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diff --git a/llvm/tools/llvm-diff/DiffConsumer.h b/llvm/tools/llvm-diff/DiffConsumer.h
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+//===-- DiffConsumer.h - Difference Consumer --------------------*- 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 Consumer
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TOOLS_LLVM_DIFF_DIFFCONSUMER_H
+#define LLVM_TOOLS_LLVM_DIFF_DIFFCONSUMER_H
+
+#include "DiffLog.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/IR/Value.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace llvm {
+class StringRef;
+  class Module;
+  class Value;
+  class Function;
+
+  /// The interface for consumers of difference data.
+  class Consumer {
+    virtual void anchor();
+  public:
+    /// 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:
+    virtual ~Consumer() {}
+  };
+
+  class DiffConsumer : public Consumer {
+  private:
+    struct DiffContext {
+      DiffContext(Value *L, Value *R)
+        : L(L), R(R), Differences(false), IsFunction(isa<Function>(L)) {}
+      Value *L;
+      Value *R;
+      bool Differences;
+      bool IsFunction;
+      DenseMap<Value*,unsigned> LNumbering;
+      DenseMap<Value*,unsigned> RNumbering;
+    };
+
+    raw_ostream &out;
+    SmallVector<DiffContext, 5> contexts;
+    bool Differences;
+    unsigned Indent;
+
+    void printValue(Value *V, bool isL);
+    void header();
+    void indent();
+
+  public:
+    DiffConsumer()
+      : out(errs()), Differences(false), Indent(0) {}
+
+    bool hadDifferences() const;
+    void enterContext(Value *L, Value *R) override;
+    void exitContext() override;
+    void log(StringRef text) override;
+    void logf(const LogBuilder &Log) override;
+    void logd(const DiffLogBuilder &Log) override;
+  };
+}
+
+#endif