[llvm] [RISCV][VLOPT] Compute demanded VLs up front. NFC (PR #124530)

[Luke Lau via llvm-commits]

================
@@ -1355,52 +1369,33 @@ bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
   if (!ST.hasVInstructions())
     return false;
 
-  SetVector<MachineInstr *> Worklist;
-  auto PushOperands = [this, &Worklist](MachineInstr &MI,
-                                        bool IgnoreSameBlock) {
-    for (auto &Op : MI.operands()) {
-      if (!Op.isReg() || !Op.isUse() || !Op.getReg().isVirtual() ||
-          !isVectorRegClass(Op.getReg(), MRI))
-        continue;
-
-      MachineInstr *DefMI = MRI->getVRegDef(Op.getReg());
-      if (!isCandidate(*DefMI))
-        continue;
-
-      if (IgnoreSameBlock && DefMI->getParent() == MI.getParent())
-        continue;
-
-      Worklist.insert(DefMI);
-    }
-  };
+  TII = ST.getInstrInfo();
 
-  // 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) {
     // Avoid unreachable blocks as they have degenerate dominance
     if (!MDT->isReachableFromEntry(&MBB))
       continue;
 
-    for (auto &MI : make_range(MBB.rbegin(), MBB.rend())) {
+    // For each instruction that defines a vector, compute what VL its
+    // downstream users demand.
+    for (const auto &MI : reverse(MBB)) {
+      if (!isCandidate(MI))
+        continue;
+      DemandedVLs.insert({&MI, computeDemandedVL(MI)});
----------------
lukel97 wrote:

Yes, because now we don't reduce the VL of anything until after we've analysed the demanded VLs for everything. 

computeDemandedVL will work out the minimum possible VL for everything up front, which previously would have required multiple iterations of tryReduceVL. It's able to propagate the VL without mutating anything because of the change to getMinimumVLForUser that peeks through the user's demandedVL.

https://github.com/llvm/llvm-project/pull/124530