[Bug tree-optimization/54073] [4.7/4.8 Regression] SciMark Monte Carlo test performance has seriously decreased in recent GCC releases

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54073

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

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--- Comment #12 from Jan Hubicka <hubicka at gcc dot gnu.org> 2012-11-13 15:54:22 UTC ---
The decision on whether to use cmov or jmp was always tricky on x86
architectures. Cmov increase dependency chains, register pressure (both values
needs to be loaded in) and has long opcode. So jump sequence, if well
predicted, flows better through the out-of-order core. If badly predicted it
is, of course, a disaster. I think more modern CPUs solved the problems with
long latency of cmov, but the dependency chains are still there.

This patch fixes a bug in a pattern rather than tweaks heuristic on
predictability. As such I think it is OK for mainline. 

We should do something about rnflow. I will look into that.
The usual wisdom is that lacking profile feedback one should handle non-loop
branhces as inpredctable and loop branches as predictable, so all handled by
ifconvert fals to the first category. With profile feedback one can see branch
probability and if it is close to 0 or REG_BR_PROB_BASE tread the branch as
predictable. We handle this with predictable_edge_p parameter passed to
BRANCH_COST (that by itself is a gross, but for years we was not able to come
with something saner)

Honza