Translating on Windows

RPython is supported on Windows platforms, starting with Windows 2000. The following text gives some hints about how to translate a interpreter written in RPython, using PyPy as an example.

PyPy supports only being translated as a 32bit program, even on 64bit Windows. See at the end of this page for what is missing for a full 64bit translation.

To build pypy-c you need a working python environment, and a C compiler. It is possible to translate with a CPython 2.6 or later, but this is not the preferred way, because it will take a lot longer to run – depending on your architecture, between two and three times as long. So head to our downloads and get the latest stable version.

Microsoft Visual Studio is preferred as a compiler, but there are reports of success with the mingw32 port of gcc.

What Compiler to use and How to find it?

The first stumbling block when building something for Python on windows is how to discover the path to the compiler, headers, and libraries. One can install many versions of the MSVC compiler tools, from stand-alone build tools to full blown Visual Studio IDE installations. Each of these use cases put the compiler at different locations, and the layout changes from time to time.

The distutils package, located in the stdlib, is the natural place to put this discovery code, but it is frozen by the python version. The pip- installable setuptools can move faster to adapt to new tools. So the first thing that will happen after building PyPy is it will install pip and download setuptools, then it will build the cffi modules used in stdlib. PyPy has a chicken and egg problem: in order to compile something we need setuptools, but in order to get setuptools we need pip which requires _ssl, and _ssl must be compiled. So PyPy vendors in a copy of in rpython/tools/

PyPy will prefer to compile with the latest MSVC compiler it can find, which is a departure from CPython’s desire to compile with Visual Studio 9.

Translating PyPy with Visual Studio

We routinely test translation of PyPy using Visual Studio 2019, MSVC160. Other configurations may work as well.

The translation scripts will set up the appropriate environment variables for the compiler, so you do not need to run vcvars before translation. They will pick the most recent Visual Studio compiler they can find. In addition, the target architecture (32 bits, 64 bits) is automatically selected. A 32 bit build can only be built using a 32 bit Python and vice versa. By default the interpreter is built using the Multi-threaded DLL (/MD) runtime environment.

If you wish to override this detection method to use a different compiler (mingw or a different version of MSVC):

  • set up the PATH and other environment variables as needed

  • set the CC environment variable to compiler exe to be used, for a different version of MSVC SET CC=cl.exe.

Note: The RPython translator does currently not support 64 bit Python, and translation will fail in this case.

Python and a C compiler are all you need to build pypy, but it will miss some modules that relies on third-party libraries. See below how to get and build them.

Please see the non-windows instructions for more information, especially note that translation is RAM-hungry. A standard translation requires around 4GB, so special preparations are necessary, or you may want to use the following method to reduce memory usage at the price of a slower translation:

pypy --jit loop_longevity=300 ../../rpython/bin/rpython -Ojit targetpypystandalone
# This is done as part of translation
PYTHONPATH=../.. ./pypy-c ../../lib_pypy/pypy_tools/

Preparing Windows for the large build

Normally 32bit programs are limited to 2GB of memory on Windows. It is possible to raise this limit to almost 4GB on Windows 64bit.

You need to execute:

editbin /largeaddressaware translator.exe

where translator.exe is the pypy.exe or cpython.exe you will use to translate with. This is done by default during PyPy translation, so it should Just Work.

Installing external packages

We uses a subrepository inside pypy to hold binary compiled versions of the build dependencies for windows. As part of the rpython setup stage, environment variables will be set to use these dependencies. The repository has a README file on how to replicate, and a branch for each supported platform. You may run the utility to checkout the proper branch for your platform and PyPy version.

Using the mingw compiler

You can compile an RPython program with the mingw compiler, using the –cc=mingw32 option; gcc.exe must be on the PATH. If the -cc flag does not begin with “ming”, it should be the name of a valid gcc-derivative compiler, i.e. x86_64-w64-mingw32-gcc for the 64 bit compiler creating a 64 bit target.

