[PATCH] D41927: DAGCombine: Let truncates negate extension through extract-subvector

[Zvi Rackover via Phabricator via llvm-commits]

zvi created this revision.
zvi added reviewers: RKSimon, craig.topper, spatel, efriedma.

Fold cases such as:
(v8i8 truncate (v8i32 extract_subvector (v16i32 sext (v16i8 V), Idx)))
->
(v8i8 extract_subvector (v16i8 V), Idx)

This can be generalized to cases where the truncate and extend do not
fully cancel each other out, but it may require querying the target
about profitability.


https://reviews.llvm.org/D41927

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/X86/madd.ll
  test/CodeGen/X86/trunc-subvector.ll

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