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

This optimizes profile updates and visits, where we want to access contexts for a specific function. These are all the current update cases. We do so by maintaining a list of contexts for each function, preserving preorder traversal. The list is updated whenever contexts are `std::move`-d or deleted.

Author: mtrofin

llvm/include/llvm/Analysis/CtxProfAnalysis.h

@@ -35,7 +35,7 @@ class PGOContextualProfile {
     uint32_t NextCounterIndex = 0;
     uint32_t NextCallsiteIndex = 0;
     const std::string Name;
-
+    PGOCtxProfContext Index;
     FunctionInfo(StringRef Name) : Name(Name) {}
   };
   std::optional<PGOCtxProfContext::CallTargetMapTy> Profiles;
@@ -50,6 +50,8 @@ class PGOContextualProfile {
   // its state piecemeal.
   PGOContextualProfile() = default;
 
+  void initIndex();
+
 public:
   PGOContextualProfile(const PGOContextualProfile &) = delete;
   PGOContextualProfile(PGOContextualProfile &&) = default;
@@ -94,7 +96,7 @@ class PGOContextualProfile {
   using ConstVisitor = function_ref<void(const PGOCtxProfContext &)>;
   using Visitor = function_ref<void(PGOCtxProfContext &)>;
 
-  void update(Visitor, const Function *F = nullptr);
+  void update(Visitor, const Function &F);
   void visit(ConstVisitor, const Function *F = nullptr) const;
 
   const CtxProfFlatProfile flatten() const;

llvm/include/llvm/ProfileData/PGOCtxProfReader.h

@@ -22,20 +22,74 @@
 #include <map>
 
 namespace llvm {
+class PGOContextualProfile;
+class PGOCtxProfContext;
+
+namespace internal {
+// When we traverse the contextual profile, we typically want to visit contexts
+// pertaining to a specific function. To avoid traversing the whole tree, we
+// want to keep a per-function list - which will be in preorder - of that
+// function's contexts. This happens in PGOContextualProfile. For memory use
+// efficiency, we want to make PGOCtxProfContext an intrusive double-linked list
+// node. We need to handle the cases where PGOCtxProfContext nodes are moved and
+// deleted: in both cases, we need to update the index (==list). We can do that
+// directly from the node in the list, without knowing who the "parent" of the
+// list is. That makes the ADT ilist overkill here. Finally, IndexNode is meant
+// to be an implementation detail of PGOCtxProfContext, and the only reason it's
+// factored out is to avoid implementing move semantics for all its members.
+class IndexNode {
+  // This class' members are intentionally private - it's a convenience
+  // implementation detail.
+  friend class ::llvm::PGOCtxProfContext;
+  friend class ::llvm::PGOContextualProfile;
+
+  IndexNode *Previous = nullptr;
+  IndexNode *Next = nullptr;
+
+  ~IndexNode() {
+    if (Next)
+      Next->Previous = Previous;
+    if (Previous)
+      Previous->Next = Next;
+  }
+
+  IndexNode(const IndexNode &Other) = delete;
+
+  IndexNode(IndexNode &&Other) {
+    // Copy the neighbor info
+    Next = Other.Next;
+    Previous = Other.Previous;
+
+    // Update the neighbors to point to this object
+    if (Other.Next)
+      Other.Next->Previous = this;
+    if (Other.Previous)
+      Other.Previous->Next = this;
+
+    // Make sure the dtor is a noop
+    Other.Next = nullptr;
+    Other.Previous = nullptr;
+  }
+  IndexNode() = default;
+};
+} // namespace internal
+
 /// A node (context) in the loaded contextual profile, suitable for mutation
 /// during IPO passes. We generally expect a fraction of counters and
 /// callsites to be populated. We continue to model counters as vectors, but
 /// callsites are modeled as a map of a map. The expectation is that, typically,
 /// there is a small number of indirect targets (usually, 1 for direct calls);
 /// but potentially a large number of callsites, and, as inlining progresses,
 /// the callsite count of a caller will grow.
-class PGOCtxProfContext final {
+class PGOCtxProfContext final : public internal::IndexNode {
 public:
   using CallTargetMapTy = std::map<GlobalValue::GUID, PGOCtxProfContext>;
   using CallsiteMapTy = std::map<uint32_t, CallTargetMapTy>;
 
