[PATCH] D109149: [DAG] Fold select_cc setgt X, -1, C, ~C -> xor (ashr X, BW-1), C

[Dave Green via Phabricator via llvm-commits]

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Given a select_cc producing a constant and a negation of the constant for a comparison more than zero, we can produce an xor with ashr instead, which produces smaller code. The ashr either sets all bits or clear all bits depending on if the value is negative. This is then xor'd with the constant to optionally negate the value.
https://alive2.llvm.org/ce/z/DTFaBZ

To prevent some AMDGPU regressions, a fold for `select_cc seteq (ashr X, BW-1), -1, A, B` -> `select_cc setlt X, 0, A, B` was also added, which could be produced from xor -1 not instructions.


https://reviews.llvm.org/D109149

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/test/CodeGen/AArch64/select-constant-xor.ll
  llvm/test/CodeGen/AMDGPU/fp_to_sint.ll
  llvm/test/CodeGen/AMDGPU/fp_to_uint.ll
  llvm/test/CodeGen/AMDGPU/select-constant-xor.ll
  llvm/test/CodeGen/AMDGPU/selectcc-icmp-select-float.ll
  llvm/test/CodeGen/ARM/select-constant-xor.ll
  llvm/test/CodeGen/PowerPC/select-constant-xor.ll
  llvm/test/CodeGen/PowerPC/smulfixsat.ll
  llvm/test/CodeGen/PowerPC/testComparesi32ltu.ll
  llvm/test/CodeGen/RISCV/select-constant-xor.ll
  llvm/test/CodeGen/X86/pr16031.ll
  llvm/test/CodeGen/X86/select-constant-xor.ll
  llvm/test/CodeGen/X86/smul_fix_sat.ll
  llvm/test/CodeGen/X86/sshl_sat.ll

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