[RISCV] Handle redundant SplitF64+BuildPairF64 pairs in a DAGCombine

r343712 performed this optimisation during instruction selection. As Eli 
Friedman pointed out in post-commit review, implementing this as a DAGCombine 
might allow opportunities for further optimisations.

llvm-svn: 343741

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 69569752183..550c9061eb4 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -494,6 +494,24 @@ SDValue RISCVTargetLowering::LowerRETURNADDR(SDValue Op,
   return DAG.getCopyFromReg(DAG.getEntryNode(), DL, Reg, XLenVT);
 }
 
+SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
+                                               DAGCombinerInfo &DCI) const {
+  switch (N->getOpcode()) {
+  default:
+    break;
+  case RISCVISD::SplitF64: {
+    // If the input to SplitF64 is just BuildPairF64 then the operation is
+    // redundant. Instead, use BuildPairF64's operands directly.
+    SDValue Op0 = N->getOperand(0);
+    if (Op0->getOpcode() != RISCVISD::BuildPairF64)
+      break;
+    return DCI.CombineTo(N, Op0.getOperand(0), Op0.getOperand(1));
+  }
+  }
+
+  return SDValue();
+}
+
 static MachineBasicBlock *emitSplitF64Pseudo(MachineInstr &MI,
                                              MachineBasicBlock *BB) {
   assert(MI.getOpcode() == RISCV::SplitF64Pseudo && "Unexpected instruction");