[llvm] [CGData] Outlined Hash Tree (PR #89792)

[Ellis Hoag via llvm-commits]

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+//===- OutlinedHashTree.h --------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===---------------------------------------------------------------------===//
+//
+// This defines the OutlinedHashTree class. It contains sequences of stable
+// hash values of instructions that have been outlined. This OutlinedHashTree
+// can be used to track the outlined instruction sequences across modules.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGENDATA_OUTLINEDHASHTREE_H
+#define LLVM_CODEGENDATA_OUTLINEDHASHTREE_H
+
+#include "llvm/ADT/StableHashing.h"
+#include "llvm/ObjectYAML/YAML.h"
+#include "llvm/Support/raw_ostream.h"
+
+#include <unordered_map>
+#include <vector>
+
+namespace llvm {
+
+/// A HashNode is an entry in an OutlinedHashTree, holding a hash value
+/// and a collection of Successors (other HashNodes). If a HashNode has
+/// a positive terminal value (Terminals > 0), it signifies the end of
+/// a hash sequence with that occurrence count.
+struct HashNode {
+  /// The hash value of the node.
+  stable_hash Hash;
+  /// The number of terminals in the sequence ending at this node.
+  unsigned Terminals;
+  /// The successors of this node.
+  std::unordered_map<stable_hash, std::unique_ptr<HashNode>> Successors;
+};
+
+/// HashNodeStable is the serialized, stable, and compact representation
+/// of a HashNode.
+struct HashNodeStable {
+  llvm::yaml::Hex64 Hash;
+  unsigned Terminals;
+  std::vector<unsigned> SuccessorIds;
+};
+
+class OutlinedHashTree {
+
+  using EdgeCallbackFn =
+      std::function<void(const HashNode *, const HashNode *)>;
+  using NodeCallbackFn = std::function<void(const HashNode *)>;
+
+  using HashSequence = std::vector<stable_hash>;
+  using HashSequencePair = std::pair<std::vector<stable_hash>, unsigned>;
+
+public:
+  /// Walks every edge and node in the OutlinedHashTree and calls CallbackEdge
+  /// for the edges and CallbackNode for the nodes with the stable_hash for
+  /// the source and the stable_hash of the sink for an edge. These generic
+  /// callbacks can be used to traverse a OutlinedHashTree for the purpose of
+  /// print debugging or serializing it.
+  void walkGraph(NodeCallbackFn CallbackNode,
+                 EdgeCallbackFn CallbackEdge = nullptr,
+                 bool SortedWalk = false) const;
+
+  /// Release all hash nodes except the root hash node.
+  void clear() {
+    assert(getRoot()->Hash == 0 && getRoot()->Terminals == 0);
+    getRoot()->Successors.clear();
+  }
+
+  /// \returns true if the hash tree has only the root node.
+  bool empty() { return size() == 1; }
+
+  /// \returns the size of a OutlinedHashTree by traversing it. If
+  /// \p GetTerminalCountOnly is true, it only counts the terminal nodes
+  /// (meaning it returns the the number of hash sequences in the
+  /// OutlinedHashTree).
+  size_t size(bool GetTerminalCountOnly = false) const;
+
+  /// \returns the depth of a OutlinedHashTree by traversing it.
+  size_t depth() const;
+
+  /// \returns the root hash node of a OutlinedHashTree.
+  const HashNode *getRoot() const { return Root.get(); }
+  HashNode *getRoot() { return Root.get(); }
+
+  /// Inserts a \p Sequence into the this tree. The last node in the sequence
+  /// will increase Terminals.
+  void insert(const HashSequencePair &SequencePair);
+
+  /// Merge a \p OtherTree into this Tree.
+  void merge(const OutlinedHashTree *OtherTree);
+
+  /// \returns the matching count if \p Sequence exists in the OutlinedHashTree.
+  unsigned find(const HashSequence &Sequence) const;
+
+  OutlinedHashTree() { Root = std::make_unique<HashNode>(); }
+
+private:
+  std::unique_ptr<HashNode> Root;
----------------
ellishg wrote:

Does this need to be a `unique_ptr`?
```suggestion
  HashNode Root;
```

https://github.com/llvm/llvm-project/pull/89792