31 Mar 2013

/Wp64

31 Mar 2013

/Wp64 (Detect 64-Bit Portability Issues) is the key of Visual C++ compiler. The key appeared in Visual Studio 2003 and was intended for preparing migration of applications on 64-bit systems. In Visual Studio 2008 /Wp64 key is considered obsolete for it is high time we began to compile 64-bit applications and not to prepare for them.

When defining /Wp64 key the compiler detects some potential errors which can occur when compiling C/C++ code for 64-bit systems. Test of the code consists in that the types marked by the key word __w64 in 32-bit code are interpreted as 64-bit types during the test.

For example, we have the following code:

typedef int MyInt32;
#ifdef _WIN64
  typedef __int64 MySSizet;
#else
  typedef int MySSizet;
#endif
void foo() {
  MyInt32 value32 = 10;
  MySSizet size = 20;
  value32 = size;
}

"value32 = size;" expression on a 64-bit system will cause value cutoff and, consequently, to a potential error. We want to diagnose it. But when compiling a 32-bit application everything is correct and we won't get a warning message.

To prepare for 64-bit systems we should add /Wp64 key and insert the key word __w64 when defining MySSizet type in a 32-bit version. As the result we have the following code:

typedef int MyInt32;
#ifdef _WIN64
  typedef __int64 MySSizet;
#else
  typedef int __w64 MySSizet; // Add __w64 keyword
#endif
void foo() {
  MyInt32 value32 = 10;
  MySSizet size = 20;
  value32 = size; // C4244 64-bit int assigned to 32-bit int
}

Now we'll get the warning message C4244 which will help us to prepare for porting the code on a 64-bit platform.

You should understand that /Wp64's abilities are rather limited. A more detailed and specialized code test can be performed by the commercial tool Viva64 developed specially for simplifying port of applications on 64-bit systems.