[LLVMbugs] [Bug 2577] Use static profile info from __builtin_expect in optimizer to improve code

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Sun Oct 16 23:22:24 PDT 2011


Chandler Carruth <chandlerc at gmail.com> changed:

           What    |Removed                     |Added
             Status|RESOLVED                    |REOPENED
                 CC|                            |chandlerc at gmail.com
         Resolution|FIXED                       |

--- Comment #4 from Chandler Carruth <chandlerc at gmail.com> 2011-10-17 01:22:23 CDT ---
This is not yet done... It's a bit hard to prove to yourself that it is not
done by running a command because we have few optimizations using the
information, but I've added a testing layer that demonstrates that in fact
metadata produced by the LLVM expect intrinsic does not get incorporated into
the branch probability analysis, or consequentially into the block frequency
analysis, or any optimizations we do have using them.

I'm mailing patches to cfe-commits which will bridge this gap in the
infrastructure, and I've already fixed a critical bug in branch probabilities
that would have led to turning this on producing exactly *backwards*
probabilities, pessimizing code instead of optimizing it. See r142168 for that
bit of fun.

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