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

[Ellis Hoag via llvm-commits]

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@@ -0,0 +1,107 @@
+//===- 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;
----------------
ellishg wrote:

I would love to see `DenseMap` used here, but I believe it asserts when the `tombstoneKey` is used (I think `-1` or `-2`) and `stable_hash` could be that value. Let's add a comment here to explain that.

https://discourse.llvm.org/t/what-is-gettombstonekey/37616

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