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

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``

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.

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(); }
 };
 

include/llvm/Support/Mutex.h

@@ -15,13 +15,11 @@
 #define LLVM_SUPPORT_MUTEX_H
 
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/Threading.h"
 #include <cassert>
 
 namespace llvm
 {
-  // Forward declare.
-  bool llvm_is_multithreaded();
-
   namespace sys
   {
     /// @brief Platform agnostic Mutex class.

include/llvm/Support/Threading.h

@@ -7,21 +7,40 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file defines helper functions for running LLVM in a multi-threaded
-// environment.
+// TThis file defines llvm_start_multithreaded() and friends.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_THREADING_H
 #define LLVM_SUPPORT_THREADING_H
 
-#include "llvm/Support/Mutex.h"
-
 namespace llvm {
-  /// llvm_is_multithreaded - returns true if LLVM is compiled with support
-  /// for multiple threads, 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();
 
+  /// acquire_global_lock - Acquire the global lock.  This is a no-op if called
+  /// before llvm_start_multithreaded().
+  void llvm_acquire_global_lock();
+
+  /// release_global_lock - Release the global lock.  This is a no-op if called
+  /// before llvm_start_multithreaded().
+  void llvm_release_global_lock();
+
   /// llvm_execute_on_thread - Execute the given \p UserFn on a separate
   /// thread, passing it the provided \p UserData.
   ///

lib/IR/Core.cpp

@@ -2702,10 +2702,11 @@ void LLVMDisposePassManager(LLVMPassManagerRef PM) {
 /*===-- Threading ------------------------------------------------------===*/
 
 LLVMBool LLVMStartMultithreaded() {
-  return LLVMIsMultithreaded();
+  return llvm_start_multithreaded();
 }
 
 void LLVMStopMultithreaded() {
+  llvm_stop_multithreaded();
 }
 
 LLVMBool LLVMIsMultithreaded() {

lib/Support/ManagedStatic.cpp

@@ -14,36 +14,17 @@
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/Atomic.h"
-#include "llvm/Support/MutexGuard.h"
 #include <cassert>
-#include <mutex>
 using namespace llvm;
 
 static const ManagedStaticBase *StaticList = nullptr;
 
-// ManagedStatics can get created during execution of static constructors.  As a
-// result, we cannot use a global static std::mutex object for the lock since it
-// may not have been constructed.  Instead, we do a call-once initialization of
-// a pointer to a mutex.  This also means that we must not "initialize" the
-// mutex with nullptr, otherwise it might get reset to nullptr after being
-// initialized by std::call_once.
-static std::once_flag MutexInitializationFlag;
-static std::recursive_mutex *ManagedStaticMutex;
-
-namespace {
-  void InitializeManagedStaticMutex() {
-    std::call_once(MutexInitializationFlag,
-        []() { ManagedStaticMutex = new std::recursive_mutex(); });
-  }
-}
-
 void ManagedStaticBase::RegisterManagedStatic(void *(*Creator)(),
                                               void (*Deleter)(void*)) const {
   assert(Creator);
   if (llvm_is_multithreaded()) {
-    InitializeManagedStaticMutex();
+    llvm_acquire_global_lock();
 
-    std::lock_guard<std::recursive_mutex> Lock(*ManagedStaticMutex);
     if (!Ptr) {
       void* tmp = Creator();
 
@@ -62,6 +43,8 @@ void ManagedStaticBase::RegisterManagedStatic(void *(*Creator)(),
       Next = StaticList;
       StaticList = this;
     }
+
+    llvm_release_global_lock();
   } else {
     assert(!Ptr && !DeleterFn && !Next &&
            "Partially initialized ManagedStatic!?");
@@ -92,9 +75,8 @@ void ManagedStaticBase::destroy() const {
 
 /// llvm_shutdown - Deallocate and destroy all ManagedStatic variables.
 void llvm::llvm_shutdown() {
-  InitializeManagedStaticMutex();
-  std::lock_guard<std::recursive_mutex> Lock(*ManagedStaticMutex);
-
   while (StaticList)
     StaticList->destroy();
+
+  if (llvm_is_multithreaded()) llvm_stop_multithreaded();
 }

lib/Support/Threading.cpp

@@ -7,8 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file implements helper functions for running LLVM in a multi-threaded
-// environment.
+// This file implements llvm_start_multithreaded() and friends.
 //
 //===----------------------------------------------------------------------===//
 
@@ -20,14 +19,50 @@
 
 using namespace llvm;
 
-bool llvm::llvm_is_multithreaded() {
+static bool multithreaded_mode = false;
+
+static sys::Mutex* global_lock = nullptr;
+
+bool llvm::llvm_start_multithreaded() {
 #if LLVM_ENABLE_THREADS != 0
+  assert(!multithreaded_mode && "Already multithreaded!");
+  multithreaded_mode = true;
+  global_lock = new sys::Mutex(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;
+  delete global_lock;
+#endif
+}
+
+bool llvm::llvm_is_multithreaded() {
+  return multithreaded_mode;
+}
+
+void llvm::llvm_acquire_global_lock() {
+  if (multithreaded_mode) global_lock->acquire();
+}
+
+void llvm::llvm_release_global_lock() {
+  if (multithreaded_mode) global_lock->release();
+}
+
 #if LLVM_ENABLE_THREADS != 0 && defined(HAVE_PTHREAD_H)
 #include <pthread.h>
 

lib/Support/Timer.cpp

@@ -18,7 +18,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/ManagedStatic.h"
-#include "llvm/Support/MutexGuard.h"
+#include "llvm/Support/Mutex.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
@@ -84,13 +84,14 @@ static TimerGroup *getDefaultTimerGroup() {
   sys::MemoryFence();
   if (tmp) return tmp;
 
-  sys::SmartScopedLock<true> Lock(*TimerLock);
+  llvm_acquire_global_lock();
   tmp = DefaultTimerGroup;
   if (!tmp) {
     tmp = new TimerGroup("Miscellaneous Ungrouped Timers");
     sys::MemoryFence();
     DefaultTimerGroup = tmp;
   }
+  llvm_release_global_lock();
 
   return tmp;
 }

unittests/Support/ManagedStatic.cpp

@@ -47,13 +47,15 @@ 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);
   pthread_join(t1, nullptr);
   pthread_join(t2, nullptr);
   free(p1);
   free(p2);
+  llvm_stop_multithreaded();
 }
 #endif