[llvm] [VPlan] Extract reverse operation for reverse accesses (PR #146525)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Rebased and ping. There are currently two points that may require further discussion:
> 
>     1. When converting to vp.reverse, should we eagerly convert all reverse operations, or should we only convert those whose results are used by a vp.load or consumed by a vp.store?
>        The current implementation uses the latter (more conservative) strategy, but the former would be simpler.
> 

>From https://github.com/llvm/llvm-project/pull/146525#discussion_r2348512773, it sounds like converting them all would be simpler and also valid/correct, if so good to go with the simpler option to start with?

>     2. We currently do not convert reverse operations defined outside the loop region into vp.reverse.
>        Besides the fact that EVL is not available in that region, such reversed stored values should be uniform.
>        If we think this is unsafe, we could instead move that reverse operation back into the vector loop and still convert it to vp.reverse. Later, this can be cleaned up by a rule like
>        `Reverse(Splat(V)) --> Splat(V)`
>        in SimplifyRecipes to eliminate the unnecessary reverse.
 
EVL would be available in the middle block, but not blocks before the loop region. Do we have such cases? Can we verify? Any reverse hoisted to the loop should have a single-scalar operand, if so we the reverse can be removed independently I think


https://github.com/llvm/llvm-project/pull/146525