Get rid of llvm_start/stop_multithreading() functions

[zturner]

Summary

With the removal of the LLVM global lock in r211277, these functions are now essentially no-ops that serve no useful purpose.

[rnk]

lgtm

[zturner]

Closed by commit rL211287 (authored by @zturner).

docs/ProgrammersManual.rst

 
 Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic
 intrinsics in order to support threaded operation.  If you need a
 multhreading-capable LLVM on a platform without a suitably modern system
 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:
 
 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``
 ----------------------------------------
 
 ``LLVMContext`` is an opaque class in the LLVM API which clients can use to
 operate multiple, isolated instances of LLVM concurrently within the same
 address space.  For instance, in a hypothetical compile-server, the compilation

include/llvm-c/Core.h

 /**
  * @defgroup LLVMCCoreThreading Threading
  *
  * Handle the structures needed to make LLVM safe for multithreading.
  *
  * @{
  */
 
-/** 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.
     @see llvm::llvm_is_multithreaded */
 LLVMBool LLVMIsMultithreaded(void);
 
 /**
  * @}

include/llvm/Support/ManagedStatic.h

 
 /// llvm_shutdown - Deallocate and destroy all ManagedStatic variables.
 void llvm_shutdown();
 
 /// llvm_shutdown_obj - This is a simple helper class that calls
 /// 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(); }
 };
 
 }
 
 #endif

include/llvm/Support/Threading.h

 //===-- llvm/Support/Threading.h - Control multithreading mode --*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
-// TThis file defines llvm_start_multithreaded() and friends.
+// This file declares helper functions for running LLVM in a multi-threaded
+// environment.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_THREADING_H
 #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();
+#include "llvm/Support/Mutex.h"
 
-  /// llvm_is_multithreaded - Check whether LLVM is executing in thread-safe
-  /// mode or not.
+namespace llvm {
+  /// 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
   /// thread, passing it the provided \p UserData.
   ///
   /// This function does not guarantee that the code will actually be executed
   /// on a separate thread or honoring the requested stack size, but tries to do
   /// so where system support is available.

lib/IR/Core.cpp

 
 void LLVMDisposePassManager(LLVMPassManagerRef PM) {
   delete unwrap(PM);
 }
 
 /*===-- Threading ------------------------------------------------------===*/
 
 LLVMBool LLVMStartMultithreaded() {
-  return llvm_start_multithreaded();
+  return LLVMIsMultithreaded();
 }
 
 void LLVMStopMultithreaded() {
-  llvm_stop_multithreaded();
 }
 
 LLVMBool LLVMIsMultithreaded() {
   return llvm_is_multithreaded();
 }

lib/Support/Threading.cpp

 //===-- llvm/Support/Threading.cpp- Control multithreading mode --*- C++ -*-==//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
-// This file implements llvm_start_multithreaded() and friends.
+// This file defines helper functions for running LLVM in a multi-threaded
+// environment.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/Threading.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/Atomic.h"
 #include "llvm/Support/Mutex.h"
 #include <cassert>
 
 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>
 
 struct ThreadInfo {
   void (*UserFn)(void *);
   void *UserData;
 };
 static void *ExecuteOnThread_Dispatch(void *Arg) {

unittests/Support/ManagedStatic.cpp

 
 TEST(Initialize, MultipleThreads) {
   // Run this test under tsan: http://code.google.com/p/data-race-test/
 
   pthread_attr_t a1, a2;
   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
 
 } // anonymous namespace