[FR] Switch Windows builds to use LLVM (opensource Apache 2.0) instead of Visual Studio modules - clang-cl.exe and lld-link.exe

[mattslezak-shell]

What's the problem this feature will solve?

The current builds of Python for Windows use the MSVC 14.X libraries which are covered under the Microsoft Visual Studio licenses. This means that any extension modules that are generated also fall under the same Microsoft licenses. Which is contrary to the the whole point of Python in general - the whole library should be OPEN SOURCE. So the very easy thing to do on Windows is to switch to the LLVM compilers that are 100% compatible with the MSVC builds. Since they already exist under LLVM (clang-cl.exe and lld-link.exe) the project SHOULD move toward their adoption, rather than continuing to use closed source build tools.

Describe the solution you'd like

Simply switch the default build tools for Python modules to use the LLVM clang-cl.exe and lld-link.exe so that Python on Windows is actually open-source, i.e. Apache 2.0 license.

Alternative Solutions

Well if you use MSVC to build Python it's always going to fall under the commercial license when people try to extend the functionality via compiled modules.

Additional context

No response

Code of Conduct

• I agree to follow the PSF Code of Conduct

[mattslezak-shell]

"""distutils._msvccompiler

Contains MSVCCompiler, an implementation of the abstract CCompiler class
for Microsoft Visual Studio 2015.

The module is compatible with VS 2015 and later. You can find legacy support
for older versions in distutils.msvc9compiler and distutils.msvccompiler.
"""

# Written by Perry Stoll
# hacked by Robin Becker and Thomas Heller to do a better job of
#   finding DevStudio (through the registry)
# ported to VS 2005 and VS 2008 by Christian Heimes
# ported to VS 2015 by Steve Dower
# switched to use LLVM (Apache 2.0 License) `clang-cl.exe` compiler and linker `lld-link.exe` for Windows by Matt Slezak

import os
import subprocess
import contextlib
import warnings
import unittest.mock
with contextlib.suppress(ImportError):
    import winreg

from distutils.errors import DistutilsExecError, DistutilsPlatformError, \
                             CompileError, LibError, LinkError
from distutils.ccompiler import CCompiler, gen_lib_options
from distutils import log
from distutils.util import get_platform

from itertools import count

def _find_vc2015():
    try:
        key = winreg.OpenKeyEx(
            winreg.HKEY_LOCAL_MACHINE,
            r"Software\Microsoft\VisualStudio\SxS\VC7",
            access=winreg.KEY_READ | winreg.KEY_WOW64_32KEY
        )
    except OSError:
        log.debug("Visual C++ is not registered")
        return None, None

    best_version = 0
    best_dir = None
    with key:
        for i in count():
            try:
                v, vc_dir, vt = winreg.EnumValue(key, i)
            except OSError:
                break
            if v and vt == winreg.REG_SZ and os.path.isdir(vc_dir):
                try:
                    version = int(float(v))
                except (ValueError, TypeError):
                    continue
                if version >= 14 and version > best_version:
                    best_version, best_dir = version, vc_dir
    return best_version, best_dir

def _find_vc2017():
    """Returns "15, path" based on the result of invoking vswhere.exe
    If no install is found, returns "None, None"

    The version is returned to avoid unnecessarily changing the function
    result. It may be ignored when the path is not None.

    If vswhere.exe is not available, by definition, VS 2017 is not
    installed.
    """
    root = os.environ.get("ProgramFiles(x86)") or os.environ.get("ProgramFiles")
    if not root:
        return None, None

    try:
        path = subprocess.check_output([
            os.path.join(root, "Microsoft Visual Studio", "Installer", "vswhere.exe"),
            "-latest",
            "-prerelease",
            "-requires", "Microsoft.VisualStudio.Component.VC.Tools.x86.x64",
            "-property", "installationPath",
            "-products", "*",
        ], encoding="mbcs", errors="strict").strip()
    except (subprocess.CalledProcessError, OSError, UnicodeDecodeError):
        return None, None

    path = os.path.join(path, "VC", "Auxiliary", "Build")
    if os.path.isdir(path):
        return 15, path

    return None, None

PLAT_SPEC_TO_RUNTIME = {
    'x86' : 'x86',
    'x86_amd64' : 'x64',
    'x86_arm' : 'arm',
    'x86_arm64' : 'arm64'
}

def _find_vcvarsall(plat_spec):
    # bpo-38597: Removed vcruntime return value
    _, best_dir = _find_vc2017()

    if not best_dir:
        best_version, best_dir = _find_vc2015()

    if not best_dir:
        log.debug("No suitable Visual C++ version found")
        return None, None