You probably want to set the CPATH, LIBRARY_PATH, and PATH environment variables to the header files, lib or dlls, and dlls respectively of the locally installed packages if they are not in the mingw directory heirarchy.

libffi for the mingw compiler

To enable the _rawffi (and ctypes) module, you need to compile a mingw version of libffi. Here is one way to do this, wich should allow you to try to build for win64 or win32:

  1. Download and unzip a mingw32 build or mingw64 build, say into c:mingw

  2. If you do not use cygwin, you will need msys to provide make, autoconf tools and other goodies.

    1. Download and unzip a msys for mingw, say into c:msys

    2. Edit the c:msysetcfstab file to mount c:mingw

  3. Download and unzip the libffi source files, and extract them in the base directory.

  4. Run c:msysmsys.bat or a cygwin shell which should make you feel better since it is a shell prompt with shell tools.

  5. From inside the shell, cd to the libffi directory and do:

    sh ./configure
    cp .libs/libffi-5.dll <somewhere on the PATH>

If you can’t find the dll, and the libtool issued a warning about “undefined symbols not allowed”, you will need to edit the libffi Makefile in the toplevel directory. Add the flag -no-undefined to the definition of libffi_la_LDFLAGS

If you wish to experiment with win64, you must run configure with flags:

sh ./configure --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32

or such, depending on your mingw64 download.

hacking on PyPy with the mingw compiler

Since hacking on PyPy means running tests, you will need a way to specify the mingw compiler when hacking (as opposed to translating). As of March 2012, –cc is not a valid option for However if you set an environment variable CC to the compiler exe, testing will use it.

What is missing for a full 64-bit translation

The main blocker is that we assume that the integer type of RPython is large enough to (occasionally) contain a pointer value cast to an integer. The simplest fix is to make sure that it is so, but it will give the following incompatibility between CPython and PyPy on Win64:

CPython: sys.maxint == 2**31-1, sys.maxsize == 2**63-1

PyPy: sys.maxint == sys.maxsize == 2**63-1

…and, correspondingly, PyPy supports ints up to the larger value of sys.maxint before they are converted to long. The first decision that someone needs to make is if this incompatibility is reasonable.

Assuming that it is, the first thing to do is probably to hack CPython until it fits this model: replace the field in PyIntObject with a long long field, and change the value of sys.maxint. This might just work, even if half-brokenly: I’m sure you can crash it because of the precision loss that undoubtedly occurs everywhere, but try not to. :-)

Such a hacked CPython is what you’ll use in the next steps. We’ll call it CPython64/64.

It is probably not too much work if the goal is only to get a translated PyPy executable, and to run all tests before translation. But you need to start somewhere, and you should start with some tests in rpython/translator/c/test/, like and try to have them pass on top of CPython64/64.

Keep in mind that this runs small translations, and some details may go wrong. The most obvious one is to check that it produces C files that use the integer type Signed — but what is Signed defined to? It should be equal to long on every other platform, but on Win64 it should be something like long long.

What is more generally needed is to review all the C files in rpython/translator/c/src for the word long, because this means a 32-bit integer even on Win64. Replace it with Signed most of the times. You can replace one with the other without breaking anything on any other platform, so feel free to.

Then, these two C types have corresponding RPython types: rffi.LONG and lltype.Signed respectively. The first should really correspond to the C long. Add tests that check that integers cast to one type or the other really have 32 and 64 bits respectively, on Win64.

Once these basic tests work, you need to review rpython/rlib/ for uses of rffi.LONG versus lltype.Signed. The goal would be to fix some more LONG-versus-Signed issues, by fixing the tests — as always run on top of CPython64/64. Note that there was some early work done in rpython/rlib/rarithmetic with the goal of running all the tests on Win64 on the regular CPython, but I think by now that it’s a bad idea. Look only at CPython64/64.

The major intermediate goal is to get a translation of PyPy with -O2 with a minimal set of modules, starting with --no-allworkingmodules; you need to use CPython64/64 to run this translation too. Check carefully the warnings of the C compiler at the end. By default, MSVC reports a lot of mismatches of integer sizes as warnings instead of errors.

Then you need to review pypy/module/*/ for LONG-versus-Signed issues. At some time during this review, we get a working translated PyPy on Windows 64 that includes all --translationmodules, i.e. everything needed to run translations. Once we have that, the hacked CPython64/64 becomes much less important, because we can run future translations on top of this translated PyPy. As soon as we get there, please distribute the translated PyPy. It’s an essential component for anyone else that wants to work on Win64! We end up with a strange kind of dependency — we need a translated PyPy in order to translate a PyPy —, but I believe it’s ok here, as Windows executables are supposed to never be broken by newer versions of Windows.

Happy hacking :-)