[PATCH] D84108: [SimplifyCFG][LoopRotate] SimplifyCFG: disable common instruction hoisting by default, enable late in pipeline

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

Just a drive-by general comment...
Before we added SimplifyCFGOptions, there was a named "LateSimplifyCFG" pass that implicitly enabled more transforms. There was push back on llvm-dev about passes that use options to change behavior because that made it harder to test the pass managers.

Given the known problems with SimplifyCFG (it does poison-unsafe transforms, it does not reliably reach fix-point, it makes questionable use of the cost model, etc.), the best long-term solution would be to break it up into multiple passes - canonicalization-only early and potentially various optimization passes for later in the pipeline. It's a tough job because (as shown in the data here), that's probably going to uncover lots of other problems. If we now have a fairly reliable way to test that with llvm-compile-time-tracker and there's work underway to find/fuzz our way to better pass pipeline configs, then we should be able to finally untangle this knot.


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https://reviews.llvm.org/D84108