[llvm] [VPlan] Speed up VPSlotTracker by using ModuleSlotTracker (PR #139881)

[Florian Hahn via llvm-commits]

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@@ -382,14 +383,25 @@ class VPSlotTracker {
   /// Number to assign to the next VPValue without underlying value.
   unsigned NextSlot = 0;
 
+  /// Cache slot indexes to avoid recomputing them on each printAsOperand call.
+  std::unique_ptr<ModuleSlotTracker> MST;
+
   void assignName(const VPValue *V);
   void assignNames(const VPlan &Plan);
   void assignNames(const VPBasicBlock *VPBB);
 
 public:
   VPSlotTracker(const VPlan *Plan = nullptr) {
-    if (Plan)
+    if (Plan) {
+      // This check is required to support unit tests with incomplete IR.
+      if (Function *F =
+              Plan->getScalarHeader()->getIRBasicBlock()->getParent()) {
+        Module *M = F->getParent();
+        MST = std::make_unique<ModuleSlotTracker>(M);
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fhahn wrote:

IIUC this may slow down the case where numbering the function isn't neccessary, e.g. because all values are named or we only have VPValues w/o underlying IR. Could we delay `incorporateFunction` until it is definitely needed, i.e. printing an unnamed IR value? 

Then we should get the best of both worlds, because otherwise printAsOperand would call `incorporateFunction` each time.

https://github.com/llvm/llvm-project/pull/139881