    vcvarsall = os.path.join(best_dir, "vcvarsall.bat")
    if not os.path.isfile(vcvarsall):
        log.debug("%s cannot be found", vcvarsall)
        return None, None

    return vcvarsall, None

def _get_vc_env(plat_spec):
    if os.getenv("DISTUTILS_USE_SDK"):
        return {
            key.lower(): value
            for key, value in os.environ.items()
        }

    vcvarsall, _ = _find_vcvarsall(plat_spec)
    if not vcvarsall:
        raise DistutilsPlatformError("Unable to find vcvarsall.bat")

    try:
        out = subprocess.check_output(
            'cmd /u /c "{}" {} && set'.format(vcvarsall, plat_spec),
            stderr=subprocess.STDOUT,
        ).decode('utf-16le', errors='replace')
    except subprocess.CalledProcessError as exc:
        log.error(exc.output)
        raise DistutilsPlatformError("Error executing {}"
                .format(exc.cmd))

    env = {
        key.lower(): value
        for key, _, value in
        (line.partition('=') for line in out.splitlines())
        if key and value
    }

    return env

def _find_exe(exe, paths=None):
    """Return path to an MSVC executable program.

    Tries to find the program in several places: first, one of the
    MSVC program search paths from the registry; next, the directories
    in the PATH environment variable.  If any of those work, return an
    absolute path that is known to exist.  If none of them work, just
    return the original program name, 'exe'.
    """
    if not paths:
        paths = os.getenv('path').split(os.pathsep)
    for p in paths:
        fn = os.path.join(os.path.abspath(p), exe)
        if os.path.isfile(fn):
            return fn
    return exe

# A map keyed by get_platform() return values to values accepted by
# 'vcvarsall.bat'. Always cross-compile from x86 to work with the
# lighter-weight MSVC installs that do not include native 64-bit tools.
PLAT_TO_VCVARS = {
    'win32' : 'x86',
    'win-amd64' : 'x86_amd64',
    'win-arm32' : 'x86_arm',
    'win-arm64' : 'x86_arm64'
}

class MSVCCompiler(CCompiler) :
    """Concrete class that implements an interface to Microsoft Visual C++,
       as defined by the CCompiler abstract class."""

    compiler_type = 'msvc'

    # Just set this so CCompiler's constructor doesn't barf.  We currently
    # don't use the 'set_executables()' bureaucracy provided by CCompiler,
    # as it really isn't necessary for this sort of single-compiler class.
    # Would be nice to have a consistent interface with UnixCCompiler,
    # though, so it's worth thinking about.
    executables = {}

    # Private class data (need to distinguish C from C++ source for compiler)
    _c_extensions = ['.c']
    _cpp_extensions = ['.cc', '.cpp', '.cxx']
    _rc_extensions = ['.rc']
    _mc_extensions = ['.mc']

    # Needed for the filename generation methods provided by the
    # base class, CCompiler.
    src_extensions = (_c_extensions + _cpp_extensions +
                      _rc_extensions + _mc_extensions)
    res_extension = '.res'
    obj_extension = '.obj'
    static_lib_extension = '.lib'
    shared_lib_extension = '.dll'
    static_lib_format = shared_lib_format = '%s%s'
    exe_extension = '.exe'


    def __init__(self, verbose=0, dry_run=0, force=0):
        super().__init__(verbose, dry_run, force)
        # target platform (.plat_name is consistent with 'bdist')
        self.plat_name = None
        self.initialized = False

    def initialize(self, plat_name=None):
        # multi-init means we would need to check platform same each time...
        assert not self.initialized, "don't init multiple times"
        if plat_name is None:
            plat_name = get_platform()
        # sanity check for platforms to prevent obscure errors later.
        if plat_name not in PLAT_TO_VCVARS:
            raise DistutilsPlatformError("--plat-name must be one of {}"
                                         .format(tuple(PLAT_TO_VCVARS)))

        # Get the vcvarsall.bat spec for the requested platform.
        plat_spec = PLAT_TO_VCVARS[plat_name]

        vc_env = _get_vc_env(plat_spec)
        if not vc_env:
            raise DistutilsPlatformError("Unable to find a compatible "
                "Visual Studio installation.")

