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author | Zachary Turner <zturner@google.com> | 2014-06-16 22:49:41 +0000 |
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committer | Zachary Turner <zturner@google.com> | 2014-06-16 22:49:41 +0000 |
commit | 5ea46694d0b48eebcf04d9e1fc0dac6382356f32 (patch) | |
tree | 686459ef65febf5538b05e7f68c26f04c6a5d7c8 /docs | |
parent | 298ff80849de645ab7afb92004759350309de400 (diff) | |
download | llvm-5ea46694d0b48eebcf04d9e1fc0dac6382356f32.tar.gz llvm-5ea46694d0b48eebcf04d9e1fc0dac6382356f32.tar.bz2 llvm-5ea46694d0b48eebcf04d9e1fc0dac6382356f32.tar.xz |
Revert r211066, 211067, 211068, 211069, 211070.
These were committed accidentally from the wrong branch before having
a review sign-off.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211072 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs')
-rw-r--r-- | docs/ProgrammersManual.rst | 51 |
1 files changed, 47 insertions, 4 deletions
diff --git a/docs/ProgrammersManual.rst b/docs/ProgrammersManual.rst index b35364d06a..4973e5c667 100644 --- a/docs/ProgrammersManual.rst +++ b/docs/ProgrammersManual.rst @@ -2170,24 +2170,67 @@ compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and using the resultant compiler to build a copy of LLVM with multithreading support. +.. _startmultithreaded: + +Entering and Exiting Multithreaded Mode +--------------------------------------- + +In order to properly protect its internal data structures while avoiding +excessive locking overhead in the single-threaded case, the LLVM must intialize +certain data structures necessary to provide guards around its internals. To do +so, the client program must invoke ``llvm_start_multithreaded()`` before making +any concurrent LLVM API calls. To subsequently tear down these structures, use +the ``llvm_stop_multithreaded()`` call. You can also use the +``llvm_is_multithreaded()`` call to check the status of multithreaded mode. + +Note that both of these calls must be made *in isolation*. That is to say that +no other LLVM API calls may be executing at any time during the execution of +``llvm_start_multithreaded()`` or ``llvm_stop_multithreaded``. It is the +client's responsibility to enforce this isolation. + +The return value of ``llvm_start_multithreaded()`` indicates the success or +failure of the initialization. Failure typically indicates that your copy of +LLVM was built without multithreading support, typically because GCC atomic +intrinsics were not found in your system compiler. In this case, the LLVM API +will not be safe for concurrent calls. However, it *will* be safe for hosting +threaded applications in the JIT, though :ref:`care must be taken +<jitthreading>` to ensure that side exits and the like do not accidentally +result in concurrent LLVM API calls. + .. _shutdown: Ending Execution with ``llvm_shutdown()`` ----------------------------------------- When you are done using the LLVM APIs, you should call ``llvm_shutdown()`` to -deallocate memory used for internal structures. +deallocate memory used for internal structures. This will also invoke +``llvm_stop_multithreaded()`` if LLVM is operating in multithreaded mode. As +such, ``llvm_shutdown()`` requires the same isolation guarantees as +``llvm_stop_multithreaded()``. + +Note that, if you use scope-based shutdown, you can use the +``llvm_shutdown_obj`` class, which calls ``llvm_shutdown()`` in its destructor. + .. _managedstatic: Lazy Initialization with ``ManagedStatic`` ------------------------------------------ ``ManagedStatic`` is a utility class in LLVM used to implement static -initialization of static resources, such as the global type tables. In a -single-threaded environment, it implements a simple lazy initialization scheme. -When LLVM is compiled with support for multi-threading, however, it uses +initialization of static resources, such as the global type tables. Before the +invocation of ``llvm_shutdown()``, it implements a simple lazy initialization +scheme. Once ``llvm_start_multithreaded()`` returns, however, it uses double-checked locking to implement thread-safe lazy initialization. +Note that, because no other threads are allowed to issue LLVM API calls before +``llvm_start_multithreaded()`` returns, it is possible to have +``ManagedStatic``\ s of ``llvm::sys::Mutex``\ s. + +The ``llvm_acquire_global_lock()`` and ``llvm_release_global_lock`` APIs provide +access to the global lock used to implement the double-checked locking for lazy +initialization. These should only be used internally to LLVM, and only if you +know what you're doing! + .. _llvmcontext: Achieving Isolation with ``LLVMContext`` |