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macOS: disable fixup chains when linking extension modules

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On macOS, extension library builds usually rely on the flag ``-undefined
dynamic_lookup`` to get the linker to temporarily accept missing CPython
API symbols. As the name implies, those symbols are then dynamically
resolved when Python imports such an extension library. Binaries
produced in this way are more portable since they don't contain a
hardcoded path to the CPython library. This is critical when
distributing binary wheels on PyPI, etc..

When targeting macOS>=12, XCode recently started to generate a somewhat
ominous warning:

``-undefined dynamic_lookup may not work with chained fixups``

For further detail on what chained fixups are, see the following
informative WWDC
https://developer.apple.com/videos/play/wwdc2022/110362/ (the relevant
part starts about 20mins into the video)

The message that the dynamic resolution functionality became broken.
I reported this to Apple via feedback request FB11767124.

Apple investigated the request and updated the behavior of ``ld``
starting with XCode 14.3b1+: it now disables the fixup chain linker
optimization whenever ``-undefined dynamic_lookup`` is specified. The
feedback from Apple also stated that the parameter
``-Wl,-no_fixup_chains`` should be specified on pre-14.3 XCode versions
to ensure correct behavior.

This commit realizes those changes by passing a flag to *always* disable
fixup chains. Related discussion is available in the CPython repository
(https://github.com/python/cpython/issues/97524).
This commit is contained in:
Wenzel Jakob 2022-10-31 22:11:16 +01:00
parent 3cc7e4258c
commit e076c4513d
2 changed files with 8 additions and 6 deletions
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8
docs/compiling.rst
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@ -582,13 +582,13 @@ using ``pip`` or ``conda``. If it hasn't, you can also manually specify
``-I <path-to-pybind11>/include`` together with the Python includes path ``-I <path-to-pybind11>/include`` together with the Python includes path
``python3-config --includes``. ``python3-config --includes``.
On macOS: the build command is almost the same but it also requires passing On macOS: the build command is almost the same but it also requires passing the
the ``-undefined dynamic_lookup`` flag so as to ignore missing symbols when ``-undefined dynamic_lookup -Wl,no_fixup_chains`` flag to ignore missing symbols
building the module: when building the module:
.. code-block:: bash .. code-block:: bash
$ c++ -O3 -Wall -shared -std=c++11 -undefined dynamic_lookup $(python3 -m pybind11 --includes) example.cpp -o example$(python3-config --extension-suffix) $ c++ -O3 -Wall -shared -std=c++11 -undefined dynamic_lookup -Wl,no_fixup_chains $(python3 -m pybind11 --includes) example.cpp -o example$(python3-config --extension-suffix)
In general, it is advisable to include several additional build parameters In general, it is advisable to include several additional build parameters
that can considerably reduce the size of the created binary. Refer to section that can considerably reduce the size of the created binary. Refer to section

6
tools/pybind11Common.cmake
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@ -75,7 +75,8 @@ if(CMAKE_VERSION VERSION_LESS 3.13)
set_property( set_property(
TARGET pybind11::python_link_helper TARGET pybind11::python_link_helper
APPEND APPEND
PROPERTY INTERFACE_LINK_LIBRARIES "$<$<PLATFORM_ID:Darwin>:-undefined dynamic_lookup>") PROPERTY INTERFACE_LINK_LIBRARIES
"$<$<PLATFORM_ID:Darwin>:-undefined dynamic_lookup -no_fixup_chains>")
else() else()
# link_options was added in 3.13+ # link_options was added in 3.13+
# This is safer, because you are ensured the deduplication pass in CMake will not consider # This is safer, because you are ensured the deduplication pass in CMake will not consider
@ -83,7 +84,8 @@ else()
set_property( set_property(
TARGET pybind11::python_link_helper TARGET pybind11::python_link_helper
APPEND APPEND
PROPERTY INTERFACE_LINK_OPTIONS "$<$<PLATFORM_ID:Darwin>:LINKER:-undefined,dynamic_lookup>") PROPERTY INTERFACE_LINK_OPTIONS
"$<$<PLATFORM_ID:Darwin>:LINKER:-undefined,dynamic_lookup,-no_fixup_chains>")
endif() endif()
# ------------------------ Windows extras ------------------------- # ------------------------ Windows extras -------------------------