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

[Philip Reames via llvm-commits]

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@@ -1336,29 +1342,20 @@ 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())
+  // For each instruction that defines a vector, compute what VL its
+  // downstream users demand.
+  for (MachineBasicBlock *MBB : post_order(&MF)) {
+    assert(MDT->isReachableFromEntry(MBB));
+    for (MachineInstr &MI : reverse(*MBB)) {
+      if (!isCandidate(MI))
         continue;
-
-      Worklist.insert(DefMI);
+      if (auto DemandedVL = checkUsers(MI))
----------------
preames wrote:

This can become an unconditional call to insert now that the value in the map is an optional.

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