7 files changed, 99 insertions, 17 deletions
diff --git a/docs/ProgrammersManual.rst b/docs/ProgrammersManual.rst
index a7b28b36ca..df3795a34d 100644
--- a/docs/ProgrammersManual.rst
+++ b/docs/ProgrammersManual.rst
@@ -2170,13 +2170,46 @@ 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:
 
@@ -2184,11 +2217,15 @@ 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.
+
 .. _llvmcontext:
 
 Achieving Isolation with ``LLVMContext``
diff --git a/include/llvm-c/Core.h b/include/llvm-c/Core.h
index 8693a3020a..0e78ed71fa 100644
--- a/include/llvm-c/Core.h
+++ b/include/llvm-c/Core.h
@@ -2848,13 +2848,16 @@ void LLVMDisposePassManager(LLVMPassManagerRef PM);
  * @{
  */
 
-/** Deprecated: Multi-threading can only be enabled/disabled with the compile
-    time define LLVM_ENABLE_THREADS.  This function always returns
-    LLVMIsMultithreaded(). */
+/** Allocate and initialize structures needed to make LLVM safe for
+    multithreading. The return value indicates whether multithreaded
+    initialization succeeded. Must be executed in isolation from all
+    other LLVM api calls.
+    @see llvm::llvm_start_multithreaded */
 LLVMBool LLVMStartMultithreaded(void);
 
-/** Deprecated: Multi-threading can only be enabled/disabled with the compile
-    time define LLVM_ENABLE_THREADS. */
+/** Deallocate structures necessary to make LLVM safe for multithreading.
+    Must be executed in isolation from all other LLVM api calls.
+    @see llvm::llvm_stop_multithreaded */
 void LLVMStopMultithreaded(void);
 
 /** Check whether LLVM is executing in thread-safe mode or not.
diff --git a/include/llvm/Support/ManagedStatic.h b/include/llvm/Support/ManagedStatic.h
index d8fbfeb8e2..1bb8cea092 100644
--- a/include/llvm/Support/ManagedStatic.h
+++ b/include/llvm/Support/ManagedStatic.h
@@ -103,6 +103,9 @@ void llvm_shutdown();
 /// llvm_shutdown() when it is destroyed.
 struct llvm_shutdown_obj {
   llvm_shutdown_obj() { }
+  explicit llvm_shutdown_obj(bool multithreaded) {
+    if (multithreaded) llvm_start_multithreaded();
+  }
   ~llvm_shutdown_obj() { llvm_shutdown(); }
 };
 
diff --git a/include/llvm/Support/Threading.h b/include/llvm/Support/Threading.h
index 7e8758407c..d84d118679 100644
--- a/include/llvm/Support/Threading.h
+++ b/include/llvm/Support/Threading.h
@@ -7,8 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file declares helper functions for running LLVM in a multi-threaded
-// environment.
+// TThis file defines llvm_start_multithreaded() and friends.
 //
 //===----------------------------------------------------------------------===//
 
@@ -16,8 +15,22 @@
 #define LLVM_SUPPORT_THREADING_H
 
 namespace llvm {
-  /// Returns true if LLVM is compiled with support for multi-threading, and
-  /// false otherwise.
+  /// llvm_start_multithreaded - Allocate and initialize structures needed to
+  /// make LLVM safe for multithreading.  The return value indicates whether
+  /// multithreaded initialization succeeded.  LLVM will still be operational
+  /// on "failed" return, and will still be safe for hosting threading
+  /// applications in the JIT, but will not be safe for concurrent calls to the
+  /// LLVM APIs.
+  /// THIS MUST EXECUTE IN ISOLATION FROM ALL OTHER LLVM API CALLS.
+  bool llvm_start_multithreaded();
+
+  /// llvm_stop_multithreaded - Deallocate structures necessary to make LLVM
+  /// safe for multithreading.
+  /// THIS MUST EXECUTE IN ISOLATION FROM ALL OTHER LLVM API CALLS.
+  void llvm_stop_multithreaded();
+
+  /// llvm_is_multithreaded - Check whether LLVM is executing in thread-safe
+  /// mode or not.
   bool llvm_is_multithreaded();
 
   /// llvm_execute_on_thread - Execute the given \p UserFn on a separate
diff --git a/lib/IR/Core.cpp b/lib/IR/Core.cpp
index 0cb781c2c6..2c49d5b949 100644
--- a/lib/IR/Core.cpp
+++ b/lib/IR/Core.cpp
@@ -2708,10 +2708,11 @@ void LLVMDisposePassManager(LLVMPassManagerRef PM) {
 /*===-- Threading ------------------------------------------------------===*/
 
 LLVMBool LLVMStartMultithreaded() {
-  return LLVMIsMultithreaded();
+  return llvm_start_multithreaded();
 }
 
 void LLVMStopMultithreaded() {
+  llvm_stop_multithreaded();
 }
 
 LLVMBool LLVMIsMultithreaded() {
diff --git a/lib/Support/Threading.cpp b/lib/Support/Threading.cpp
index ca7f3f64aa..2358dde0e6 100644
--- a/lib/Support/Threading.cpp
+++ b/lib/Support/Threading.cpp
@@ -7,8 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file defines helper functions for running LLVM in a multi-threaded
-// environment.
+// This file implements llvm_start_multithreaded() and friends.
 //
 //===----------------------------------------------------------------------===//
 
@@ -20,14 +19,38 @@
 
 using namespace llvm;
 
-bool llvm::llvm_is_multithreaded() {
+static bool multithreaded_mode = false;
+
+bool llvm::llvm_start_multithreaded() {
 #if LLVM_ENABLE_THREADS != 0
+  assert(!multithreaded_mode && "Already multithreaded!");
+  multithreaded_mode = true;
+
+  // We fence here to ensure that all initialization is complete BEFORE we
+  // return from llvm_start_multithreaded().
+  sys::MemoryFence();
   return true;
 #else
   return false;
 #endif
 }
 
+void llvm::llvm_stop_multithreaded() {
+#if LLVM_ENABLE_THREADS != 0
+  assert(multithreaded_mode && "Not currently multithreaded!");
+
+  // We fence here to insure that all threaded operations are complete BEFORE we
+  // return from llvm_stop_multithreaded().
+  sys::MemoryFence();
+
+  multithreaded_mode = false;
+#endif
+}
+
+bool llvm::llvm_is_multithreaded() {
+  return multithreaded_mode;
+}
+
 #if LLVM_ENABLE_THREADS != 0 && defined(HAVE_PTHREAD_H)
 #include <pthread.h>
 
diff --git a/unittests/Support/ManagedStatic.cpp b/unittests/Support/ManagedStatic.cpp
index 153884ba42..ad2fc977e6 100644
--- a/unittests/Support/ManagedStatic.cpp
+++ b/unittests/Support/ManagedStatic.cpp
@@ -47,6 +47,7 @@ TEST(Initialize, MultipleThreads) {
   void *p1 = test1::allocate_stack(a1);
   void *p2 = test1::allocate_stack(a2);
 
+  llvm_start_multithreaded();
   pthread_t t1, t2;
   pthread_create(&t1, &a1, test1::helper, nullptr);
   pthread_create(&t2, &a2, test1::helper, nullptr);
@@ -54,6 +55,7 @@ TEST(Initialize, MultipleThreads) {
   pthread_join(t2, nullptr);
   free(p1);
   free(p2);
+  llvm_stop_multithreaded();
 }
 #endif