Remove support for runtime multithreading

[zturner]

Summary

This is a resubmit of r210600 (Differential Revision D4076). This probably only needs a quick glance since the previous change was already reviewed, however this change differs from the previous submission in 2 important ways:

1. Conversions from sys::Mutex to std::[recursive_]mutex are removed, to reduce the scope of the changes. Those will be submitted later as a separate changelist.

2. Making ErrorHandling.cpp thread-safe was handled as a separate changelist (see D4140)

[chandlerc]

lib/Support/ErrorHandling.cpp

70–80 ↗

(On Diff #10399)

All of this feels like a totally separate thing from removing the runtime control of multithreading.

Can you try to split this up into more isolated changes? I feel like this is still a combined patch of several things that really should be submitted independently.

[emaste]

lib/Support/ErrorHandling.cpp

74 ↗

(On Diff #10399)

To be pedantic, all implementations exhibit undefined behaviour if a mutex destroyed while owned, although the nature of that behavior may be different.

[zturner]

lib/Support/ErrorHandling.cpp

70–80 ↗

(On Diff #10399)

Fair enough. Although if I'm going to do it that way, the error handling functions have to be made thread-safe first, and then the removal of llvm_start / stop multithreading changes have to go in second. Which is fine, so I'll do that.

74 ↗

(On Diff #10399)

I'm actually submitting this *before* converting to std::*mutex though. Our own mutex implementation doesn't exhibit undefined behavior. std::mutex implementations do though.

[zturner]

Changes to ErrorHandling.cpp uploaded as a separate change (D4140). That change should be submitted first.

[chandlerc]

One more thing to split out, and I think this will actually look good.

lib/Support/ManagedStatic.cpp

27

This (and the related change to the interface) is the other unrelated cleanup that I would like to see separated out from the change which has policy effect.

Consider, if we need to revert the runtime control of multithreading for a brief period because something breaks, we shouldn't also lose this cleanup change.

[zturner]

I tried this once and ran into some difficulty, because before this change,
the Mutex is a pointer which null is an expected value if
llvm_start_multithreaded has not been called. So the function would have
to return a pointer to the mutex. That would require changing MutexGuard
to take a pointer instead of a reference, which would generate a ton of
churn.

I'll have another stab at it and see if there's a way to do it in a way
that has very little churn and still contributes to this change as well.

[zturner]

Removed code that had been split out into a different patch.

include/llvm-c/Core.h

  * @{
  */
 
-/** 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

 /// 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

 #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

 //
 //===----------------------------------------------------------------------===//
 //
-// TThis file defines llvm_start_multithreaded() and friends.
+// This file defines helper functions for running LLVM in a multi-threaded
+// environment.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_THREADING_H
 #define LLVM_SUPPORT_THREADING_H
 
+#include "llvm/Support/Mutex.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_get_global_lock() - returns the llvm global lock object.
+  sys::Mutex &llvm_get_global_lock();
 
-  /// llvm_is_multithreaded - Check whether LLVM is executing in thread-safe
-  /// mode or not.
+  /// llvm_is_multithreaded() - returns true if LLVM is compiled with support
+  /// for multiple threads, and false otherwise.
   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

 /*===-- Threading ------------------------------------------------------===*/
 
 LLVMBool LLVMStartMultithreaded() {
-  return llvm_start_multithreaded();
+  return LLVMIsMultithreaded();
 }
 
 void LLVMStopMultithreaded() {
-  llvm_stop_multithreaded();
 }
 
 LLVMBool LLVMIsMultithreaded() {

lib/Support/ManagedStatic.cpp

 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/Atomic.h"
+#include "llvm/Support/MutexGuard.h"
 #include <cassert>
 using namespace llvm;
 

                                               void (*Deleter)(void*)) const {
   assert(Creator);
   if (llvm_is_multithreaded()) {
-    llvm_acquire_global_lock();
+    llvm::MutexGuard Lock(llvm_get_global_lock());
 
     if (!Ptr) {
       void* tmp = Creator();

       StaticList = this;
     }
 
-    llvm_release_global_lock();
   } else {
     assert(!Ptr && !DeleterFn && !Next &&
            "Partially initialized ManagedStatic!?");

 void llvm::llvm_shutdown() {
   while (StaticList)
     StaticList->destroy();
-
-  if (llvm_is_multithreaded()) llvm_stop_multithreaded();
 }

lib/Support/Threading.cpp

 //
 //===----------------------------------------------------------------------===//
 //
-// This file implements llvm_start_multithreaded() and friends.
+// This file implements helper functions for running LLVM in a multi-threaded
+// environment.
 //
 //===----------------------------------------------------------------------===//
 

 
 using namespace llvm;
 
-static bool multithreaded_mode = false;
-
-static sys::Mutex* global_lock = nullptr;
+sys::Mutex& llvm::llvm_get_global_lock() {
+  static sys::Mutex global_lock;
+  return global_lock;
+}
 
-bool llvm::llvm_start_multithreaded() {
+bool llvm::llvm_is_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

 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Format.h"
 #include "llvm/Support/ManagedStatic.h"
-#include "llvm/Support/Mutex.h"
+#include "llvm/Support/MutexGuard.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;

   sys::MemoryFence();
   if (tmp) return tmp;
   
-  llvm_acquire_global_lock();
+  llvm::MutexGuard Lock(llvm::llvm_get_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

   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(t2, nullptr);
   free(p1);
   free(p2);
-  llvm_stop_multithreaded();
 }
 #endif