forked from OSchip/llvm-project
115 lines
4.3 KiB
ReStructuredText
115 lines
4.3 KiB
ReStructuredText
===================
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Availability Markup
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===================
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.. contents::
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:local:
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Overview
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========
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Libc++ is used as a system library on macOS and iOS (amongst others). In order
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for users to be able to compile a binary that is intended to be deployed to an
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older version of the platform, clang provides the
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`availability attribute <https://clang.llvm.org/docs/AttributeReference.html#availability>`_
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that can be placed on declarations to describe the lifecycle of a symbol in the
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library.
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Design
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======
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When a new feature is introduced that requires dylib support, a macro should be
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created in include/__config to mark this feature as unavailable for all the
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systems. For example::
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// Define availability macros.
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#if defined(_LIBCPP_USE_AVAILABILITY_APPLE)
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#define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
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#else if defined(_LIBCPP_USE_AVAILABILITY_SOME_OTHER_VENDOR)
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#define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
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#else
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#define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
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#endif
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When the library is updated by the platform vendor, the markup can be updated.
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For example::
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#define _LIBCPP_AVAILABILITY_SHARED_MUTEX \
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__attribute__((availability(macosx,strict,introduced=10.12))) \
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__attribute__((availability(ios,strict,introduced=10.0))) \
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__attribute__((availability(tvos,strict,introduced=10.0))) \
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__attribute__((availability(watchos,strict,introduced=3.0)))
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In the source code, the macro can be added on a class if the full class requires
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type info from the library for example::
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_LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL
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class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
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: public std::logic_error {
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or on a particular symbol:
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_LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void operator delete(void* __p, std::size_t __sz) _NOEXCEPT;
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Testing
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=======
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Some parameters can be passed to lit to run the test-suite and exercising the
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availability.
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* The `platform` parameter controls the deployement target. For example lit can
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be invoked with `--param=platform=macosx10.8`. Default is the current host.
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* The `use_system_cxx_lib` parameter indicates to use another library than the
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just built one. Invoking lit with `--param=use_system_cxx_lib=true` will run
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the test-suite against the host system library. Alternatively a path to the
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directory containing a specific prebuilt libc++ can be used, for example:
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`--param=use_system_cxx_lib=/path/to/macOS/10.8/`.
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* The `with_availability` boolean parameter enables the availability markup.
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Tests can be marked as XFAIL based on multiple features made available by lit:
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* if either `use_system_cxx_lib` or `with_availability` is passed to lit,
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assuming `--param=platform=macosx10.8` is passed as well the following
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features will be available:
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- availability
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- availability=x86_64
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- availability=macosx
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- availability=x86_64-macosx
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- availability=x86_64-apple-macosx10.8
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- availability=macosx10.8
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This feature is used to XFAIL a test that *is* using a class of a method marked
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as unavailable *and* that is expected to *fail* if deployed on an older system.
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* if `use_system_cxx_lib` is passed to lit, the following features will also
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be available:
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- with_system_cxx_lib
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- with_system_cxx_lib=x86_64
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- with_system_cxx_lib=macosx
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- with_system_cxx_lib=x86_64-macosx
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- with_system_cxx_lib=x86_64-apple-macosx10.8
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- with_system_cxx_lib=macosx10.8
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This feature is used to XFAIL a test that is *not* using a class of a method
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marked as unavailable *but* that is expected to fail if deployed on an older
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system. For example if we know that it exhibits a but in the libc on a
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particular system version.
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* if `with_availability` is passed to lit, the following features will also
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be available:
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- availability_markup
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- availability_markup=x86_64
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- availability_markup=macosx
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- availability_markup=x86_64-macosx
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- availability_markup=x86_64-apple-macosx10.8
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- availability_markup=macosx10.8
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This feature is used to XFAIL a test that *is* using a class of a method
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marked as unavailable *but* that is expected to *pass* if deployed on an older
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system. For example if it is using a symbol in a statically evaluated context.
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