[PATCH] D93043: [CostModel] Add costs for llvm.experimental.vector.{extract,insert} intrinsics

[Bradley Smith via Phabricator via llvm-commits]

bsmith added a comment.

In D93043#2448152 <https://reviews.llvm.org/D93043#2448152>, @joechrisellis wrote:

> Hi @bsmith, thank you for the patch! 😄
>
> I noticed `b3e77c6d55853eea5f5c32ec8a3510c0b0e438e1` earlier:
>
>   commit b3e77c6d55853eea5f5c32ec8a3510c0b0e438e1
>   Author: Christopher Tetreault <ctetreau at quicinc.com>
>   Date:   Tue Jun 16 14:05:21 2020 -0700
>   
>       [SVE] Remove invalid calls to VectorType::getNumElements from BasicTTIImpl
>   
>       <snip>
>       If a developer makes some change in the future that causes
>       one of these asserts to fire, they should either change their code or
>       make the function they are trying to call handle scalable vectors.
>       <snip>
>
> This patch changes the interface of both `get{Extract,Insert}SubvectorOverhead` (among others) to accepting `FixedVectorType`s instead of `VectorType`s, so this patch is partially reverting some of the changes there. Not sure if this is pertinent here, but perhaps it's useful for context.

Agreed, I see this patch as doing as the highlighted part of the commit message indicates, hence not really a reversion as such.



================
Comment at: llvm/include/llvm/CodeGen/BasicTTIImpl.h:118
+  unsigned getExtractSubvectorOverhead(VectorType *VTy, int Index,
                                        FixedVectorType *SubVTy) {
     assert(VTy && SubVTy &&
----------------
joechrisellis wrote:
> `llvm.experimental.vector.extract` can also extract a scalable from a scalable -- should we also change this `FixedVectorType` to `VectorType` too?
> 
> I suppose this cost model breaks down a bit when we're extracting/inserting a scalable into a scalable, since we don't know how many elements we have to 'work on'. But I think that it might be possible for this code to crash in the same way as before if we don't address this case.
I think currently we don't generate these intrinsics as extracting or inserting scalable vectors into another scalable vector, and considering adding costs for that is much more complex it should be a different patch, if we do end up adding it.

That said, I agree that this case shouldn't crash, which it does currently. I will make those cases fall back to `BaseT::getIntrinsicInstrCost(ICA, CostKind)` as the other scalable vector cases do, as well as adding tests.


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