[llvm] [nfc][ctx_prof] Efficient profile traversal and update (PR #110052)

[Mircea Trofin via llvm-commits]

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@@ -279,16 +278,44 @@ static void preorderVisit(ProfilesTy &Profiles,
     Traverser(P);
 }
 
-void PGOContextualProfile::update(Visitor V, const Function *F) {
-  GlobalValue::GUID G = F ? getDefinedFunctionGUID(*F) : 0U;
+void PGOContextualProfile::initIndex() {
+  // Initialize the head of the index list for each function. We don't need it
+  // after this point.
+  DenseMap<GlobalValue::GUID, PGOCtxProfContext *> InsertionPoints;
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mtrofin wrote:

`InsertionPoints` is for the end of the map, where we insert, `FuncInfo` holds the beginning. We don't care about inserting later. DenseMap is a good fit here, it's just a temporary tool, no need for iterator stability.

https://github.com/llvm/llvm-project/pull/110052