[PATCH] D23897: [SelectionDAG] Generate vector_shuffle nodes for undersized result vector sizes

[Michael Kuperstein via llvm-commits]

mkuper created this revision.
mkuper added reviewers: spatel, RKSimon, craig.topper.
mkuper added a subscriber: llvm-commits.

Today, we can generate a vector_shuffle from an IR shuffle where the size of the result is exactly the sum of the sizes of the input vectors. If the output vector is smaller - e.g. a <12 x i8> being formed by a shuffle with two <8 x i8> inputs, we emit a sequence of extracts and inserts.

Instead, we can form a larger vector_shuffle, and then extract a subvector of the right size - e.g. shuffle the two <8 x i8>-s into a <16 x i8> and extract a <12 x i8>.

This solves PR29025.

This has a dependency on D23893 - forming a vector instead of a series of inserts/extracts pessimizes a test, because of a mess created by legalization, and we need D23893 for clean-up.

https://reviews.llvm.org/D23897

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  test/CodeGen/X86/pr29025.ll

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