Question

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

cl : Command line warning D9035 :
option ‘Wp64’ has been deprecated and will be removed in a future release

Answer 1

I think that/Wp64 is deprecated mainly because compiling for a 64-bit target will catch the kinds of errors it was designed to catch (/Wp64 is only valid in 32-bit compiles). The option was added back when 64-bit targets were emerging to help people migrate their programs to 64-bit and help detect code that wasn’t ’64-bit clean’.

Here’s an example of the kinds of problems with /Wp64 that Microsoft just isn’t interested in fixing – probably rightly so (from http://connect.microsoft.com/VisualStudio/feedback/details/502281/std-vector-incompatible-with-wp64-compiler-option):

Actually, the STL isn’t intentionally incompatible with /Wp64, nor is
it completely and unconditionally incompatible with /Wp64. The
underlying problem is that /Wp64 interacts extremely badly with
templates, because __w64 isn’t fully integrated into the type system.
Therefore, if vector<unsigned int> is instantiated before vector<__w64 unsigned int>, then both of them will behave like vector<unsigned int>, and vice versa. On x86, SOCKET is a typedef for __w64 unsigned int. It’s not obvious, but vector<unsigned int> is being instantiated
before your vector<SOCKET>, since vector<bool> is backed (in our
implementation) by vector<unsigned int>.

Previously (in VC9 and earlier), this bad interaction between /Wp64
and templates caused spurious warnings. In VC10, however, changes to
the STL have made this worse. Now, when vector::push_back() is given
an element of the vector itself, it figures out the element’s index
before doing other work. That index is obtained by subtracting the
element’s address from the beginning of the vector. In your repro,
this involves subtracting const SOCKET * - unsigned int *. (The latter
is unsigned int * and not SOCKET * due to the previously described
bug.) This /should/ trigger a spurious warning, saying “I’m
subtracting pointers that point to the same type on x86, but to
different types on x64”. However, there is a SECOND bug here, where
/Wp64 gets really confused and thinks this is a hard error (while
adding constness to the unsigned int *).

We agree that this bogus error message is confusing. However, since
it’s preceded by an un-silenceable command line deprecation warning
D9035, we believe that that should be sufficient. D9035 already says
that /Wp64 shouldn’t be used (although it doesn’t go on to say “this
option is super duper buggy, and completely unnecessary now”).

In the STL, we could #error when /Wp64 is used. However, that would
break customers who are still compiling with /Wp64 (despite the
deprecation warning) and aren’t triggering this bogus error. The STL
could also emit a warning, but the compiler is already emitting D9035.