[Bug tree-optimization/38401] TreeSSA-PRE load after store missed optimization

["amylaar at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #24 from amylaar at gcc dot gnu dot org  2009-02-02 20:02 -------
(In reply to comment #22)
> If you post a patch to add the option to enable/disable partial-PRE I will
> happily review and approve it for 4.4.

I experimented using Seteven Bosscher's patch as a starting point and
augmenting the test in do_regular_insertion with a speed based heuristic
to throttle the calls to insert_into_preds_of_block.  That was worse than
turning off partial-PRE altogether.  Then I added the heuristic also in
do_partial_insertion, which worked better.  Then I tried to remove the speed
heuristoc from do_regular_insertion, and taht change only very tiny, although
overall beneficial, effects.

To get meaningful results we had to modify the linking a bit to reduce
instruction cache effects: the most needed libgcc function were pulled out
early and placed next to the core benchmark objects.

applying heuristic only to partial-partial vs. not applying it at all is...
automotive: 6.55389% faster
consumer:   0.00048% worse
networking: 0.03793% faster
office:     0.07269% worse
telecom:    0.00000% faster

applying heuristic only to partial-partial vs. applying it in general is...
automotive: 0.00674% faster
consumer:   0.00076% worse
networking: 0.01746% faster
office:     0.00440% worse
telecom:    0.00002% worse

Unfortunately, there is still no word from the FSF on what they did with our
Copyright Assignment.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38401