[PATCH] D128123: [SDAG] try to replace subtract-from-constant with xor

[Sanjay Patel via Phabricator via llvm-commits]

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This is almost the same as the abandoned D48529 <https://reviews.llvm.org/D48529>, but it allows splat vector constants too.
This replaces the x86-specific code that was added with the alternate patch D48557 <https://reviews.llvm.org/D48557> with the original generic combine. 
This transform is a less restricted form of an existing InstCombine and the proposed SDAG equivalent for that in D128080 <https://reviews.llvm.org/D128080>:
https://alive2.llvm.org/ce/z/OUm6N_


https://reviews.llvm.org/D128123

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/lib/Target/X86/X86InstrCompiler.td
  llvm/test/CodeGen/AArch64/sub1.ll
  llvm/test/CodeGen/AMDGPU/ds-sub-offset.ll
  llvm/test/CodeGen/AMDGPU/setcc-multiple-use.ll
  llvm/test/CodeGen/ARM/intrinsics-overflow.ll
  llvm/test/CodeGen/ARM/usub_sat.ll
  llvm/test/CodeGen/ARM/usub_sat_plus.ll
  llvm/test/CodeGen/PowerPC/bool-math.ll
  llvm/test/CodeGen/PowerPC/select_const.ll
  llvm/test/CodeGen/RISCV/atomic-rmw.ll
  llvm/test/CodeGen/RISCV/atomic-signext.ll
  llvm/test/CodeGen/SPARC/64bit.ll

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