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Update the threading section to reflect current plans/implementation.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73521 91177308-0d34-0410-b5e6-96231b3b80d8
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Owen Anderson 2009-06-16 18:04:19 +00:00
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commit 1ad70e3bb4
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docs/ProgrammersManual.html
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@ -132,7 +132,8 @@ with another <tt>Value</tt></a> </li>
<li><a href="#threading">Threads and LLVM</a>
<ul>
<li><a href="#startmultithreaded">Entering threaded mode with <tt>llvm_start_multithreaded()</tt></a></li>
<li><a href="#startmultithreaded">Entering and Exiting Multithreaded Mode
</a></li>
<li><a href="#shutdown">Ending execution with <tt>llvm_shutdown()</tt></a></li>
<li><a href="#managedstatic">Lazy initialization with <tt>ManagedStatic</tt></a></li>
</ul>
@ -2172,27 +2173,38 @@ support.
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="startmultithreaded">Entering Threaded Mode with
<tt>llvm_start_multithreaded()</tt></a>
<a name="startmultithreaded">Entering and Exiting Multithreaded Mode</a>
</div>
<div class="doc_text">
<p>
In order to properly protect its internal data structures while avoiding
excessive locking overhead in the single-threaded case, the LLVM APIs require
that the client invoke <tt>llvm_start_multithreaded()</tt>. This call must
complete <em>before</em> any other threads attempt to invoke LLVM APIs. Any
attempts to call LLVM APIs from multiple threads before
<tt>llvm_start_multithreaded</tt> returns can and will cause corruption of
LLVM's internal data.
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 <tt>llvm_start_multithreaded()</tt> before
making any concurrent LLVM API calls. To subsequently tear down these
structures, use the <tt>llvm_stop_multithreaded()</tt> call. You can also use
the <tt>llvm_is_multithreaded()</tt> call to check the status of multithreaded
mode.
</p>
<p>
A caveat: before <tt>llvm_start_multithreaded()</tt> has been invoked, all
<tt>llvm::sys::Mutex</tt> acquisitions and releases will become no-ops. This
means that <tt>llvm_start_multithreaded()</tt> must be invoked before a threaded
application can be executed in the JIT.
Note that both of these calls must be made <em>in isolation</em>. That is to
say that no other LLVM API calls may be executing at any time during the
execution of <tt>llvm_start_multithreaded()</tt> or <tt>llvm_stop_multithreaded
</tt>. It's is the client's responsibility to enforce this isolation.
</p>
<p>
The return value of <tt>llvm_start_multithreaded()</tt> 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 <em>will</em> be safe for
hosting threaded applications in the JIT, though care must be taken to ensure
that side exits and the like do not accidentally result in concurrent LLVM API
calls.
</p>
</div>
@ -2204,9 +2216,10 @@ application can be executed in the JIT.
<div class="doc_text">
<p>
When you are done using the LLVM APIs, you should call <tt>llvm_shutdown()</tt>
to deallocate memory used for internal structures. This call must not begin
while any other threads are still issuing LLVM API calls. Doing so is likely
to result in garbage data or crashes.
to deallocate memory used for internal structures. This will also invoke
<tt>llvm_stop_multithreaded()</tt> if LLVM is operating in multithreaded mode.
As such, <tt>llvm_shutdown()</tt> requires the same isolation guarantees as
<tt>llvm_stop_multithreaded()</tt>.
</p>
<p>
@ -2235,6 +2248,13 @@ Note that, because no other threads are allowed to issue LLVM API calls before
<tt>llvm_start_multithreaded()</tt> returns, it is possible to have
<tt>ManagedStatic</tt>s of <tt>llvm::sys::Mutex</tt>s.
</p>
<p>
The <tt>llvm_acquire_global_lock()</tt> and <tt>llvm_release_global_lock</tt>
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!
</p>
</div>
<!-- *********************************************************************** -->