fix X86Disassembler Address parameter

[vtjnash]

Summary

Commit r221751 seems to have also caused the Address parameter to llvm::MCDisassembler::getInstruction to be ignored in the final address computation, since it gets both added and then subtracted as an offset. This diff removes this extra indirection so that the Address parameter works as it did in LLVM3.5 and previous.

[vtjnash]

it looks like the other disassembler targets also ignore Address and simply read the first N bytes from "Bytes", so i assume that was an intentional change in llvm3.5?

lib/Target/X86/Disassembler/X86Disassembler.cpp

X86GenericDisassembler::X86GenericDisassembler(
   case X86::Mode64Bit:
     fMode = MODE_64BIT;
     break;
   default:
     llvm_unreachable("Invalid CPU mode");
   }
 }
 
-struct Region {
-  ArrayRef<uint8_t> Bytes;
-  uint64_t Base;
-  Region(ArrayRef<uint8_t> Bytes, uint64_t Base) : Bytes(Bytes), Base(Base) {}
-};
-
-/// A callback function that wraps the readByte method from Region.
+/// A callback function that wraps the readByte method from an ArrayRef<uint8_t>
 ///
 /// @param Arg      - The generic callback parameter.  In this case, this should
-///                   be a pointer to a Region.
+///                   be a pointer to a ArrayRef<uint8_t>.
 /// @param Byte     - A pointer to the byte to be read.
 /// @param Address  - The address to be read.
 static int regionReader(const void *Arg, uint8_t *Byte, uint64_t Address) {
-  auto *R = static_cast<const Region *>(Arg);
-  ArrayRef<uint8_t> Bytes = R->Bytes;
-  unsigned Index = Address - R->Base;
-  if (Bytes.size() <= Index)
+  const ArrayRef<uint8_t> *Bytes = static_cast<const ArrayRef<uint8_t> *>(Arg);
+  if (Bytes->size() <= Address)
     return -1;
-  *Byte = Bytes[Index];
+  *Byte = Bytes->data()[Address];
   return 0;
 }
 
 /// logger - a callback function that wraps the operator<< method from
 ///   raw_ostream.
 ///
 /// @param arg      - The generic callback parameter.  This should be a pointe
 ///                   to a raw_ostream.
MCDisassembler::DecodeStatus X86GenericDisassembler::getInstruction(
   CommentStream = &CStream;
 
   InternalInstruction InternalInstr;
 
   dlog_t LoggerFn = logger;
   if (&VStream == &nulls())
     LoggerFn = nullptr; // Disable logging completely if it's going to nulls().
 
-  Region R(Bytes, Address);
-
-  int Ret = decodeInstruction(&InternalInstr, regionReader, (const void *)&R,
+  int Ret = decodeInstruction(&InternalInstr, regionReader, (const void *)&Bytes,
                               LoggerFn, (void *)&VStream,
                               (const void *)MII.get(), Address, fMode);
 
   if (Ret) {
     Size = InternalInstr.readerCursor - Address;
     return Fail;
   } else {
     Size = InternalInstr.length;