[Python-Dev] Bug in build system for cross-platform builds
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 x86_64 first. This gives you a local platform version of
> > Python you can use when building the iOS version.
> > Early in the Makefile, the variable PYTHON_FOR_BUILD is set. This points
> > 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
> > set by —host and —build flags to configure, plus the use of CC and
> > environment variables to point at the compiler and libraries for the
> > platform you’re compiling for, and a PATH variable that provides the
> > 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 PGEN_FOR_BUILD override, like HOSTPGEN in the patch used
> at <
> That might fix the pgen problem, but _freeze_importlib 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
> > execution of the parser generator, and _freeze_importlib. In both cases,
> > 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
> > the tool. By spawning the child MAKE, you get a “clean” environment, so
> > tool is built natively. However, as I understand it, it causes problems
> > 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 _freeze_importlib are compiled for ARM64, so
> > 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
> Would it be more correct to say instead that in 3.4 you did a separate
> native build step, precompiling pgen and _freeze_importlib 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 _freeze_importlib as part of the
> > artefacts of local platform. That way, you could use the version of pgen
> > _freeze_importlib that was compiled as part of the local platform build.
> > present, pgen and _freeze_importlib are used during the build process,
> > 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/_freeze_importlib 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,
> > can be used to compile pgen and _freeze_importlib; 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.
Russ Magee %-)
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