Add markup for libc++ dylib availability

Libc++ is used as a system library on macOS and iOS (amongst others). In order
for users to be able to compile a binary that is intended to be deployed to an
older version of the platform, clang provides the
availability attribute <https://clang.llvm.org/docs/AttributeReference.html#availability>_
that can be placed on declarations to describe the lifecycle of a symbol in the
library.

See docs/DesignDocs/AvailabilityMarkup.rst for more information.

Differential Revision: https://reviews.llvm.org/D31739

llvm-svn: 302172

2 files changed, 115 insertions, 0 deletions

diff --git a/libcxx/docs/DesignDocs/AvailabilityMarkup.rst b/libcxx/docs/DesignDocs/AvailabilityMarkup.rst
new file mode 100644
index 00000000000..4a85c698f23
--- /dev/null
+++ b/libcxx/docs/DesignDocs/AvailabilityMarkup.rst
@@ -0,0 +1,114 @@
+===================
+Availability Markup
+===================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+Libc++ is used as a system library on macOS and iOS (amongst others). In order
+for users to be able to compile a binary that is intended to be deployed to an
+older version of the platform, clang provides the
+`availability attribute <https://clang.llvm.org/docs/AttributeReference.html#availability>`_
+that can be placed on declarations to describe the lifecycle of a symbol in the
+library.
+
+Design
+======
+
+When a new feature is introduced that requires dylib support, a macro should be
+created in include/__config to mark this feature as unavailable for all the
+systems. For example::
+
+    // Define availability macros.
+    #if defined(_LIBCPP_USE_AVAILABILITY_APPLE)
+    #define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
+    #else  if defined(_LIBCPP_USE_AVAILABILITY_SOME_OTHER_VENDOR)
+    #define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS __attribute__((unavailable))
+	#else
+    #define _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS
+    #endif
+
+When the library is updated by the platform vendor, the markup can be updated.
+For example::
+
+    #define _LIBCPP_AVAILABILITY_SHARED_MUTEX                                  \
+      __attribute__((availability(macosx,strict,introduced=10.12)))            \
+      __attribute__((availability(ios,strict,introduced=10.0)))                \
+      __attribute__((availability(tvos,strict,introduced=10.0)))               \
+      __attribute__((availability(watchos,strict,introduced=3.0)))
+
+In the source code, the macro can be added on a class if the full class requires
+type info from the library for example::
+
+	_LIBCPP_BEGIN_NAMESPACE_EXPERIMENTAL
+	class _LIBCPP_EXCEPTION_ABI _LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS bad_optional_access
+	    : public std::logic_error {
+
+or on a particular symbol:
+
+    _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_AVAILABILITY_SIZED_NEW_DELETE void  operator delete(void* __p, std::size_t __sz) _NOEXCEPT;
+
+
+Testing
+=======
+
+Some parameters can be passed to lit to run the test-suite and exercising the
+availability.
+
+* The `platform` parameter controls the deployement target. For example lit can
+  be invoked with `--param=platform=macosx10.8`. Default is the current host.
+* The `use_system_cxx_lib` parameter indicates to use another library than the
+  just built one. Invoking lit with `--param=use_system_cxx_lib=true` will run
+  the test-suite against the host system library. Alternatively a path to the
+  directory containing a specific prebuilt libc++ can be used, for example:
+  `--param=use_system_cxx_lib=/path/to/macOS/10.8/`.
+* The `with_availability` boolean parameter enables the availability markup.
+
+Tests can be marked as XFAIL based on multiple features made available by lit:
+
+
+* if either `use_system_cxx_lib` or  `with_availability` is passed to lit,
+  assuming `--param=platform=macosx10.8` is passed as well the following
+  features will be available:
+
+  - availability
+  - availability=x86_64
+  - availability=macosx
+  - availability=x86_64-macosx
+  - availability=x86_64-apple-macosx10.8
+  - availability=macosx10.8
+
+  This feature is used to XFAIL a test that *is* using a class of a method marked
+  as unavailable *and* that is expected to *fail* if deployed on an older system.
+
+* if `use_system_cxx_lib` is passed to lit, the following features will also
+  be available:
+
+  - with_system_cxx_lib
+  - with_system_cxx_lib=x86_64
+  - with_system_cxx_lib=macosx
+  - with_system_cxx_lib=x86_64-macosx
+  - with_system_cxx_lib=x86_64-apple-macosx10.8
+  - with_system_cxx_lib=macosx10.8
+
+  This feature is used to XFAIL a test that is *not* using a class of a method
+  marked as unavailable *but* that is expected to fail if deployed on an older
+  system. For example if we know that it exhibits a but in the libc on a
+  particular system version.
+
+* if `with_availability` is passed to lit, the following features will also
+  be available:
+
+  - availability_markup
+  - availability_markup=x86_64
+  - availability_markup=macosx
+  - availability_markup=x86_64-macosx
+  - availability_markup=x86_64-apple-macosx10.8
+  - availability_markup=macosx10.8
+
+  This feature is used to XFAIL a test that *is* using a class of a method
+  marked as unavailable *but* that is expected to *pass* if deployed on an older
+  system. For example if it is using a symbol in a statically evaluated context.
diff --git a/libcxx/docs/index.rst b/libcxx/docs/index.rst
index c4cdf3b9b2c..3526b47210f 100644
--- a/libcxx/docs/index.rst
+++ b/libcxx/docs/index.rst
@@ -128,6 +128,7 @@ Design Documents
 .. toctree::
    :maxdepth: 1
 
+   DesignDocs/AvailabilityMarkup
    DesignDocs/DebugMode
    DesignDocs/CapturingConfigInfo
    DesignDocs/ABIVersioning