
  # PRED: 6 [100.0%]  (fallthru,exec) 7 [99.0%]  (false,exec)
  # ivtmp.65_38 = PHI <ivtmp.65_113(6), ivtmp.65_129(7)>
  # ivtmp.68_147 = PHI <ivtmp.68_151(6), ivtmp.68_148(7)>
  D.1360_26 = MEM[index: ivtmp.65_38];
  D.1404_30 = pow (D.1360_26, 1.5e+0);
  MEM[index: ivtmp.68_147] = D.1404_30;
  ivtmp.65_129 = ivtmp.65_38 + 1200;
  ivtmp.68_148 = ivtmp.68_147 + 1200;
  if (ivtmp.77_32 == ivtmp.65_129)
    goto <bb 8>;
  else
    goto <bb 7>;
  # SUCC: 8 [1.0%]  (true,exec) 7 [99.0%]  (false,exec)

And 3 is lower than 11, the minimum frequency a BB is considered not cold.

Predictions for bb 7
  DS theory heuristics (ignored): 0.1%
  first match heuristics: 1.0%
  combined heuristics: 1.0%
  opcode values nonequal (on trees) heuristics (ignored): 28.0%
  loop branch heuristics (ignored): 14.0%
  guessed loop iterations heuristics: 1.0%


but I see most blocks do not have a frequency at all and I also see

  # BLOCK 17 freq:10000
  # PRED: 16 [100.0%]  (fallthru,exec) 17 [99.0%]  (false,exec)
  # ivtmp.16_116 = PHI <ivtmp.16_125(16), ivtmp.16_115(17)>
  MEM[index: ivtmp.16_116] = dtd_56(D);
  ivtmp.16_115 = ivtmp.16_116 + 1200;
  if (ivtmp.27_12 == ivtmp.16_115)
    goto <bb 18>;
  else
    goto <bb 17>;
  # SUCC: 18 [1.0%]  (true,exec) 17 [99.0%]  (false,exec)

which is the block with the highest frequency (the innermost loop of the
2nd nest).

I can imagine that with a lot of inlining and exposing very deep nested
loops alongside very hot not-so-deep loops can cause them to become
artificially cold.

Interestingly the outermost loop blocks do not have any frequency
assigned (that probably means zero).


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-10-18 13:22:22
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40106