[libcxx] Introduce an externally-threaded libc++ variant.
This patch further decouples libc++ from pthread, allowing libc++ to be built
against other threading systems. There are two main use cases:
- Building libc++ against a thread library other than pthreads.
- Building libc++ with an "external" thread API, allowing a separate library to
provide the implementation of that API.
The two use cases are quite similar, the second one being sligtly more
de-coupled than the first. The cmake option LIBCXX_HAS_EXTERNAL_THREAD_API
enables both kinds of builds. One needs to place an <__external_threading>
header file containing an implementation of the "libc++ thread API" declared
in the <__threading_support> header.
For the second use case, the implementation of the libc++ thread API can
delegate to a custom "external" thread API where the implementation of this
external API is provided in a seperate library. This mechanism allows toolchain
vendors to distribute a build of libc++ with a custom thread-porting-layer API
(which is the "external" API above), platform vendors (recipients of the
toolchain/libc++) are then required to provide their implementation of this API
to be linked with (end-user) C++ programs.
Note that the second use case still requires establishing the basic types that
get passed between the external thread library and the libc++ library
(e.g. __libcpp_mutex_t). These cannot be opaque pointer types (libc++ sources
won't compile otherwise). It should also be noted that the second use case can
have a slight performance penalty; as all the thread constructs need to cross a
library boundary through an additional function call.
When the header <__external_threading> is omitted, libc++ is built with the
"libc++ thread API" (declared in <__threading_support>) as the "external" thread
API (basic types are pthread based). An implementation (pthread based) of this
API is provided in test/support/external_threads.cpp, which is built into a
separate DSO and linked in when running the libc++ test suite. A test run
therefore demonstrates the second use case (less the intermediate custom API).
Differential revision: https://reviews.llvm.org/D21968
Reviewers: bcraig, compnerd, EricWF, mclow.lists
llvm-svn: 281179
diff --git a/libcxx/lib/CMakeLists.txt b/libcxx/lib/CMakeLists.txt
index dd0f663..8ebda51 100644
--- a/libcxx/lib/CMakeLists.txt
+++ b/libcxx/lib/CMakeLists.txt
@@ -201,6 +201,23 @@
)
endif()
+if (LIBCXX_HAS_EXTERNAL_THREAD_API)
+ file(GLOB LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES ../test/support/external_threads.cpp)
+
+ if (LIBCXX_ENABLE_SHARED)
+ add_library(cxx_external_threads SHARED ${LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES})
+ else()
+ add_library(cxx_external_threads STATIC ${LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES})
+ endif()
+
+ set_target_properties(cxx_external_threads
+ PROPERTIES
+ LINK_FLAGS "${LIBCXX_LINK_FLAGS}"
+ COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
+ OUTPUT_NAME "c++external_threads"
+ )
+endif()
+
# Generate a linker script inplace of a libc++.so symlink. Rerun this command
# after cxx builds.
if (LIBCXX_ENABLE_SHARED AND LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
