[PATCH] D136442: [GISel] Add (fsub +-0.0, X) -> fneg combine

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


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Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:5251
+  const auto LHSCst = getFConstantVRegValWithLookThrough(LHS, MRI);
+  return LHSCst && LHSCst->Value.isZero();
+}
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Doing this for -0 is only valid with nsz: [[ https://alive2.llvm.org/ce/z/SK2Bs9 ]]


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Comment at: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp:5256
+  Builder.setInstrAndDebugLoc(MI);
+  Builder.buildFNeg(MI.getOperand(0).getReg(), MatchInfo);
+  eraseInst(MI);
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This changes a canonicalizing operation to a non-canonicalizing operation. You need to introduce a canonicalize unless you know denormals aren't flushed and the source isn't a signaling nan. InstCombine currently has a bug where it ignores this.

The DAG also has getNegatedExpression, which may be overkill at this moment


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136442/new/

https://reviews.llvm.org/D136442