Initialize MST lazily

Author: igogo-x86

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1441,10 +1441,23 @@ void VPSlotTracker::assignName(const VPValue *V) {
   std::string Name;
   if (UV) {
     raw_string_ostream S(Name);
-    if (MST)
+    if (MST) {
       UV->printAsOperand(S, false, *MST);
-    else
+    } else if (isa<Instruction>(UV) && !UV->hasName()) {
+      // Lazily create the ModuleSlotTracker when we first hit an unnamed
+      // instruction
+      auto *IUV = cast<Instruction>(UV);
+      // This check is required to support unit tests with incomplete IR.
+      if (IUV->getParent()) {
+        MST = std::make_unique<ModuleSlotTracker>(IUV->getModule());
+        MST->incorporateFunction(*IUV->getFunction());
+      } else {
+        MST = std::make_unique<ModuleSlotTracker>(nullptr);
+      }
+      UV->printAsOperand(S, false, *MST);
+    } else {
       UV->printAsOperand(S, false);
+    }
   } else
     Name = VPI->getName();
 

llvm/lib/Transforms/Vectorize/VPlanHelpers.h

@@ -383,7 +383,8 @@ 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.
+  /// Lazily created ModuleSlotTracker, used only when unnamed IR instructions
+  /// require slot tracking.
   std::unique_ptr<ModuleSlotTracker> MST;
 
   void assignName(const VPValue *V);
@@ -392,16 +393,8 @@ class VPSlotTracker {
 
 public:
   VPSlotTracker(const VPlan *Plan = nullptr) {
-    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);
-        MST->incorporateFunction(*F);
-      }
+    if (Plan)
       assignNames(*Plan);
-    }
   }
 
   /// Returns the name assigned to \p V, if there is one, otherwise try to