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

[Kazu Hirata via llvm-commits]

================
@@ -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;
+  for (auto &FI : FuncInfo)
+    InsertionPoints[FI.first] = &FI.second.Index;
   preorderVisit<PGOCtxProfContext::CallTargetMapTy, PGOCtxProfContext>(
-      *Profiles, V, G);
+      *Profiles, [&](PGOCtxProfContext &Ctx) {
+        auto InsertIt = InsertionPoints.find(Ctx.guid());
+        if (InsertIt == InsertionPoints.end())
+          return;
+        // Insert at the end of the list. Since we traverse in preorder, it
+        // means that when we iterate the list from the beginning, we'd
+        // encounter the contexts in the order we would have, should we have
+        // performed a full preorder traversal.
+        InsertIt->second->Next = &Ctx;
+        Ctx.Previous = InsertIt->second;
+        InsertIt->second = &Ctx;
+      });
+}
+
+void PGOContextualProfile::update(Visitor V, const Function &F) {
+  assert(isFunctionKnown(F));
+  GlobalValue::GUID G = getDefinedFunctionGUID(F);
+  for (auto *Node = FuncInfo.find(G)->second.Index.Next; Node;
+       Node = Node->Next)
+    V(*Node);
 }
 
 void PGOContextualProfile::visit(ConstVisitor V, const Function *F) const {
-  GlobalValue::GUID G = F ? getDefinedFunctionGUID(*F) : 0U;
-  preorderVisit<const PGOCtxProfContext::CallTargetMapTy,
-                const PGOCtxProfContext>(*Profiles, V, G);
+  if (!F)
+    preorderVisit<const PGOCtxProfContext::CallTargetMapTy,
+                  const PGOCtxProfContext>(*Profiles, V);
----------------
kazutakahirata wrote:

Do you mean to return when `F == nullptr`?  Otherwise, `*F` below would segfault.  I'm guessing that you either traverse the whole tree or visit those nodes that correspond to `F`, but not both.

```suggestion
  if (!F) {
    preorderVisit<const PGOCtxProfContext::CallTargetMapTy,
                  const PGOCtxProfContext>(*Profiles, V);
    return;
  }
```


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