[PATCH] D22859: [X86][SSE] Enable target shuffle combining to combine multiple shuffle inputs.

[Simon Pilgrim via llvm-commits]

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We currently only support combining target shuffles that consist of a single source input (plus elements known to be undef/zero).

This patch generalizes the recursive combining of the target shuffle to collect all the inputs, merging any duplicates along the way, into a full set of src ops and its shuffle mask.

We uncover a number of cases where we have failed to combine a unary shuffle because the input has been duplicated and separated during lowering.

This will allow us to combine to 2-input shuffles in a future patch.

Repository:
  rL LLVM

https://reviews.llvm.org/D22859

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/avx512-build-vector.ll
  test/CodeGen/X86/clear_upper_vector_element_bits.ll
  test/CodeGen/X86/vector-shuffle-256-v16.ll
  test/CodeGen/X86/vector-shuffle-256-v32.ll
  test/CodeGen/X86/vector-shuffle-combining.ll

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