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

[Michael Maitland 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)});
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michaelmaitland wrote:

Does the demanded VL of an earlier instruction always stay the same, even if we will eventually optimize the VL of a later (user) instruction?

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