[GISel] Legalize G_FSUB to G_FADD + G_FNEG even if G_FNEG is illegal

`G_FNEG` used to be legalized to `G_FSUB -0, x` causing infinite loop.
This is no longer the case after D84287.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D148187

diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index feb355f..c61ed53 100644 (file)
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -3333,10 +3333,6 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT LowerHintTy) {
     LLT Ty = MRI.getType(Res);
 
     // Lower (G_FSUB LHS, RHS) to (G_FADD LHS, (G_FNEG RHS)).
-    // First, check if G_FNEG is marked as Lower. If so, we may
-    // end up with an infinite loop as G_FSUB is used to legalize G_FNEG.
-    if (LI.getAction({G_FNEG, {Ty}}).Action == Lower)
-      return UnableToLegalize;
     auto Neg = MIRBuilder.buildFNeg(Ty, RHS);
 
     if (MI.getOpcode() == TargetOpcode::G_STRICT_FSUB)