Windows x86_64 Python on an ARM host: Builds fail with architecture mismatch

[mkoeppe]

Windows on ARM provides a transparent emulation layer that allows users to use unmodified x86_64 programs.

When using an installation of a Windows x86_64 Python, building packages that use meson-python leads to failures due to architecture mismatch.

Examples:

• pip install cysignals ... see log in Windows x86_64 Python on ARM host: Building cysignals fails with architecture mismatch passagemath/passagemath#1242
• pip install --no-binary numpy numpy

Reported originally by @xuluze in passagemath/passagemath#1242

[rgommers]

That's expected, since building for x86-64 on arm64 is a cross-compile.

If Windows is making it as simple as macOS, we may be able to support this use case by automatically generating a cross file when ARCHFLAGS is set:

meson-python/mesonpy/__init__.py

Lines 687 to 721 in 121339e

	# setuptools-like ARCHFLAGS environment variable support
	if sysconfig.get_platform().startswith('macosx-'):
	archflags = os.environ.get('ARCHFLAGS', '').strip()
	if archflags:
	# parse the ARCHFLAGS environment variable
	parser = argparse.ArgumentParser(add_help=False, allow_abbrev=False)
	parser.add_argument('-arch', action='append')
	args, unknown = parser.parse_known_args(archflags.split())
	if unknown:
	raise ConfigError(f'Unknown flag specified in $ARCHFLAGS={archflags!r}')
	arch, *other = set(args.arch)
	if other:
	raise ConfigError(f'Multi-architecture builds are not supported but $ARCHFLAGS={archflags!r}')
	macver, _, nativearch = platform.mac_ver()
	if arch != nativearch:
	x = os.environ.setdefault('_PYTHON_HOST_PLATFORM', f'macosx-{macver}-{arch}')
	if not x.endswith(arch):
	raise ConfigError(f'$ARCHFLAGS={archflags!r} and $_PYTHON_HOST_PLATFORM={x!r} do not agree')
	family = 'aarch64' if arch == 'arm64' else arch
	cross_file_data = textwrap.dedent(f'''
	[binaries]
	c = ['cc', '-arch', {arch!r}]
	cpp = ['c++', '-arch', {arch!r}]
	objc = ['cc', '-arch', {arch!r}]
	objcpp = ['c++', '-arch', {arch!r}]
	[host_machine]
	system = 'darwin'
	cpu = {arch!r}
	cpu_family = {family!r}
	endian = 'little'
	''')
	self._meson_cross_file.write_text(cross_file_data, encoding='utf-8')
	self._meson_args['setup'].extend(('--cross-file', os.fspath(self._meson_cross_file)))

It'd require someone to dig into this though, I don't know how this is working on Windows.

[mkoeppe]

meson's --vsenv option seems the right tool for this, but unfortunately it does not seem to be prepared for this type of cross builds, as it is using windows_detect_native_arch to dispatch.
https://github.com/mesonbuild/meson/blob/0376d655694747ba36aa2bfd94e5bce2d2f1efa8/mesonbuild/utils/vsenv.py#L78

[eli-schwartz]

At least once before I've wondered if that --vsenv option should support specifying the name of an architecture to try to activate. But someone who uses Windows will need to spearhead any such enhancement.

[rgommers]

That would be useful in principle. One now needs a custom activation step for this, e.g. via a GitHub Action invocation:

https://github.com/numpy/numpy/blob/d32370539ea5aa93100842a98cf21d55667b6041/.github/workflows/windows.yml#L114-L118