[GlobalISel] LegalizerHelper: Lower (G_FSUB X, Y) to (G_FADD X, (G_FNEG Y))

Summary: No test case as none of the in-tree targets with GlobalISel support has this condition.

Reviewers: qcolombet, aditya_nandakumar, dsanders, t.p.northover, ab

Reviewed By: qcolombet

Subscribers: dberris, rovka, kristof.beyls, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D30786

llvm-svn: 297512

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index bab92e25a5b..af4747f7f7f 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -570,6 +570,24 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
     MI.eraseFromParent();
     return Legalized;
   }
+  case TargetOpcode::G_FSUB: {
+    // 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}).first == LegalizerInfo::Lower)
+      return UnableToLegalize;
+    unsigned Res = MI.getOperand(0).getReg();
+    unsigned LHS = MI.getOperand(1).getReg();
+    unsigned RHS = MI.getOperand(2).getReg();
+    unsigned Neg = MRI.createGenericVirtualRegister(Ty);
+    MIRBuilder.buildInstr(TargetOpcode::G_FNEG).addDef(Neg).addUse(RHS);
+    MIRBuilder.buildInstr(TargetOpcode::G_FADD)
+        .addDef(Res)
+        .addUse(LHS)
+        .addUse(Neg);
+    MI.eraseFromParent();
+    return Legalized;
+  }
   }
 }