tree f5d73b5c563f5547cb29b0f96d42c7cb77031e55
parent f2334a7ef255ffe906d864d7ba078aba54646abb
author Sanjay Patel <spatel@rotateright.com> 1585848089 -0400
committer Sanjay Patel <spatel@rotateright.com> 1585848622 -0400

[VectorCombine] transform bitcasted shuffle to narrower elements

bitcast (shuf V, MaskC) --> shuf (bitcast V), MaskC'

We do not attempt this in InstCombine because we do not want to change
types and create new shuffle ops that are potentially not lowered as
well as the original code. Here, we can check the cost model to see if
it is worthwhile.

I've aggressively enabled this transform even if the types are the same
size and/or equal cost because moving the bitcast allows InstCombine to
make further simplifications.

In the motivating cases from PR35454:
https://bugs.llvm.org/show_bug.cgi?id=35454
...this is enough to let instcombine and the backend eliminate the
redundant shuffles, but we probably want to extend VectorCombine to
handle the inverse pattern (shuffle-of-bitcast) to get that
simplification directly in IR.

Differential Revision: https://reviews.llvm.org/D76727