 private:
   friend class PGOCtxProfileReader;
+  friend class PGOContextualProfile;
+
   GlobalValue::GUID GUID = 0;
   SmallVector<uint64_t, 16> Counters;
   CallsiteMapTy Callsites;
@@ -47,11 +101,15 @@ class PGOCtxProfContext final {
   getOrEmplace(uint32_t Index, GlobalValue::GUID G,
                SmallVectorImpl<uint64_t> &&Counters);
 
+  // Create a bogus context object, used for anchoring the index double linked
+  // list - see IndexNode
+  PGOCtxProfContext() = default;
+
 public:
   PGOCtxProfContext(const PGOCtxProfContext &) = delete;
   PGOCtxProfContext &operator=(const PGOCtxProfContext &) = delete;
   PGOCtxProfContext(PGOCtxProfContext &&) = default;
-  PGOCtxProfContext &operator=(PGOCtxProfContext &&) = default;
+  PGOCtxProfContext &operator=(PGOCtxProfContext &&) = delete;
 
   GlobalValue::GUID guid() const { return GUID; }
   const SmallVectorImpl<uint64_t> &counters() const { return Counters; }

llvm/lib/Analysis/CtxProfAnalysis.cpp

@@ -184,6 +184,7 @@ PGOContextualProfile CtxProfAnalysis::run(Module &M,
   // If we made it this far, the Result is valid - which we mark by setting
   // .Profiles.
   Result.Profiles = std::move(*MaybeCtx);
+  Result.initIndex();
   return Result;
 }
 
@@ -265,11 +266,9 @@ CtxProfAnalysis::getSelectInstrumentation(SelectInst &SI) {
 
 template <class ProfilesTy, class ProfTy>
 static void preorderVisit(ProfilesTy &Profiles,
-                          function_ref<void(ProfTy &)> Visitor,
-                          GlobalValue::GUID Match = 0) {
+                          function_ref<void(ProfTy &)> Visitor) {
   std::function<void(ProfTy &)> Traverser = [&](auto &Ctx) {
-    if (!Match || Ctx.guid() == Match)
-      Visitor(Ctx);
+    Visitor(Ctx);
     for (auto &[_, SubCtxSet] : Ctx.callsites())
       for (auto &[__, Subctx] : SubCtxSet)
         Traverser(Subctx);
@@ -278,16 +277,44 @@ 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 &[Guid, FI] : FuncInfo)
+    InsertionPoints[Guid] = &FI.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(*reinterpret_cast<PGOCtxProfContext *>(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)
+    return preorderVisit<const PGOCtxProfContext::CallTargetMapTy,
+                         const PGOCtxProfContext>(*Profiles, V);
+  assert(isFunctionKnown(*F));
+  GlobalValue::GUID G = getDefinedFunctionGUID(*F);
+  for (const auto *Node = FuncInfo.find(G)->second.Index.Next; Node;
+       Node = Node->Next)
+    V(*reinterpret_cast<const PGOCtxProfContext *>(Node));
 }
 
 const CtxProfFlatProfile PGOContextualProfile::flatten() const {

llvm/lib/Transforms/Utils/CallPromotionUtils.cpp

@@ -655,7 +655,7 @@ CallBase *llvm::promoteCallWithIfThenElse(CallBase &CB, Function &Callee,
     Ctx.counters()[IndirectID] = IndirectCount;
 
   };
-  CtxProf.update(ProfileUpdater, &Caller);
+  CtxProf.update(ProfileUpdater, Caller);
   return &DirectCall;
 }
 

llvm/lib/Transforms/Utils/InlineFunction.cpp

@@ -2375,7 +2375,7 @@ llvm::InlineResult llvm::InlineFunction(CallBase &CB, InlineFunctionInfo &IFI,
     assert(Deleted);
     (void)Deleted;
   };
-  CtxProf.update(Updater, &Caller);
+  CtxProf.update(Updater, Caller);
   return Ret;
 }