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diff --git a/llvm/tools/llvm-diff/DifferenceEngine.h b/llvm/tools/llvm-diff/DifferenceEngine.h
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+//===-- DifferenceEngine.h - Module comparator ------------------*- 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 header defines the interface to the LLVM difference engine,
+// which structurally compares functions within a module.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TOOLS_LLVM_DIFF_DIFFERENCEENGINE_H
+#define LLVM_TOOLS_LLVM_DIFF_DIFFERENCEENGINE_H
+
+#include "DiffConsumer.h"
+#include "DiffLog.h"
+#include "llvm/ADT/StringRef.h"
+#include <utility>
+
+namespace llvm {
+  class Function;
+  class GlobalValue;
+  class Instruction;
+  class LLVMContext;
+  class Module;
+  class Twine;
+  class Value;
+
+  /// A class for performing structural comparisons of LLVM assembly.
+  class DifferenceEngine {
+  public:
+    /// A RAII object for recording the current context.
+    struct Context {
+      Context(DifferenceEngine &Engine, Value *L, Value *R) : Engine(Engine) {
+        Engine.consumer.enterContext(L, R);
+      }
+
+      ~Context() {
+        Engine.consumer.exitContext();
+      }
+
+    private:
+      DifferenceEngine &Engine;
+    };
+
+    /// An oracle for answering whether two values are equivalent as
+    /// operands.
+    class Oracle {
+      virtual void anchor();
+    public:
+      virtual bool operator()(Value *L, Value *R) = 0;
+
+    protected:
+      virtual ~Oracle() {}
+    };
+
+    DifferenceEngine(Consumer &consumer)
+      : consumer(consumer), globalValueOracle(nullptr) {}
+
+    void diff(Module *L, Module *R);
+    void diff(Function *L, Function *R);
+    void log(StringRef text) {
+      consumer.log(text);
+    }
+    LogBuilder logf(StringRef text) {
+      return LogBuilder(consumer, text);
+    }
+    Consumer& getConsumer() const { return consumer; }
+
+    /// Installs an oracle to decide whether two global values are
+    /// equivalent as operands.  Without an oracle, global values are
+    /// considered equivalent as operands precisely when they have the
+    /// same name.
+    void setGlobalValueOracle(Oracle *oracle) {
+      globalValueOracle = oracle;
+    }
+
+    /// Determines whether two global values are equivalent.
+    bool equivalentAsOperands(GlobalValue *L, GlobalValue *R);
+
+  private:
+    Consumer &consumer;
+    Oracle *globalValueOracle;
+  };
+}
+
+#endif