20 issues of porting C++ code on the 64-bit platform
by Andrey Karpov, Evgeniy Ryzhkov

Contents   Introduction   Issues 1-4   Issues 5-9   Issues 10-14   Issues 15-20   Conclusion   Printable version   Discuss this article

Observing the previous kind of errors related to mixing of simple integer types and memsize types, we surveyed only simple expressions. But similar problems may occur while using other C++ constructions too. extern int Width, Height, Depth; size_t GetIndex(int x, int y, int z) { return x + y * Width + z * Width * Height; } ... MyArray[GetIndex(x, y, z)] = 0.0f; In case if you work with large arrays (more than INT_MAX items) the given code may behave incorrectly and we’ll address not those items of the array MyArray we wanted. In spite the fact that we return the value of size_t type "x + y * Width + z * Width * Height" expression is calculated with the use of int type. We suppose you have already guessed that the corrected code will look as follows:

extern int Width, Height, Depth;
size_t GetIndex(int x, int y, int z) {
  return (size_t)(x) +
         (size_t)(y) * (size_t)(Width) +
         (size_t)(z) * (size_t)(Width) * (size_t)(Height);
}

extern char *begin, *end;
unsigned GetSize() {
  return end - begin;
}

And here it is one more example but now we’ll observe not the returned value but the formal function argument.

void foo(ptrdiff_t delta);
int i = -2;
unsigned k = 1;
foo(i + k);

During the port of 32-bit programs on the 64-bit platform the change of the logic of its work may be found which is related to the use of overload functions. If the function is overlapped for 32-bit and 64-bit values the access to it with the argument of memsize type will be compiled into different calls on different systems. This method may be useful as, for example, in the following code:

static size_t GetBitCount(const unsigned __int32 &) {
  return 32;
}
static size_t GetBitCount(const unsigned __int64 &) {
  return 64;
}
size_t a;
size_t bitCount = GetBitCount(a);

class MyStack {
...
public:
  void Push(__int32 &);
  void Push(__int64 &);
  void Pop(__int32 &);
  void Pop(__int64 &);
} stack;
ptrdiff_t value_1;
stack.Push(value_1);
...
int value_2;
stack.Pop(value_2);

We think you understand this kind of errors and that you should pay attention to the call of overload functions transferring actual arguments of memsize type.

17. Data alignment

Processors work more efficiently when they deal with data aligned properly. As a rule the 32-bit data item must be aligned at the border multiple 4 bytes and the 64-bit item at the border 8 bytes. The try to work with unaligned data on processors IA-64 (Itanium) as it is shown in the following example, will cause exception. #pragma pack (1) // Also set by key /Zp in MSVC struct AlignSample { unsigned size; void *pointer; } object; void foo(void *p) { object.pointer = p; // Alignment fault } If you have to work with unaligned data on Itanium you should indicate this to the compiler. For example, you may use a special macro UNALIGNED:

#pragma pack (1) // Also set by key /Zp in MSVC
struct AlignSample {
  unsigned size;
  void *pointer;
} object;
void foo(void *p) {
  *(UNALIGNED void *)&object.pointer = p; //Very slow
}

On the architecture x64 during the access to unaligned data exception does not occur but you should avoid them either. Firstly, because of the essential slowing down of the speed of the access to these data, and secondly, because of a high probability of porting the program on the platform IA-64 in future.

Let’s look at one more example of the code which does not take into account the data alignment.

struct MyPointersArray {
  DWORD m_n;
  PVOID m_arr[1];
} object;
...
malloc( sizeof(DWORD) + 5 * sizeof(PVOID) );
...

_{Picture 7. Alignment of the data in memory on systems Win32 and Win64}

The correct calculation of the size should look as follows:

struct MyPointersArray {
  DWORD m_n;
  PVOID m_arr[1];
} object;
...
malloc( FIELD_OFFSET(struct MyPointersArray, m_arr) +
        5 * sizeof(PVOID) );
...

Always use these macros to get a shift in the structure without relying on your knowledge of the sizes of types and the alignment. Here it is the example of the code with the correct calculation of the structure member address:

struct TFoo {
  DWORD_PTR whatever;
  int value;
} object;
int *valuePtr = 
  (int *)((size_t)(&object) + offsetof(TFoo, value)); // OK

Catch and handle exceptions when integer types participate is not a good programming practice while working in C++ language. For such aims you should use more informative types, for example classes derived from class std::exception. But sometimes one has to work with less quality code as it is shown further.

char *ptr1;
char *ptr2;
try {
  try {
    throw ptr2 - ptr1;
  }
  catch (int) {
    std::cout << "catch 1: on x86" << std::endl;
  }
}
catch (ptrdiff_t) {
  std::cout << "catch 2: on x64" << std::endl;
}

While developing a 64-bit application, be sure to bear in mind the changes of the environment in which it will be performed. Some functions will become outdated and it will be necessary to replace them with some variants. GetWindowLong is a good example of such function in the Windows operation system. Pay your attention to the constants referring to the interaction with the environment in which the program is functioning. In Windows the lines containing "system32" or "Program Files" will be suspect.

20. Explicit type conversions

Be accurate with explicit type conversions. They may change the logic of the program execution when types change their capacity of cause the loss of significant bits. It is difficult to adduce typical examples of errors related to the explicit type conversion for they are very different and specific for different programs. You have become acquainted with some errors related to the explicit type conversion earlier.

Conclusion