Why is /Wp64 deprecated?

Question

Why is the /Wp64 flag in Visual C++ deprecated?

cl : Command line warning D9035 :
option \'Wp64\' has been deprecated and will be r

Answer 1

/Wp64 on 32-bit builds is a waste of time. It is deprecated, and this deprecation makes sense. The way /Wp64 worked on 32-bit builds is it would look for a _w64 annotation on a type. This _w64 annotation would tell the compiler that, even though this type is 32-bits in 32-bit mode, it is 64-bits in 64-bit mode. This turned out to be really flakey, especially where templates are involved.

/Wp64 on 64-bit builds is extremely useful. The documentation (http://msdn.microsoft.com/en-us/library/vstudio/yt4xw8fh.aspx) claims that it is on by default in 64-bit builds, but this is not true. Compiler warnings C4311 and C4312 are only emitted if /Wp64 is explicitly set. Those two warnings indicate when a 32-bit value is put into a pointer, or vice-versa. These are very important for code correctness, and claim to be at warning level 1. I have found bugs in very widespread code that would have been stopped if the developers had turned on /Wp64 for 64-bit builds. Unfortunately, you also get the command line warning that you have observed. I know of no way to squelch this warning, and I have learned to live with it. On the bright side, if you build with as warnings as errors, this command line warning doesn't turn into an error.