[PATCH] D93818: [LangRef] Update shufflevector's semantics to return poison if the mask is undef

Mon Jun 7 22:24:06 PDT 2021

aqjune added a comment.

Hi all,

Before diving into further updates in InstCombine (D94380 <https://reviews.llvm.org/D94380> and D93817 <https://reviews.llvm.org/D93817>), what about pinning down this LangRef update first & letting people know via llvm-dev mail?

(Since this patch has been a while, D93586 <https://reviews.llvm.org/D93586>'s description will be helpful to remind the context)

Since we have `poison` constant now, there is no big obstacle for making this semantic change.
I'd like to share the status & semantic change at llvm-dev so people don't accidentally introduce `undef` placeholders anymore.

The remaining updates at D94380 <https://reviews.llvm.org/D94380> and D93817 <https://reviews.llvm.org/D93817> are about non-syntactic changes; before them, many syntactic updates to make the placeholder `poison` have been done so far (D9381 <https://reviews.llvm.org/D9381> has links to such patches).
Actually, the two patches are still correct after this LangRef change; they're for recovering performance after `poison` is used as a don't-care vector.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D93818/new/

https://reviews.llvm.org/D93818