Extend assembler with ability to produce comments for the generated code.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index b4b0ee8744d57c605991194f5274429714ceea8b..af9e1ed40719d7435d2b3b1ce4f0dfe946d825ca 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -296,7 +296,7 @@ class Label : public ValueObject {
 
 class Assembler : public ValueObject {
  public:
-  Assembler() : buffer_(), prolog_offset_(-1) { }
+  Assembler() : buffer_(), prolog_offset_(-1), comments_() { }
   ~Assembler() { }
 
   static const bool kNearJump = true;
@@ -530,6 +530,9 @@ class Assembler : public ValueObject {
   void Align(int alignment, int offset);
   void Bind(Label* label);
 
+  void Comment(const char* format, ...);
+  const Code::Comments& GetCodeComments() const;
+
   int CodeSize() const { return buffer_.Size(); }
   int prolog_offset() const { return prolog_offset_; }
   const ZoneGrowableArray<int>& GetPointerOffsets() const {
@@ -552,6 +555,23 @@ class Assembler : public ValueObject {
   AssemblerBuffer buffer_;
   int prolog_offset_;
 
+  class CodeComment : public ZoneAllocated {
+   public:
+    CodeComment(intptr_t pc_offset, const String& comment)
+        : pc_offset_(pc_offset), comment_(comment) { }
+
+    intptr_t pc_offset() const { return pc_offset_; }
+    const String& comment() const { return comment_; }
+
+   private:
+    intptr_t pc_offset_;
+    const String& comment_;
+
+    DISALLOW_COPY_AND_ASSIGN(CodeComment);
+  };
+
+  GrowableArray<CodeComment*> comments_;
+
   inline void EmitUint8(uint8_t value);
   inline void EmitInt32(int32_t value);
   inline void EmitInt64(int64_t value);