[PATCH] D94972: [SLP]Need to shrink the load vector after reordering.

[Alexey Bataev via Phabricator via llvm-commits]

ABataev added a comment.

In D94972#2507235 <https://reviews.llvm.org/D94972#2507235>, @echristo wrote:

> Is the summary correct? The patch that this is fixing isn't reverted...
>
> It's not clear what this is fixing and why it's fixing it.
>
> Thanks!
>
> -eric

Here is the reverting commit https://reviews.llvm.org/rGbcbdeafa9cb3469a7bf367c17f32d913076a4d55

Before D92668 <https://reviews.llvm.org/D92668>, we could reuse the argument of the shuffle as the instruction and in most cases, we did this. And it masked the issue that exists in the code for the case when there is no shuffle after vectorization of the node. After D92668 <https://reviews.llvm.org/D92668> we could not reuse the argument of the shuffle anymore and should resize the vectorized instructions always if required. But originally we did it incorrectly. If some of the scalars are duplicated in the vectorization tree entry, we do not vectorize them but instead generate a mask for the reuses. But if there are several users of the same entry, they may have different vectorization factors. This is especially important for PHI nodes. In this case, we need to adapt the resulting instruction for the user vectorization factor and have to reshuffle it again to take only unique elements of the vector. Before this patch, the compiler incorrectly returned reduced vector instruction with the same elements, not with the unique ones.

  block:
  %phi = phi <2 x > { .., %entry} {%shuffle, %block}
  %2 = shuffle <2 x > %phi, %poison, <4 x > <0, 0, 1, 1>
  ... (use %2)
  %shuffle = shuffle <2 x> %2, poison, <2 x> {0, 2} <----------------- Currently the compiler emits {0, 1} here but insteasd it should be {0, 2}, otherwise %2 instruction becomes incorrect after the first iteration
  br %block


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D94972/new/

https://reviews.llvm.org/D94972