Remove support for LLVM runtime multi-threading.

After a number of previous small iterations, the functions
llvm_start_multithreaded() and llvm_stop_multithreaded() have
been reduced essentially to no-ops.  This change removes them
entirely.

Reviewed by: rnk, dblaikie

Differential Revision: http://reviews.llvm.org/D4216

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211287 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ProgrammersManual.rst

@@ -2170,46 +2170,13 @@ 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.  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.
+deallocate memory used for internal structures.
 
 .. _managedstatic:
 
@@ -2217,15 +2184,11 @@ 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.  Before the
-invocation of ``llvm_shutdown()``, it implements a simple lazy initialization
-scheme.  Once ``llvm_start_multithreaded()`` returns, however, it uses
+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
 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.
-
 .. _llvmcontext:
 
 Achieving Isolation with ``LLVMContext``