[Bug target/27855] reassociation pass produces ~30% slower matrix multiplication code

["pinskia at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #6 from pinskia at gcc dot gnu dot org  2006-06-03 02:38 -------
What reassociation is doing is "scheduling" the instructions further down
before the use but it exands the life time of some variables.

e.g.:
  D.1563_59 = rA0_49 * rB0_50;
  rC0_0_60 = D.1563_59 + rC0_0_516;
  D.1564_61 = rA1_52 * rB0_50;
  rC1_0_62 = D.1564_61 + rC1_0_517;
  D.1565_63 = rA2_54 * rB0_50;
  rC2_0_64 = D.1565_63 + rC2_0_518;
  D.1566_65 = rA3_56 * rB0_50;
  rC3_0_66 = D.1566_65 + rC3_0_519;
  D.1567_67 = rA4_58 * rB0_50;
  rC4_0_68 = D.1567_67 + rC4_0_520;

into:
  D.1563_59 = rB0_50 * rA0_49;
  D.1564_61 = rA1_52 * rB0_50;
  D.1565_63 = rA2_54 * rB0_50;
  D.1566_65 = rA3_56 * rB0_50;
  D.1567_67 = rA4_58 * rB0_50;
..... (with loads, etc here)
 D.1563_477 = rB0_468 * rA0_466;
  rC0_0_60 = D.1563_59 + rC0_0_516;
  rC0_0_82 = rC0_0_60 + D.1563_81;
  rC0_0_104 = rC0_0_82 + D.1563_103;
  rC0_0_126 = rC0_0_104 + D.1563_125;
  rC0_0_148 = rC0_0_126 + D.1563_147;
  rC0_0_170 = rC0_0_148 + D.1563_169;
  rC0_0_192 = rC0_0_170 + D.1563_191;
  rC0_0_214 = rC0_0_192 + D.1563_213;
  rC0_0_236 = rC0_0_214 + D.1563_235;
  rC0_0_258 = rC0_0_236 + D.1563_257;
  rC0_0_280 = rC0_0_258 + D.1563_279;
  rC0_0_302 = rC0_0_280 + D.1563_301;
  rC0_0_324 = rC0_0_302 + D.1563_323;
  rC0_0_346 = rC0_0_324 + D.1563_345;
  rC0_0_368 = rC0_0_346 + D.1563_367;
  rC0_0_390 = rC0_0_368 + D.1563_389;
  rC0_0_412 = rC0_0_390 + D.1563_411;
  rC0_0_434 = rC0_0_412 + D.1563_433;
  rC0_0_456 = rC0_0_434 + D.1563_455;
  rC0_0_478 = rC0_0_456 + D.1563_477;


Which in of itself not supressing and not nothing which reassociate should
handle special.  This is what we get with a semi bad register allocation which
does nothing to reduce spilling.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27855