diff --git a/tools/llvm-diff/DiffLog.h b/tools/llvm-diff/DiffLog.h
new file mode 100644
index 0000000..43e318a
--- /dev/null
+++ b/tools/llvm-diff/DiffLog.h
@@ -0,0 +1,80 @@
+//===-- DiffLog.h - Difference Log Builder and accessories ------*- 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 log builder.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LLVM_DIFFLOG_H_
+#define _LLVM_DIFFLOG_H_
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace llvm {
+  class Instruction;
+  class Value;
+  class Consumer;
+
+  /// Trichotomy assumption
+  enum DiffChange { DC_match, DC_left, DC_right };
+
+  /// A temporary-object class for building up log messages.
+  class LogBuilder {
+    Consumer &consumer;
+
+    /// 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(Consumer &c, StringRef Format)
+      : consumer(c), Format(Format) {}
+
+    LogBuilder &operator<<(Value *V) {
+      Arguments.push_back(V);
+      return *this;
+    }
+
+    ~LogBuilder();
+
+    StringRef getFormat() const;
+    unsigned getNumArguments() const;
+    Value *getArgument(unsigned I) const;
+  };
+
+  /// A temporary-object class for building up diff messages.
+  class DiffLogBuilder {
+    typedef std::pair<Instruction*,Instruction*> DiffRecord;
+    SmallVector<DiffRecord, 20> Diff;
+
+    Consumer &consumer;
+
+  public:
+    DiffLogBuilder(Consumer &c) : consumer(c) {}
+    ~DiffLogBuilder();
+
+    void addMatch(Instruction *L, Instruction *R);
+    // HACK: VS 2010 has a bug in the stdlib that requires this.
+    void addLeft(Instruction *L);
+    void addRight(Instruction *R);
+
+    unsigned getNumLines() const;
+    DiffChange getLineKind(unsigned I) const;
+    Instruction *getLeft(unsigned I) const;
+    Instruction *getRight(unsigned I) const;
+  };
+
+}
+
+#endif