        self._paths = vc_env.get('path', '')
        paths = self._paths.split(os.pathsep)
        self.cc = _find_exe("clang-cl.exe", paths)
        self.linker = _find_exe("lld-link.exe", paths)
        self.lib = _find_exe("lib.exe", paths)
        self.rc = _find_exe("rc.exe", paths)   # resource compiler
        self.mc = _find_exe("mc.exe", paths)   # message compiler
        self.mt = _find_exe("mt.exe", paths)   # message compiler

        for dir in vc_env.get('include', '').split(os.pathsep):
            if dir:
                self.add_include_dir(dir.rstrip(os.sep))

        for dir in vc_env.get('lib', '').split(os.pathsep):
            if dir:
                self.add_library_dir(dir.rstrip(os.sep))

        self.preprocess_options = None
        # bpo-38597: Always compile with dynamic linking
        # Future releases of Python 3.x will include all past
        # versions of vcruntime*.dll for compatibility.
        self.compile_options = [
            '/nologo', '/O2', '/W3', '/GL', '/DNDEBUG', '/MD'
        ]

        self.compile_options_debug = [
            '/nologo', '/Od', '/MDd', '/Zi', '/W3', '/D_DEBUG'
        ]

        ldflags = [
            '/nologo', '/INCREMENTAL:NO'#, '/LTCG'
        ]

        ldflags_debug = [
            '/nologo', '/INCREMENTAL:NO', '/DEBUG:FULL' #, '/LTCG'
        ]

        self.ldflags_exe = [*ldflags, '/MANIFEST:EMBED,ID=1']
        self.ldflags_exe_debug = [*ldflags_debug, '/MANIFEST:EMBED,ID=1']
        self.ldflags_shared = [*ldflags, '/DLL', '/MANIFEST:EMBED,ID=2', '/MANIFESTUAC:NO']
        self.ldflags_shared_debug = [*ldflags_debug, '/DLL', '/MANIFEST:EMBED,ID=2', '/MANIFESTUAC:NO']
        self.ldflags_static = [*ldflags]
        self.ldflags_static_debug = [*ldflags_debug]

        self._ldflags = {
            (CCompiler.EXECUTABLE, None): self.ldflags_exe,
            (CCompiler.EXECUTABLE, False): self.ldflags_exe,
            (CCompiler.EXECUTABLE, True): self.ldflags_exe_debug,
            (CCompiler.SHARED_OBJECT, None): self.ldflags_shared,
            (CCompiler.SHARED_OBJECT, False): self.ldflags_shared,
            (CCompiler.SHARED_OBJECT, True): self.ldflags_shared_debug,
            (CCompiler.SHARED_LIBRARY, None): self.ldflags_static,
            (CCompiler.SHARED_LIBRARY, False): self.ldflags_static,
            (CCompiler.SHARED_LIBRARY, True): self.ldflags_static_debug,
        }

        self.initialized = True

    # -- Worker methods ------------------------------------------------

    def object_filenames(self,
                         source_filenames,
                         strip_dir=0,
                         output_dir=''):
        ext_map = {
            **{ext: self.obj_extension for ext in self.src_extensions},
            **{ext: self.res_extension for ext in self._rc_extensions + self._mc_extensions},
        }

        output_dir = output_dir or ''

        def make_out_path(p):
            base, ext = os.path.splitext(p)
            if strip_dir:
                base = os.path.basename(base)
            else:
                _, base = os.path.splitdrive(base)
                if base.startswith((os.path.sep, os.path.altsep)):
                    base = base[1:]
            try:
                # XXX: This may produce absurdly long paths. We should check
                # the length of the result and trim base until we fit within
                # 260 characters.
                return os.path.join(output_dir, base + ext_map[ext])
            except LookupError:
                # Better to raise an exception instead of silently continuing
                # and later complain about sources and targets having
                # different lengths
                raise CompileError("Don't know how to compile {}".format(p))

        return list(map(make_out_path, source_filenames))


    def compile(self, sources,
                output_dir=None, macros=None, include_dirs=None, debug=0,
                extra_preargs=None, extra_postargs=None, depends=None):

        if not self.initialized:
            self.initialize()
        compile_info = self._setup_compile(output_dir, macros, include_dirs,
                                           sources, depends, extra_postargs)
        macros, objects, extra_postargs, pp_opts, build = compile_info

        compile_opts = extra_preargs or []
        compile_opts.append('/c')
        if debug:
            compile_opts.extend(self.compile_options_debug)
        else:
            compile_opts.extend(self.compile_options)


        add_cpp_opts = False

        for obj in objects:
            try:
                src, ext = build[obj]
            except KeyError:
                continue
            if debug:
                # pass the full pathname to MSVC in debug mode,
                # this allows the debugger to find the source file
                # without asking the user to browse for it
                src = os.path.abspath(src)

