[PATCH] D94444: [RFC][Scalable] Add scalable shuffle intrinsic to extract evens from a pair of vectors

[Paul Walker via Phabricator via llvm-commits]

paulwalker-arm added a comment.

Considering:

  <A x Elt> llvm.experimental.vector.extract.elements(<B x Elt> %invec, i32 index, i32 stride)

Suitable restrictions are that "A = B/stride" and that "B % stride == 0".  When combined with the original restriction that "0 <= index <= stride", I think this nicely joins up our use case of having an intrinsic that effectively extracts field "index" from a vector of structs[1] where each struct has "stride" fields.

[1] invec is not actually a vector of structs at the LLVM level, but logically this is what the vector represents.

There is the option to have no restrictions and allow a use case like:

  <2 x Elt> llvm.experimental.vector.extract.elements(<vscale x 4 x Elt> %invec, i32 7, i32 13)

but I honestly think that's needlessly looking for trouble.  Besides, this extreme use case has the same interface and so once a restricted variant is available it would be easy enough for others to soften restrictions if they see a genuine reason to do so.  The important part is to ensure the intrinsic's return and parameter types are good enough to allow these future use cases, which I believe we've achieved in this instance.


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  https://reviews.llvm.org/D94444/new/

https://reviews.llvm.org/D94444