Revert "Revert 220932.": "Removing the static initializer in ManagedStatic.cpp by using llvm_call_once to initialize the ManagedStatic mutex"

This reverts commit r221331 and reinstate r220932 as discussed in D19271.
Original commit message was:

This patch adds an llvm_call_once which is a wrapper around
std::call_once on platforms where it is available and devoid
of bugs. The patch also migrates the ManagedStatic mutex to
be allocated using llvm_call_once.

These changes are philosophically equivalent to the changes
added in r219638, which were reverted due to a hang on Win32
which was the result of a bug in the Windows implementation
of std::call_once.

Differential Revision: http://reviews.llvm.org/D5922

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 269577

include/llvm/Support/Threading.h

@@ -15,6 +15,14 @@
 #ifndef LLVM_SUPPORT_THREADING_H
 #define LLVM_SUPPORT_THREADING_H
 
+#include "llvm/Config/llvm-config.h" // for LLVM_ON_UNIX
+
+#if defined(LLVM_ON_UNIX)
+#include <mutex>
+#else
+#include "llvm/Support/Atomic.h"
+#endif
+
 namespace llvm {
   /// Returns true if LLVM is compiled with support for multi-threading, and
   /// false otherwise.
@@ -34,6 +42,34 @@ namespace llvm {
   /// the thread stack.
   void llvm_execute_on_thread(void (*UserFn)(void*), void *UserData,
                               unsigned RequestedStackSize = 0);
+
+#if defined(LLVM_ON_UNIX)
+typedef std::once_flag once_flag;
+#define LLVM_DEFINE_ONCE_FLAG(flag) static once_flag flag
+#else
+enum InitStatus {
+  Done = -1,
+  Uninitialized = 0,
+  Wait = 1
+};
+typedef volatile sys::cas_flag once_flag;
+
+#define LLVM_DEFINE_ONCE_FLAG(flag) static once_flag flag = Uninitialized
+#endif
+
+/// \brief Execute the function specified as a parameter once.
+///
+/// Typical usage:
+/// \code
+///   void foo() {...};
+///   ...
+///   LLVM_DEFINE_ONCE_FLAG(flag);
+///   call_once(flag, foo);
+/// \endcode
+///
+/// \param flag Flag used for tracking whether or not this has run.
+/// \param UserFn Function to call once.
+void call_once(once_flag &flag, void (*UserFn)(void));
 }
 
 #endif

lib/Support/ManagedStatic.cpp

@@ -16,16 +16,23 @@
 #include "llvm/Support/Atomic.h"
 #include "llvm/Support/Mutex.h"
 #include "llvm/Support/MutexGuard.h"
+#include "llvm/Support/Threading.h"
 #include <cassert>
 using namespace llvm;
 
 static const ManagedStaticBase *StaticList = nullptr;
+static sys::Mutex *ManagedStaticMutex = nullptr;
+LLVM_DEFINE_ONCE_FLAG(mutex_init_flag);
 
-static sys::Mutex& getManagedStaticMutex() {
+static void initializeMutex() {
+  ManagedStaticMutex = new sys::Mutex();
+}
+
+static sys::Mutex* getManagedStaticMutex() {
   // We need to use a function local static here, since this can get called
   // during a static constructor and we need to guarantee that it's initialized
   // correctly.
-  static sys::Mutex ManagedStaticMutex;
+  call_once(mutex_init_flag, initializeMutex);
   return ManagedStaticMutex;
 }
 
@@ -33,7 +40,7 @@ void ManagedStaticBase::RegisterManagedStatic(void *(*Creator)(),
                                               void (*Deleter)(void*)) const {
   assert(Creator);
   if (llvm_is_multithreaded()) {
-    MutexGuard Lock(getManagedStaticMutex());
+    MutexGuard Lock(*getManagedStaticMutex());
 
     if (!Ptr) {
       void* tmp = Creator();
@@ -83,7 +90,7 @@ void ManagedStaticBase::destroy() const {
 
 /// llvm_shutdown - Deallocate and destroy all ManagedStatic variables.
 void llvm::llvm_shutdown() {
-  MutexGuard Lock(getManagedStaticMutex());
+  MutexGuard Lock(*getManagedStaticMutex());
 
   while (StaticList)
     StaticList->destroy();

lib/Support/Threading.cpp

@@ -110,3 +110,10 @@ void llvm::llvm_execute_on_thread(void (*Fn)(void*), void *UserData,
 }
 
 #endif
+
+#if defined(LLVM_ON_UNIX)
+#include "Unix/Threading.inc"
+#else
+#include "Windows/Threading.inc"
+#endif
+

lib/Support/Unix/Threading.inc

@@ -0,0 +1,5 @@
+#include <thread>
+
+void llvm::call_once(once_flag& flag, void (*fptr)(void)) {
+  std::call_once(flag, fptr);
+}

lib/Support/Windows/Threading.inc

@@ -0,0 +1,24 @@
+ #include <windows.h>
+
+#ifdef MemoryFence
+// WinNT.h seems to define a MemoryFence macro.
+#undef MemoryFence
+#endif
+
+void llvm::call_once(once_flag &flag, void (*fptr)(void)) {
+  while (flag != Done) {
+    if (flag == Wait) {
+      ::Sleep(1);
+      continue;
+    }
+
+    sys::cas_flag old_val = sys::CompareAndSwap(&flag, Wait, Uninitialized);
+    if (old_val == Uninitialized) {
+      fptr();
+      sys::MemoryFence();
+      flag = Done;
+      return;
+    }
+  }
+  sys::MemoryFence();
+}