[PATCH] D75689: [VectorCombine] fold extract-extract-op with different extraction indexes

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D75689#1909516 <https://reviews.llvm.org/D75689#1909516>, @RKSimon wrote:

> LGTM - You might want to add some test cases with 256/512-bit vectors (with indices coming from the same/different 128-bit subvectors) as well to get a broader range of extraction costs.


Yes, will add some larger vector tests.

> I'm not certain a SK_Splat will make much difference - the shuffle masks with a single non-undef index would be in most cases a lot cheaper even than that, especially on early SSE targets, so maybe we need to either consider another shuffle enum type or just provide the ability to compute costs from raw shuffle masks?

There's no clear answer on how far we should take the modeling. We've said in the past that we shouldn't try that hard -- because this is IR, we can never know exactly how it will be lowered for every target.
The idea of trying different options and picking a winner sounds like VPlan to me (I haven't kept up with its current state, so need to review that.)


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