[RISCV][VLOpt] Reorganize visit order and worklist management (#123973)

This implements a suggestion by Craig in PR #123878. We can move the
worklist management out of the per-instruction work and do it once at
the end of scanning all the instructions. This should reduce repeat
visitation of the same instruction when no changes can be made.

Note that this does not remove the inherent O(N^2) in the algorithm.
We're still potentially visiiting every user of every def.

I also included a guard for unreachable blocks since that had been
mentioned as a possible cause. It seems we've rulled that out, but
guarding for this case is still a good idea.

Author: preames

llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

@@ -1292,53 +1292,60 @@ std::optional<MachineOperand> RISCVVLOptimizer::checkUsers(MachineInstr &MI) {
   return CommonVL;
 }
 
-bool RISCVVLOptimizer::tryReduceVL(MachineInstr &OrigMI) {
-  SetVector<MachineInstr *> Worklist;
-  Worklist.insert(&OrigMI);
+bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) {
+  LLVM_DEBUG(dbgs() << "Trying to reduce VL for " << MI << "\n");
 
-  bool MadeChange = false;
-  while (!Worklist.empty()) {
-    MachineInstr &MI = *Worklist.pop_back_val();
-    LLVM_DEBUG(dbgs() << "Trying to reduce VL for " << MI << "\n");
+  if (!isVectorRegClass(MI.getOperand(0).getReg(), MRI))
+    return false;
 
-    if (!isVectorRegClass(MI.getOperand(0).getReg(), MRI))
-      continue;
+  auto CommonVL = checkUsers(MI);
+  if (!CommonVL)
+    return false;
 
-    auto CommonVL = checkUsers(MI);
-    if (!CommonVL)
-      continue;
+  assert((CommonVL->isImm() || CommonVL->getReg().isVirtual()) &&
+         "Expected VL to be an Imm or virtual Reg");
 
-    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);
 
-    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;
+  }
 
-    if (!RISCV::isVLKnownLE(*CommonVL, VLOp)) {
-      LLVM_DEBUG(dbgs() << "    Abort due to CommonVL not <= VLOp.\n");
-      continue;
-    }
+  if (CommonVL->isImm()) {
+    LLVM_DEBUG(dbgs() << "  Reduce VL from " << VLOp << " to "
+                      << CommonVL->getImm() << " for " << MI << "\n");
+    VLOp.ChangeToImmediate(CommonVL->getImm());
+    return true;
+  }
+  const MachineInstr *VLMI = MRI->getVRegDef(CommonVL->getReg());
+  if (!MDT->dominates(VLMI, &MI))
+    return false;
+  LLVM_DEBUG(
+      dbgs() << "  Reduce VL from " << VLOp << " to "
+             << printReg(CommonVL->getReg(), MRI->getTargetRegisterInfo())
+             << " for " << MI << "\n");
 
-    if (CommonVL->isImm()) {
-      LLVM_DEBUG(dbgs() << "  Reduce VL from " << VLOp << " to "
-                        << CommonVL->getImm() << " for " << MI << "\n");
-      VLOp.ChangeToImmediate(CommonVL->getImm());
-    } else {
-      const MachineInstr *VLMI = MRI->getVRegDef(CommonVL->getReg());
-      if (!MDT->dominates(VLMI, &MI))
-        continue;
-      LLVM_DEBUG(
-          dbgs() << "  Reduce VL from " << VLOp << " to "
-                 << printReg(CommonVL->getReg(), MRI->getTargetRegisterInfo())
-                 << " for " << MI << "\n");
+  // All our checks passed. We can reduce VL.
+  VLOp.ChangeToRegister(CommonVL->getReg(), false);
+  return true;
+}
 
-      // All our checks passed. We can reduce VL.
-      VLOp.ChangeToRegister(CommonVL->getReg(), false);
-    }
+bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
+  if (skipFunction(MF.getFunction()))
+    return false;
+
+  MRI = &MF.getRegInfo();
+  MDT = &getAnalysis<MachineDominatorTreeWrapperPass>().getDomTree();
 
-    MadeChange = true;
+  const RISCVSubtarget &ST = MF.getSubtarget<RISCVSubtarget>();
+  if (!ST.hasVInstructions())
+    return false;
 
-    // Now add all inputs to this instruction to the worklist.
+  SetVector<MachineInstr *> Worklist;
+  auto PushOperands = [this, &Worklist](MachineInstr &MI,
+                                        bool IgnoreSameBlock) {
     for (auto &Op : MI.operands()) {
       if (!Op.isReg() || !Op.isUse() || !Op.getReg().isVirtual())
         continue;
@@ -1351,34 +1358,40 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &OrigMI) {
       if (!isCandidate(*DefMI))
         continue;
 
+      if (IgnoreSameBlock && DefMI->getParent() == MI.getParent())
+        continue;
+
       Worklist.insert(DefMI);
     }
-  }
-
-  return MadeChange;
-}
-
-bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
-  if (skipFunction(MF.getFunction()))
-    return false;
-
-  MRI = &MF.getRegInfo();
-  MDT = &getAnalysis<MachineDominatorTreeWrapperPass>().getDomTree();
-
-  const RISCVSubtarget &ST = MF.getSubtarget<RISCVSubtarget>();
-  if (!ST.hasVInstructions())
-    return false;
+  };
 
+  // Do a first pass eagerly rewriting in roughly reverse instruction
+  // order, populate the worklist with any instructions we might need to
+  // revisit.  We avoid adding definitions to the worklist if they're
+  // in the same block - we're about to visit them anyways.
   bool MadeChange = false;
   for (MachineBasicBlock &MBB : MF) {
-    // Visit instructions in reverse order.
+    // Avoid unreachable blocks as they have degenerate dominance
+    if (!MDT->isReachableFromEntry(&MBB))
+      continue;
+
     for (auto &MI : make_range(MBB.rbegin(), MBB.rend())) {
       if (!isCandidate(MI))
         continue;
-
-      MadeChange |= tryReduceVL(MI);
+      if (!tryReduceVL(MI))
+        continue;
+      MadeChange = true;
+      PushOperands(MI, /*IgnoreSameBlock*/ true);
     }
   }
 
+  while (!Worklist.empty()) {
+    assert(MadeChange);
+    MachineInstr &MI = *Worklist.pop_back_val();
+    if (!tryReduceVL(MI))
+      continue;
+    PushOperands(MI, /*IgnoreSameBlock*/ false);
+  }
+
   return MadeChange;
 }