forked from OSchip/llvm-project
48ecba350e
This moves SinkIntoLoop from MachineLICM to MachineSink. The motivation for this work is that hoisting is a canonicalisation transformation, but we do not really have a good story to sink instructions back if that is better, e.g. to reduce live-ranges, register pressure and spilling. This has been discussed a few times on the list, the latest thread is: https://lists.llvm.org/pipermail/llvm-dev/2020-December/147184.html There it was pointed out that we have the LoopSink IR pass, but that works on IR, lacks register pressure informatiom, and is focused on profile guided optimisations, and then we have MachineLICM and MachineSink that both perform sinking. MachineLICM is more about hoisting and CSE'ing of hoisted instructions. It also contained a very incomplete and disabled-by-default SinkIntoLoop feature, which we now move to MachineSink. Getting loop-sinking to do something useful is going to be at least a 3-step approach: 1) This is just moving the code and is almost a NFC, but contains a bug fix. This uses helper function `isLoopInvariant` that was factored out in D94082 and added to MachineLoop. 2) A first functional change to make loop-sink a little bit less restrictive, which it really is at the moment, is the change in D94308. This lets it do more (alias) analysis using functions in MachineSink, making it a bit more powerful. Nothing changes much: still off by default. But it shows that MachineSink is a better home for this, and it starts using its functionality like `hasStoreBetween`, and in the next step we can use `isProfitableToSinkTo`. 3) This is the going to be he interesting step: decision making when and how many instructions to sink. This will be driven by the register pressure, and deciding if reducing live-ranges and loop sinking will help in better performance. 4) Once we are happy with 3), this should be enabled by default, that should be the end goal of this exercise. Differential Revision: https://reviews.llvm.org/D93694 |
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