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Subject: [Bug target/53346] [4.6/4.7/4.8 Regression] Bad vectorization in the proc cptrf2 of rnflow.f90
Date: Wed, 14 Nov 2012 22:19:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53346

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

           What    |Removed                     |Added
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                 CC|                            |hubicka at gcc dot gnu.org

--- Comment #21 from Jan Hubicka <hubicka at gcc dot gnu.org> 2012-11-14 22:18:53 UTC ---
Well, as I wrote to the other PR, the main problem of cmov is extension of
dependency chain.  For well predicted sequence with conditional jump there is
no update of rbs so the loop executes faster, because the
loads/stores/comparisons executes "in parallel". The load in the next iteration
can then happen speculatively before the condition from previous iteration is
resolved. With cmov in it, there is dependence on rbx for all the other
computations in the loop.

I guess there is no localy available information suggesting suggesting that the
particular branch is well predictable, at least without profile feedback (where
we won't disable the conversion anyway).

I wonder
 1) why the conversion to cmov do not happen on RTL if conversion pass
 2) whether we can do something to detect similar patterns and possibly disable
cmovs on them...