            # Anaconda/conda-forge customisation, we want our pdbs to be
            # relocatable:
            # https://developercommunity.visualstudio.com/comments/623156/view.html
            d1trimfile_opts = []
            if 'SRC_DIR' in os.environ and os.path.basename(self.cc) == "clang-cl.exe":
                d1trimfile_opts.append("/d1trimfile:" + os.environ['SRC_DIR'])

            if ext in self._c_extensions:
                input_opt = "/Tc" + src
            elif ext in self._cpp_extensions:
                input_opt = "/Tp" + src
                add_cpp_opts = True
            elif ext in self._rc_extensions:
                # compile .RC to .RES file
                input_opt = src
                output_opt = "/fo" + obj
                try:
                    self.spawn([self.rc] + pp_opts + [output_opt, input_opt])
                except DistutilsExecError as msg:
                    raise CompileError(msg)
                continue
            elif ext in self._mc_extensions:
                # Compile .MC to .RC file to .RES file.
                #   * '-h dir' specifies the directory for the
                #     generated include file
                #   * '-r dir' specifies the target directory of the
                #     generated RC file and the binary message resource
                #     it includes
                #
                # For now (since there are no options to change this),
                # we use the source-directory for the include file and
                # the build directory for the RC file and message
                # resources. This works at least for win32all.
                h_dir = os.path.dirname(src)
                rc_dir = os.path.dirname(obj)
                try:
                    # first compile .MC to .RC and .H file
                    self.spawn([self.mc, '-h', h_dir, '-r', rc_dir, src])
                    base, _ = os.path.splitext(os.path.basename (src))
                    rc_file = os.path.join(rc_dir, base + '.rc')
                    # then compile .RC to .RES file
                    self.spawn([self.rc, "/fo" + obj, rc_file])

                except DistutilsExecError as msg:
                    raise CompileError(msg)
                continue
            else:
                # how to handle this file?
                raise CompileError("Don't know how to compile {} to {}"
                                   .format(src, obj))

            args = [self.cc] + compile_opts + pp_opts + d1trimfile_opts
            if add_cpp_opts:
                args.append('/EHsc')
            args.append(input_opt)
            args.append("/Fo" + obj)
            args.extend(extra_postargs)

            try:
                self.spawn(args)
            except DistutilsExecError as msg:
                raise CompileError(msg)

        return objects


    def create_static_lib(self,
                          objects,
                          output_libname,
                          output_dir=None,
                          debug=0,
                          target_lang=None):

        if not self.initialized:
            self.initialize()
        objects, output_dir = self._fix_object_args(objects, output_dir)
        output_filename = self.library_filename(output_libname,
                                                output_dir=output_dir)

        if self._need_link(objects, output_filename):
            lib_args = objects + ['/OUT:' + output_filename]
            if debug:
                pass # XXX what goes here?
            try:
                log.debug('Executing "%s" %s', self.lib, ' '.join(lib_args))
                self.spawn([self.lib] + lib_args)
            except DistutilsExecError as msg:
                raise LibError(msg)
        else:
            log.debug("skipping %s (up-to-date)", output_filename)


    def link(self,
             target_desc,
             objects,
             output_filename,
             output_dir=None,
             libraries=None,
             library_dirs=None,
             runtime_library_dirs=None,
             export_symbols=None,
             debug=0,
             extra_preargs=None,
             extra_postargs=None,
             build_temp=None,
             target_lang=None):

        if not self.initialized:
            self.initialize()
        objects, output_dir = self._fix_object_args(objects, output_dir)
        fixed_args = self._fix_lib_args(libraries, library_dirs,
                                        runtime_library_dirs)
        libraries, library_dirs, runtime_library_dirs = fixed_args

        if runtime_library_dirs:
            self.warn("I don't know what to do with 'runtime_library_dirs': "
                       + str(runtime_library_dirs))

        lib_opts = gen_lib_options(self,
                                   library_dirs, runtime_library_dirs,
                                   libraries)
        if output_dir is not None:
            output_filename = os.path.join(output_dir, output_filename)

        if self._need_link(objects, output_filename):
            ldflags = self._ldflags[target_desc, debug]

            export_opts = ["/EXPORT:" + sym for sym in (export_symbols or [])]

            ld_args = (ldflags + lib_opts + export_opts +
                       objects + ['/OUT:' + output_filename])

