[PATCH] D121354: [SLP] Fix lookahead operand reordering for splat loads.

[Vasileios Porpodas via Phabricator via llvm-commits]

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Splat loads are inexpensive in X86. For a 2-lane vector we need just one
instruction: `movddup (%reg), xmm0`. Using the standard Splat score leads
to worse code. This patch adds a new score dedicated for splat loads.

Please note that a splat is usually three IR instructions:

- It is usually a load and 2 inserts: %ld = load double, double* %gep %ins1 = insertelement <2 x double> poison, double %ld, i32 0 %ins2 = insertelement <2 x double> %ins1, double %ld, i32 1

- But it can also be a load, an insert and a shuffle: %ld = load double, double* %gep %ins = insertelement <2 x double> poison, double %ld, i32 0 %shf = shufflevector <2 x double> %ins, <2 x double> poison, <2 x i32> zeroinitializer

Because of this some of the lit tests contain more IR instructions.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121354

Files:
  llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
  llvm/test/Transforms/SLPVectorizer/X86/lookahead.ll
  llvm/test/Transforms/SLPVectorizer/X86/operandorder.ll

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