[PATCH] D118066: [SimplifyCFG] Don't speculatively execute preductably-taken block

[Sotiris Apostolakis via Phabricator via llvm-commits]

apostolakis added a comment.

I agree with nikic and ebrevnov in that it is better to avoid converting more selects to branches in SimplifyCFG since they might block other IR-level optimizations.

In general, I think we need to do the inverse. Flatten the CFG more aggressively, allow a simpler control flow that will facilitate IR-level optimizations and then at the end of middle-end make the decision on whether to convert the selects to branches.
I have a RFC on cmov/branch decision-making <https://llvm.discourse.group/t/rfc-cmov-vs-branch-optimization/6040> that proposes extending the logic on CodeGenPrepare or having a pass just before it (essentially at the end of middle-end) . 
In the current proposal, I did not change SimplifyCFG but from some preliminary experiments, preventing SimplifyCFG from making such decisions and deferring this for later further improves performance.  
Besides, isn't SimplifyCFG supposed to be a canonicalization pass that enables others, rather than an optimization pass.


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