Add hybrid assembler concept to ARM assembler.

src/IceFixups.h

 //===- subzero/src/IceFixups.h - Assembler fixup kinds ----------*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 27 matching lines @@
   void set_kind(FixupKind Kind) { kind_ = Kind; }
 
   RelocOffsetT offset() const;
   IceString symbol(const GlobalContext *Ctx) const;
 
   static const Constant *NullSymbol;
   bool isNullSymbol() const { return value_ == NullSymbol; }
 
   void set_value(const Constant *Value) { value_ = Value; }
 
-  void emit(GlobalContext *Ctx, RelocOffsetT OverrideOffset) const;
+  /// Emits fixup, then returns the number of bytes to skip.
+  virtual size_t emit(GlobalContext *Ctx, RelocOffsetT OverrideOffset,
+                      bool IsPCRel) const;
 
 private:
   intptr_t position_ = 0;
   FixupKind kind_ = 0;
   const Constant *value_ = nullptr;
 };
 
+/// Extends a fixup to be textual. That is, it emits text instead of a sequence
+/// of bytes. This class is used as a fallback for unimplemented emitIAS
+/// methods, allowing them to generate compilable assembly code.
+class AssemblerTextFixup : public AssemblerFixup {
+  AssemblerTextFixup() = delete;
+  AssemblerTextFixup(const AssemblerTextFixup &) = delete;
+  AssemblerTextFixup &operator=(const AssemblerTextFixup &) = delete;
+
+public:
+  AssemblerTextFixup(const std::string &Message, size_t NumBytes)
+      : AssemblerFixup(), Message(Message), NumBytes(NumBytes) {}
+  ~AssemblerTextFixup() = default;
+  virtual size_t emit(GlobalContext *Ctx, RelocOffsetT OverrideOffset,
+                      bool isPcRel) const;
+
+private:
+  const std::string Message;
+  const size_t NumBytes;
+};
+
 using FixupList = std::vector<AssemblerFixup>;
 using FixupRefList = std::vector<AssemblerFixup *>;
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEFIXUPS_H