            # The MSVC linker generates .lib and .exp files, which cannot be
            # suppressed by any linker switches. The .lib files may even be
            # needed! Make sure they are generated in the temporary build
            # directory. Since they have different names for debug and release
            # builds, they can go into the same directory.
            build_temp = os.path.dirname(objects[0])
            if export_symbols is not None:
                (dll_name, dll_ext) = os.path.splitext(
                    os.path.basename(output_filename))
                implib_file = os.path.join(
                    build_temp,
                    self.library_filename(dll_name))
                ld_args.append ('/IMPLIB:' + implib_file)

            if extra_preargs:
                ld_args[:0] = extra_preargs
            if extra_postargs:
                ld_args.extend(extra_postargs)

            output_dir = os.path.dirname(os.path.abspath(output_filename))
            self.mkpath(output_dir)
            try:
                log.debug('Executing "%s" %s', self.linker, ' '.join(ld_args))
                self.spawn([self.linker] + ld_args)
            except DistutilsExecError as msg:
                raise LinkError(msg)
        else:
            log.debug("skipping %s (up-to-date)", output_filename)

    def spawn(self, cmd):
        env = dict(os.environ, PATH=self._paths)
        with self._fallback_spawn(cmd, env) as fallback:
            return super().spawn(cmd, env=env)
        return fallback.value

    @contextlib.contextmanager
    def _fallback_spawn(self, cmd, env):
        """
        Discovered in pypa/distutils#15, some tools monkeypatch the compiler,
        so the 'env' kwarg causes a TypeError. Detect this condition and
        restore the legacy, unsafe behavior.
        """
        bag = type('Bag', (), {})()
        try:
            yield bag
        except TypeError as exc:
            if "unexpected keyword argument 'env'" not in str(exc):
                raise
        else:
            return
        warnings.warn(
            "Fallback spawn triggered. Please update distutils monkeypatch.")
        with unittest.mock.patch.dict('os.environ', env):
            bag.value = super().spawn(cmd)

    # -- Miscellaneous methods -----------------------------------------
    # These are all used by the 'gen_lib_options() function, in
    # ccompiler.py.

    def library_dir_option(self, dir):
        return "/LIBPATH:" + dir

    def runtime_library_dir_option(self, dir):
        raise DistutilsPlatformError(
              "don't know how to set runtime library search path for MSVC")

    def library_option(self, lib):
        return self.library_filename(lib)

    def find_library_file(self, dirs, lib, debug=0):
        # Prefer a debugging library if found (and requested), but deal
        # with it if we don't have one.
        if debug:
            try_names = [lib + "_d", lib]
        else:
            try_names = [lib]
        for dir in dirs:
            for name in try_names:
                libfile = os.path.join(dir, self.library_filename(name))
                if os.path.isfile(libfile):
                    return libfile
        else:
            # Oops, didn't find it in *any* of 'dirs'
            return None

[imba-tjd]

Hi.

• How do you know clang-cl is 100% compatiple?
• Is there a reference to say using msvc will fall under the commercial license?
• How about CPython? It uses msvc. Doesn't it have the same problem?

[mattslezak-shell]

Clang-cl has test suites with NumPy and SciPy and those at least pass all tests. I can't say for sure that EVERY library will work 100%, just that having the option to select a different linker and compiler would be helpful for some users. Every extension module I've built using clang-cl and lld-link works without issue alongside OpenMP and MSVC compiled Python (PYD files). I believe Python itself has no issue as the library is opensource, so the Build Tools are allowed. When using the Build Tools for commercial applications which are distributed, they fall under the Visual Studio license, which means if you distribute them, then you need to buy Visual Studio. So for the small subset of Python users that create compiled libraries and deploy them for commercial use, or use the compiled modules within a company >5 users, having an option to use LLVM tools instead at least gets past these restrictions. I already know how to "hack" the _msvccompiler.py to use LLVM tools, but some people don't. If it's easy to add that option then why not. Probably just switching every build over to these tools wouldn't make much sense, but adding an option to select them instead would.

[jaraco]

This project isn't wed to any particular compiler and is open to support additional compilers. Ideally, it should also present one preferred compiler for each platform such that users get a consistent, predictable experience, and for Windows, that's likely to be the MS compiler. If presenting alternate compilers, there should be a straightforward way to opt into that compiler on an individual or environment basis. It should also include tests, which may prove somewhat difficult to integrate (see cygwin jobs in the CI runs on pypa/distutils for an example). It might even be desirable to refactor the code to allow for modular compilers.

Since most of the compiler logic is currently maintained in distutils, probably the implementation will need to happen in pypa/distutils.

Please feel free to work on a solution, though be aware that this feature request is rather a big one and not something that will be solved in a simple patch. I'm happy to help guide as you work through concepts and an implementation.

[isuruf]

See #7 for a previous PR.

Note that using clang-cl.exe does not mean that the extension module will not link to MSVC libraries. Clang-cl generated executables and the extension modules still link to MSVC libraries