[Python-Dev] Bug in build system for cross-platform builds (original) (raw)

Russell Keith-Magee russell at keith-magee.com
Fri Mar 11 18:16:29 EST 2016

On Sat, Mar 12, 2016 at 6:38 AM, Martin Panter <vadmium+py at gmail.com> wrote:

Hi Russell. Sorry for the minor ~1 month delay in replying :)

I have been doing some experimenting to see what is involved in cross-compiling Python (Native host = Linux, target = Windows via mingw and some patches). So I have a slightly better understanding of the problem than before. On 16 February 2016 at 01:41, Russell Keith-Magee <russell at keith-magee.com> wrote: > In order to build for a host platform, you have to compile for a local > platform first - for example, to compile an iOS ARM64 binary, you have to > compile for OS X x8664 first. This gives you a local platform version of > Python you can use when building the iOS version. > > Early in the Makefile, the variable PYTHONFORBUILD is set. This points at > the CPU-local version of Python that can be invoked, which is used for > module builds, and for compiling the standard library source code. This is > set by —host and —build flags to configure, plus the use of CC and LDFLAGS > environment variables to point at the compiler and libraries for the > platform you’re compiling for, and a PATH variable that provides the local > platform’s version of Python. So far I haven’t succeeded with my Min GW cross build and am temporarily giving up due to incompatibilities. But my attempts looked a bit like this: make clean # Work around confusion with existing in-source build mkdir native (cd native/ && ../configure) make -C native/ Parser/pgen mkdir mingw (cd mingw/ && ../configure --host=i486-mingw32 --build=x86) make -C mingw/ PGEN=../native/Parser/pgen Actually it was not as smooth as the above commands, because pgen tends to get overwritten with a cross-compiled version. Perhaps we could add a PGENFORBUILD override, like HOSTPGEN in the patch used at <_ _https://wayback.archive.org/web/20160131224915/http://randomsplat.com/id5-cross-compiling-python-for-embedded-linux.html_ _>. That might fix the pgen problem, but freezeimportlib still remains. I suppose the same thing might be possible for _freeze_importlib as well…

There are two places where special handling is required: the compilation and > execution of the parser generator, and freezeimportlib. In both cases, the > tool needs to be compiled for the local platform, and then executed. > Historically (i.e., Py3.4 and earlier), this has been done by spawning a > child MAKE to compile the tool; this runs the compilation phase with the > local CPU environment, before returning to the master makefile and executing > the tool. By spawning the child MAKE, you get a “clean” environment, so the > tool is built natively. However, as I understand it, it causes problems with > parallel builds due to race conditions on build rules. The change in > Python3.5 simplified the rule so that child MAKE calls weren’t used, but > that means that pgen and freezeimportlib are compiled for ARM64, so they > won’t run on the local platform.

You suggest that the child Make command happened to compile pgen etc natively, rather than with the cross compiler. But my understanding is that when you invoke $(MAKE), all the environment variables, configure settings, etc, including the cross compiler, would be inherited by the child. Would it be more correct to say instead that in 3.4 you did a separate native build step, precompiling pgen and freezeimportlib for the native build host? Then you hoped that the child Make was not invoked in the cross-compilation stage and your precompiled executables would not be rebuilt?

Yes - as far as I can make out (with my admittedly hazy understanding), that appears to be what is going on. Although it’s not that I “hoped” the build wouldn’t happen on the second pass - it was the behavior that was previously relied, and on was altered.

> As best as I can work out, the solution is to: > > (1) Include the parser generator and freezeimportlib as part of the > artefacts of local platform. That way, you could use the version of pgen and > freezeimportlib that was compiled as part of the local platform build. At > present, pgen and freezeimportlib are used during the build process, but > aren’t preserved at the end of the build; or

I don’t understand. After I run Make, it looks like I get working executables leftover at Programs/freezeimportlib and Parser/pgen. Do you mean to install these programs with “make install” or something?

Making them part of the installable artefacts would be one option, but they don’t have to be installed, just preserved.

For example, as a nasty hack, I’ve been able to use this approach to get the build working for 3.5. After the native build, I copy _freeze_importlib to a “safe” location. I then copy it back into place prior to the target build. It works, but it’s in no way suitable for a final build.

> (2) Include some concept of the “local compiler” in the build process, which > can be used to compile pgen and freezeimportlib; or

On the surface solution (2) sounds like the ideal fix. But I guess the local native compiler might also require a separate set of CPPFLAGS, pyconfig.h settings etc. In other words it is sounding like a whole separate “configure” run. I am thinking it might be simplest to just require this native “configure” run to be done manually.

That run is going to happen anyway, since you have to compile and build for the native platform.

Yours, Russ Magee %-) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20160312/0948ab33/attachment-0001.html>

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