Move VL=1 check to tryReduceVL

Author: lukel97

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -1143,16 +1143,6 @@ bool RISCVVLOptimizer::isCandidate(const MachineInstr &MI) const {
   if (MI.getNumDefs() != 1)
     return false;
 
-  unsigned VLOpNum = RISCVII::getVLOpNum(Desc);
-  const MachineOperand &VLOp = MI.getOperand(VLOpNum);
-
-  // If the VL is 1, then there is no need to reduce it. This is an
-  // optimization, not needed to preserve correctness.
-  if (VLOp.isImm() && VLOp.getImm() == 1) {
-    LLVM_DEBUG(dbgs() << "  Not a candidate because VL is already 1\n");
-    return false;
-  }
-
   if (MI.mayRaiseFPException()) {
     LLVM_DEBUG(dbgs() << "Not a candidate because may raise FP exception\n");
     return false;
@@ -1285,16 +1275,23 @@ std::optional<MachineOperand> RISCVVLOptimizer::checkUsers(MachineInstr &MI) {
 bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) {
   LLVM_DEBUG(dbgs() << "Trying to reduce VL for " << MI << "\n");
 
+  unsigned VLOpNum = RISCVII::getVLOpNum(MI.getDesc());
+  MachineOperand &VLOp = MI.getOperand(VLOpNum);
+
+  // If the VL is 1, then there is no need to reduce it. This is an
+  // optimization, not needed to preserve correctness.
+  if (VLOp.isImm() && VLOp.getImm() == 1) {
+    LLVM_DEBUG(dbgs() << "  Abort due to VL == 1, no point in reducing.\n");
+    return false;
+  }
+
   auto CommonVL = checkUsers(MI);
   if (!CommonVL)
     return false;
 
   assert((CommonVL->isImm() || CommonVL->getReg().isVirtual()) &&
          "Expected VL to be an Imm or virtual Reg");
 
-  unsigned VLOpNum = RISCVII::getVLOpNum(MI.getDesc());
-  MachineOperand &VLOp = MI.getOperand(VLOpNum);
-
   if (!RISCV::isVLKnownLE(*CommonVL, VLOp)) {
     LLVM_DEBUG(dbgs() << "    Abort due to CommonVL not <= VLOp.\n");
     return false;
@@ -1345,6 +1342,8 @@ bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
         continue;
 
       MachineInstr *DefMI = MRI->getVRegDef(Op.getReg());
+      if (!isCandidate(*DefMI))
+        continue;
 
       if (IgnoreSameBlock && DefMI->getParent() == MI.getParent())
         continue;
@@ -1376,8 +1375,7 @@ bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
   while (!Worklist.empty()) {
     assert(MadeChange);
     MachineInstr &MI = *Worklist.pop_back_val();
-    if (!isCandidate(MI))
-      continue;
+    assert(isCandidate(MI));
     if (!tryReduceVL(MI))
       continue;
     PushOperands(MI, /*IgnoreSameBlock*/ false);