The /G switch ( /G3, /G4, /G5, /G6, /G7, and /GB ) was used optimize the code for the specific processor architecture like Pentium, Pentium II, Pentium III and IV etc. This was possible with Visual C++ prior to VC++ 8.0 (Visual Studio 2005). But this compiler option is no more supported. The compiler now generates code in a blended mode which works well with all architectures. This is especially need to be taken care while works with makesfiles on switching over from previous version of  Visual C++ versions.

See more information on breaking changes with Visual C++ 2005 Compiler