[Bug target/34452] Multiply-by-constant pessimation

[tege-gcc at swox dot com]

------- Comment #2 from tege-gcc at swox dot com  2007-12-13 10:52 -------
It does make sense to bluff somewhat about the costs of the few 3
operand instructions that we have:

  lea
  mul  const, regx, regy

Exactly what cost to assign is not obvious.  I think the nominal
cost - epsilon is probably better than the nominal cost - 1 + epsilon.
The latter is what Honza does.

But teaching synth_mult about 2.5 operandness might be the best way to
make constant multiplication code come out optimal.  I'm not sure how
to do that, though.  Peeking at the constraints flags?  :-)

If I understand Honza right, he's trying to please reload more
than synth_mult with his tweak.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34452