Re-apply r211287: Remove support for LLVM runtime multi-threading.

I'll fix the problems in libclang and other projects in ways that don't
require <mutex> until we sort out the cygwin situation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211900 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``

include/llvm-c/Core.h

@@ -2848,16 +2848,13 @@ void LLVMDisposePassManager(LLVMPassManagerRef PM);
  * @{
  */
 
-/** 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 */
+/** Deprecated: Multi-threading can only be enabled/disabled with the compile
+    time define LLVM_ENABLE_THREADS.  This function always returns
+    LLVMIsMultithreaded(). */
 LLVMBool LLVMStartMultithreaded(void);
 
-/** 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 */
+/** Deprecated: Multi-threading can only be enabled/disabled with the compile
+    time define LLVM_ENABLE_THREADS. */
 void LLVMStopMultithreaded(void);
 
 /** Check whether LLVM is executing in thread-safe mode or not.

include/llvm/Support/ManagedStatic.h

@@ -103,9 +103,6 @@ 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/Threading.h

@@ -7,7 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// TThis file defines llvm_start_multithreaded() and friends.
+// This file declares helper functions for running LLVM in a multi-threaded
+// environment.
 //
 //===----------------------------------------------------------------------===//
 
@@ -15,22 +16,8 @@
 #define LLVM_SUPPORT_THREADING_H
 
 namespace llvm {
-  /// 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.
+  /// Returns true if LLVM is compiled with support for multi-threading, and
+  /// false otherwise.
   bool llvm_is_multithreaded();
 
   /// llvm_execute_on_thread - Execute the given \p UserFn on a separate

lib/IR/Core.cpp

@@ -2708,11 +2708,10 @@ void LLVMDisposePassManager(LLVMPassManagerRef PM) {
 /*===-- Threading ------------------------------------------------------===*/
 
 LLVMBool LLVMStartMultithreaded() {
-  return llvm_start_multithreaded();
+  return LLVMIsMultithreaded();
 }
 
 void LLVMStopMultithreaded() {
-  llvm_stop_multithreaded();
 }
 
 LLVMBool LLVMIsMultithreaded() {

lib/Support/Threading.cpp

@@ -7,7 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file implements llvm_start_multithreaded() and friends.
+// This file defines helper functions for running LLVM in a multi-threaded
+// environment.
 //
 //===----------------------------------------------------------------------===//
 
@@ -19,38 +20,14 @@
 
 using namespace llvm;
 
-static bool multithreaded_mode = false;
-
-bool llvm::llvm_start_multithreaded() {
+bool llvm::llvm_is_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>
 

unittests/Support/ManagedStatic.cpp

@@ -47,7 +47,6 @@ 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);
@@ -55,7 +54,6 @@ TEST(Initialize, MultipleThreads) {
   pthread_join(t2, nullptr);
   free(p1);
   free(p2);
-  llvm_stop_multithreaded();
 }
 #endif