Threading.h revision 314564 
1//===-- llvm/Support/Threading.h - Control multithreading mode --*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file declares helper functions for running LLVM in a multi-threaded
11// environment.
12//
13//===----------------------------------------------------------------------===//
14
15#ifndef LLVM_SUPPORT_THREADING_H
16#define LLVM_SUPPORT_THREADING_H
17
18#include "llvm/Config/llvm-config.h" // for LLVM_ON_UNIX
19#include "llvm/Support/Compiler.h"
20#include <ciso646> // So we can check the C++ standard lib macros.
21#include <functional>
22
23// std::call_once from libc++ is used on all Unix platforms. Other
24// implementations like libstdc++ are known to have problems on NetBSD,
25// OpenBSD and PowerPC.
26#if defined(LLVM_ON_UNIX) && (defined(_LIBCPP_VERSION) ||                      \
27    !(defined(__NetBSD__) || defined(__OpenBSD__) || defined(__ppc__)))
28#define LLVM_THREADING_USE_STD_CALL_ONCE 1
29#else
30#define LLVM_THREADING_USE_STD_CALL_ONCE 0
31#endif
32
33#if LLVM_THREADING_USE_STD_CALL_ONCE
34#include <mutex>
35#else
36#include "llvm/Support/Atomic.h"
37#endif
38
39namespace llvm {
40  /// Returns true if LLVM is compiled with support for multi-threading, and
41  /// false otherwise.
42  bool llvm_is_multithreaded();
43
44  /// llvm_execute_on_thread - Execute the given \p UserFn on a separate
45  /// thread, passing it the provided \p UserData and waits for thread
46  /// completion.
47  ///
48  /// This function does not guarantee that the code will actually be executed
49  /// on a separate thread or honoring the requested stack size, but tries to do
50  /// so where system support is available.
51  ///
52  /// \param UserFn - The callback to execute.
53  /// \param UserData - An argument to pass to the callback function.
54  /// \param RequestedStackSize - If non-zero, a requested size (in bytes) for
55  /// the thread stack.
56  void llvm_execute_on_thread(void (*UserFn)(void*), void *UserData,
57                              unsigned RequestedStackSize = 0);
58
59#if LLVM_THREADING_USE_STD_CALL_ONCE
60
61  typedef std::once_flag once_flag;
62
63  /// This macro is the only way you should define your once flag for LLVM's
64  /// call_once.
65#define LLVM_DEFINE_ONCE_FLAG(flag) static once_flag flag
66
67#else
68
69  enum InitStatus { Uninitialized = 0, Wait = 1, Done = 2 };
70  typedef volatile sys::cas_flag once_flag;
71
72  /// This macro is the only way you should define your once flag for LLVM's
73  /// call_once.
74#define LLVM_DEFINE_ONCE_FLAG(flag) static once_flag flag = Uninitialized
75
76#endif
77
78  /// \brief Execute the function specified as a parameter once.
79  ///
80  /// Typical usage:
81  /// \code
82  ///   void foo() {...};
83  ///   ...
84  ///   LLVM_DEFINE_ONCE_FLAG(flag);
85  ///   call_once(flag, foo);
86  /// \endcode
87  ///
88  /// \param flag Flag used for tracking whether or not this has run.
89  /// \param F Function to call once.
90  template <typename Function, typename... Args>
91  void call_once(once_flag &flag, Function &&F, Args &&... ArgList) {
92#if LLVM_THREADING_USE_STD_CALL_ONCE
93    std::call_once(flag, std::forward<Function>(F),
94                   std::forward<Args>(ArgList)...);
95#else
96    // For other platforms we use a generic (if brittle) version based on our
97    // atomics.
98    sys::cas_flag old_val = sys::CompareAndSwap(&flag, Wait, Uninitialized);
99    if (old_val == Uninitialized) {
100      std::forward<Function>(F)(std::forward<Args>(ArgList)...);
101      sys::MemoryFence();
102      TsanIgnoreWritesBegin();
103      TsanHappensBefore(&flag);
104      flag = Done;
105      TsanIgnoreWritesEnd();
106    } else {
107      // Wait until any thread doing the call has finished.
108      sys::cas_flag tmp = flag;
109      sys::MemoryFence();
110      while (tmp != Done) {
111        tmp = flag;
112        sys::MemoryFence();
113      }
114    }
115    TsanHappensAfter(&flag);
116#endif
117  }
118
119  /// Get the amount of currency to use for tasks requiring significant
120  /// memory or other resources. Currently based on physical cores, if
121  /// available for the host system, otherwise falls back to
122  /// thread::hardware_concurrency().
123  /// Returns 1 when LLVM is configured with LLVM_ENABLE_THREADS=OFF
124  unsigned heavyweight_hardware_concurrency();
125}
126
127#endif
128