[Bug rtl-optimization/33928] [4.3/4.4/4.5 Regression] 30% performance slowdown in floating-point code caused by r118475

[steven at gcc dot gnu dot org]

------- Comment #77 from steven at gcc dot gnu dot org  2009-05-07 17:50 -------
Re. comment #75: Just the fact that an option is enabled in both releases
doesn't mean the pass behind it is doing the same thing in both releases. What
the scheduler does, depends heavily on the code you feed it.  Sometimes it is
pure (good or bad) luck that changes  the behavior of a pass in the compiler. 
The interactions between all the pieces are just very complicated (which is
why, IMHO, retargetable-compiler engineering is so difficult: controlling the
pipeline is undoable).

Re. comment #76:
Sad as it may be, I think this is the best short-term solution.
Alternatively we could re-work fwprop to work on regions and use the
partial-CFG dataflow stuff, similar to what the RTL loop optimizers (like
loop-invariant) do.  To be honest, I'd much prefer the latter, but the
DIY-fwprop thing is probably easier in the short term.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33928