[PATCH] D141778: [DAGCombiner][X86] `mergeConsecutiveStores()`: support merging splat-stores of the same value

[Roman Lebedev via Phabricator via llvm-commits]

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`getStoreMergeCandidates()` is quite obious for this kind
of source - just look for identical store values.
`tryStoreMergeOfConstants()` basically already supports it,
we just need to bypass some checks that are constant-specific.

For X86, this does appear to be always better,
as long as we store at least a single full vector register.
But the test changes for other targets were more questionable,
so i didn't enable it elsewhere.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141778

Files:
  llvm/include/llvm/CodeGen/TargetLowering.h
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/lib/Target/X86/X86ISelLowering.h
  llvm/test/CodeGen/X86/MergeConsecutiveStores.ll
  llvm/test/CodeGen/X86/elementwise-store-of-scalar-splat.ll
  llvm/test/CodeGen/X86/legalize-shl-vec.ll
  llvm/test/CodeGen/X86/wide-scalar-shift-by-byte-multiple-legalization.ll

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