gh-90905: Allow cross-compilation on macOS

[zanieb]

Adds support for cross-compiling x86_64 from arm64 macOS. This is particularly useful due to the decreasing availability of x86_64 machines to perform builds on.

The changes here are loosely based on the patch that python-build-standalone uses for cross-compiling.

I tested compiling x86_64 from an arm64 machine with the following script:

#!/usr/bin/env bash

set -ex

ROOT=$(pwd)
OUT=${ROOT}/../cpython-out

# Assume that x86_64 Homebrew is installed
ibrew() { arch -x86_64 /usr/local/bin/brew "$@"; }
export -f ibrew

mkdir -p ${OUT}

# Compile the "build" aarch64 Python

make clean
rm -rf ${OUT}/cpython-aarch64

# Exclude modules we don't need that require system dependencies
./configure \
    py_cv_module__openssl=n/a \
    py_cv_module__hashlib=n/a \
    py_cv_module__gdbm=n/a \
    --without-ensurepip \
    --prefix ${OUT}/cpython-aarch64

make -j8
make -j sharedinstall
make -j install

# Build the "host" x86_64 Python

make clean
rm -rf ${OUT}/cpython-x86_64

CFLAGS="-arch x86_64" \
LDFLAGS="-arch x86_64" \
MACOSX_DEPLOYMENT_TARGET=10.15 \
GDBM_CFLAGS="-I$(ibrew --prefix gdbm)/include" \
GDBM_LIBS="-L$(ibrew --prefix gdbm)/lib -lgdbm" \
PKG_CONFIG="$(ibrew --prefix pkg-config)/bin/pkg-config" \
./configure \
    --with-pydebug \
    --with-system-libmpdec \
    --with-openssl="$(ibrew --prefix openssl@3)" \
    --build=aarch64-apple-darwin \
    --host=x86_64-apple-darwin \
    --prefix ${OUT}/cpython-x86_64 \
    --with-build-python=${OUT}/cpython-aarch64/bin/python3 \
    --disable-ipv6 \
    ac_cv_file__dev_ptc=no \
    ac_cv_file__dev_ptmx=no \
    ac_cv_func_sendfile=no

make -j8
make -j sharedinstall
make -j install

${OUT}/cpython-x86_64/bin/python3 -c "import platform; print(platform.machine())"

• Issue: Support cross compilation on macOS #90905

[zanieb]

This seems a bit too easy. I'm curious if I'm missing something?

The justification for the configure flags are as follows:

--disable-ipv6

configure: error: You must get working getaddrinfo() function or pass the "--disable-ipv6" option to configure.

ac_cv_file__dev_ptmx=no

checking for /dev/ptmx... not set
configure: error: set ac_cv_file__dev_ptmx to yes/no in your CONFIG_SITE file when cross compiling

ac_cv_file__dev_ptc=no

checking for /dev/ptc... not set
configure: error: set ac_cv_file__dev_ptc to yes/no in your CONFIG_SITE file when cross compiling

ac_cv_func_sendfile=no

./Modules/posixmodule.c:11871:15: error: call to undeclared function 'sendfile'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    ret = sendfile(in_fd, out_fd, offset, &sbytes, &sf, flags);
            ^

It's unclear to me if it's expected that the -arch needs to be added to CFLAGS and LDFLAGS explicitly.

I also checked the Platform in the sysconfig, as I know we have an extra patch for this in python-build-standalone but it doesn't look necessary here:

❯ ../cpython-out/cpython-x86_64/bin/python3 -m sysconfig | grep Platform
Platform: "macosx-10.15-x86_64"

[corona10]

I think that @ned-deily and @ronaldoussoren have interests with this patch.