[Python-Dev] Moving Python 3.5 on Windows to a new compiler

[Steve Dower]

Hi all

I would like to propose moving Python 3.5 to use Visual C++ 14.0 as the main compiler. The first CTP of Visual Studio "14" was released earlier this week: http://blogs.msdn.com/b/vcblog/archive/2014/06/03/visual-studio-14-ctp.aspx

The major feature of interest in this version of MSVC is a new policy to maintain binary compatibility for the CRT into the future. (There will be a blog about this soon, but I didn't want to hold up getting the discussion started here.)

What this means for Python is that C extensions for Python 3.5 and later can be built using any version of MSVC from 14.0 and later. Those who are aware of the current state of affairs where you need to use a matching compiler will hopefully see how big an improvement this will be. It is also likely that other compilers will have an easier time providing compatibility with this new CRT, making it simpler and more reliable to build extensions with LLVM or GCC against an MSVC CPython. 

The other major benefit is that both products are at points in their development where changes can be made. Being a Microsoft employee, I have the ability to test Python builds regularly against the daily MSVC builds and to file bugs directly to the VC team (crashes, incorrect code generation, incorrect linking, performance regressions, etc.). This is a great opportunity to make sure that our needs are covered by the compiler team - it's also a good chance to raise any particular missing features that would be beneficial.

My internal testing shows that the core code is almost fully compatible and builds successfully with only trivial modifications (some CRT variables are now macros with a leading underscore). The project files need updating, but I am willing to do this as part of any migration. There may also be some work required for external dependencies, since I did not test these, but I am also willing to do that.

Basically, what I am offering to do is:

* Update the files in PCBuild to work with Visual Studio "14"
* Make any code changes necessary to build with VC14
* Regularly test the latest Python source with the latest MSVC builds and report issues/suggestions to the MSVC team
* Keep all changes in a separate (public) repo until early next year when we're getting close to the final VS "14" release 

What I am asking anyone else to do is:

* Nothing

Thoughts/comments/concerns?

Cheers,
Steve