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diff --git a/tools/llvm-diff/DifferenceEngine.h b/tools/llvm-diff/DifferenceEngine.h
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+//===-- DifferenceEngine.h - Module comparator ------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This header defines the interface to the LLVM difference engine,
+// which structurally compares functions within a module.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LLVM_DIFFERENCE_ENGINE_H_
+#define _LLVM_DIFFERENCE_ENGINE_H_
+
+#include <utility>
+#include <llvm/ADT/SmallVector.h>
+#include <llvm/ADT/StringRef.h>
+
+namespace llvm {
+  class LLVMContext;
+  class Module;
+  class Function;
+  class Twine;
+  class Value;
+  class GlobalValue;
+
+  /// A class for performing structural comparisons of LLVM assembly.
+  class DifferenceEngine {
+  public:
+    /// A temporary-object class for building up log messages.
+    class LogBuilder {
+      DifferenceEngine &Engine;
+
+      /// The use of a stored StringRef here is okay because
+      /// LogBuilder should be used only as a temporary, and as a
+      /// temporary it will be destructed before whatever temporary
+      /// might be initializing this format.
+      StringRef Format;
+
+      SmallVector<Value*, 4> Arguments;
+
+    public:
+      LogBuilder(DifferenceEngine &Engine, StringRef Format)
+        : Engine(Engine), Format(Format) {}
+
+      LogBuilder &operator<<(Value *V) {
+        Arguments.push_back(V);
+        return *this;
+      }
+
+      ~LogBuilder() {
+        Engine.consumer.logf(*this);
+      }
+
+      StringRef getFormat() const { return Format; }
+
+      unsigned getNumArguments() const { return Arguments.size(); }
+      Value *getArgument(unsigned I) const { return Arguments[I]; }
+    };
+
+    enum DiffChange { DC_match, DC_left, DC_right };
+
+    /// A temporary-object class for building up diff messages.
+    class DiffLogBuilder {
+      typedef std::pair<Instruction*,Instruction*> DiffRecord;
+      SmallVector<DiffRecord, 20> Diff;
+
+      DifferenceEngine &Engine;
+
+    public:
+      DiffLogBuilder(DifferenceEngine &Engine) : Engine(Engine) {}
+      ~DiffLogBuilder() { Engine.consumer.logd(*this); }
+
+      void addMatch(Instruction *L, Instruction *R) {
+        Diff.push_back(DiffRecord(L, R));
+      }
+      void addLeft(Instruction *L) { Diff.push_back(DiffRecord(L, 0)); }
+      void addRight(Instruction *R) { Diff.push_back(DiffRecord(0, R)); }
+
+      unsigned getNumLines() const { return Diff.size(); }
+      DiffChange getLineKind(unsigned I) const {
+        return (Diff[I].first ? (Diff[I].second ? DC_match : DC_left)
+                              : DC_right);
+      }
+      Instruction *getLeft(unsigned I) const { return Diff[I].first; }
+      Instruction *getRight(unsigned I) const { return Diff[I].second; }
+    };
+
+    /// The interface for consumers of difference data.
+    struct Consumer {
+      /// Record that a local context has been entered.  Left and
+      /// Right are IR "containers" of some sort which are being
+      /// considered for structural equivalence: global variables,
+      /// functions, blocks, instructions, etc.
+      virtual void enterContext(Value *Left, Value *Right) = 0;
+
+      /// Record that a local context has been exited.
+      virtual void exitContext() = 0;
+
+      /// Record a difference within the current context.
+      virtual void log(StringRef Text) = 0;
+
+      /// Record a formatted difference within the current context.
+      virtual void logf(const LogBuilder &Log) = 0;
+
+      /// Record a line-by-line instruction diff.
+      virtual void logd(const DiffLogBuilder &Log) = 0;
+
+    protected:
+      ~Consumer() {}
+    };
+
+    /// 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.
+    struct Oracle {
+      virtual bool operator()(Value *L, Value *R) = 0;
+
+    protected:
+      ~Oracle() {}
+    };
+
+    DifferenceEngine(LLVMContext &context, Consumer &consumer)
+      : context(context), consumer(consumer), globalValueOracle(0) {}
+
+    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(*this, text);
+    }
+
+    /// 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:
+    LLVMContext &context;
+    Consumer &consumer;
+    Oracle *globalValueOracle;
+  };
+}
+
+#endif