[PATCH] D113291: [AggressiveInstCombine] Lower Table Based CTTZ and enable it for AARCH64 in -O3

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

Hello - We were having a discussion about a very similar patch in D125755 <https://reviews.llvm.org/D125755>. I think the outcome for this patch is that either:

- We need to do this later (maybe in CodeGenPrepare).
- We need to do this unconditionally without the call to TTI.preferCTTZLowering() and have the reverse transform later for targets that do not have a cheaper alternative.
- We need to argue some more :)

There are more details about why in D125755 <https://reviews.llvm.org/D125755>. I would go for the first option if it doesn't lead to worse performance, as for the second I'm not sure when it would be profitable to transform back and emit the table. You may not want to do that for non-hot ctzs? It sounds like it may be difficult to get right, but maybe I'm overestimating it.


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