Enhance address mode recovery

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 13 matching lines @@
 using FixupKind = uint32_t;
 
 /// Assembler fixups are positions in generated code/data that hold relocation
 /// information that needs to be processed before finalizing the code/data.
 struct AssemblerFixup {
   AssemblerFixup &operator=(const AssemblerFixup &) = delete;
 
 public:
   AssemblerFixup() = default;
   AssemblerFixup(const AssemblerFixup &) = default;
-  intptr_t position() const { return position_; }
-  void set_position(intptr_t Position) { position_ = Position; }
+  intptr_t position() const {
+    assert(position_was_set_);
+    return position_;
+  }
+  void set_position(intptr_t Position) {
+    position_ = Position;
+    position_was_set_ = true;
+  }
 
   FixupKind kind() const { return kind_; }
   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; }
 
   /// Emits fixup, then returns the number of bytes to skip.
   virtual size_t emit(GlobalContext *Ctx, RelocOffsetT OverrideOffset,
                       bool IsPCRel) const;
 
 private:
+  bool position_was_set_ = false;
   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;
@@ skipping 11 matching lines @@
   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