[Bug tree-optimization/23181] [4.1 Regression] Dominator opts slows down bresenham line drawing by roughly 20%

[hubicka at ucw dot cz]

------- Additional Comments From hubicka at ucw dot cz  2005-08-01 15:14 -------
Subject: Re:  [4.1 Regression] Dominator opts slows down bresenham line drawing by roughly 20%


One problem is  that DOM manages to convert

a += 1;
if (....)
   a += 10;

to

a1 = a + 1
  if (....)
    a1 = a + 11
a1 = a

hence the extra move even on 3-address architectures.  There are two
such additions done in parallel so we won't get cmove.
We either need to go the all way around and produce if-then-else and
save one instruction or avoid this transformation as it is dificult to
undo.

Honza


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23181