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6568ff645c
Previously we could not use it because std::once_flag's default constructor was not constexpr. Today, all supported versions of VS correctly mark it constexpr. I confirmed that MSVC 2015 does not emit any problematic racy dynamic initialization code, so we should be safe to use this now. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295013 91177308-0d34-0410-b5e6-96231b3b80d8
133 lines
4.5 KiB
C++
133 lines
4.5 KiB
C++
//===-- llvm/Support/Threading.h - Control multithreading mode --*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file declares helper functions for running LLVM in a multi-threaded
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// environment.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_SUPPORT_THREADING_H
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#define LLVM_SUPPORT_THREADING_H
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#include "llvm/Config/llvm-config.h" // for LLVM_ON_UNIX
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#include "llvm/Support/Compiler.h"
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#include <ciso646> // So we can check the C++ standard lib macros.
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#include <functional>
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#if defined(_MSC_VER)
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// MSVC's call_once implementation worked since VS 2015, which is the minimum
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// supported version as of this writing.
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#define LLVM_THREADING_USE_STD_CALL_ONCE 1
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#elif defined(LLVM_ON_UNIX) && \
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(defined(_LIBCPP_VERSION) || \
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!(defined(__NetBSD__) || defined(__OpenBSD__) || defined(__ppc__)))
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// std::call_once from libc++ is used on all Unix platforms. Other
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// implementations like libstdc++ are known to have problems on NetBSD,
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// OpenBSD and PowerPC.
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#define LLVM_THREADING_USE_STD_CALL_ONCE 1
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#else
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#define LLVM_THREADING_USE_STD_CALL_ONCE 0
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#endif
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#if LLVM_THREADING_USE_STD_CALL_ONCE
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#include <mutex>
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#else
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#include "llvm/Support/Atomic.h"
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#endif
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namespace llvm {
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/// Returns true if LLVM is compiled with support for multi-threading, and
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/// false otherwise.
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bool llvm_is_multithreaded();
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/// llvm_execute_on_thread - Execute the given \p UserFn on a separate
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/// thread, passing it the provided \p UserData and waits for thread
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/// completion.
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///
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/// This function does not guarantee that the code will actually be executed
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/// on a separate thread or honoring the requested stack size, but tries to do
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/// so where system support is available.
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///
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/// \param UserFn - The callback to execute.
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/// \param UserData - An argument to pass to the callback function.
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/// \param RequestedStackSize - If non-zero, a requested size (in bytes) for
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/// the thread stack.
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void llvm_execute_on_thread(void (*UserFn)(void*), void *UserData,
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unsigned RequestedStackSize = 0);
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#if LLVM_THREADING_USE_STD_CALL_ONCE
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typedef std::once_flag once_flag;
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#else
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enum InitStatus { Uninitialized = 0, Wait = 1, Done = 2 };
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/// \brief The llvm::once_flag structure
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///
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/// This type is modeled after std::once_flag to use with llvm::call_once.
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/// This structure must be used as an opaque object. It is a struct to force
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/// autoinitialization and behave like std::once_flag.
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struct once_flag {
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volatile sys::cas_flag status = Uninitialized;
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};
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#endif
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/// \brief Execute the function specified as a parameter once.
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///
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/// Typical usage:
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/// \code
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/// void foo() {...};
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/// ...
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/// static once_flag flag;
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/// call_once(flag, foo);
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/// \endcode
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///
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/// \param flag Flag used for tracking whether or not this has run.
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/// \param F Function to call once.
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template <typename Function, typename... Args>
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void call_once(once_flag &flag, Function &&F, Args &&... ArgList) {
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#if LLVM_THREADING_USE_STD_CALL_ONCE
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std::call_once(flag, std::forward<Function>(F),
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std::forward<Args>(ArgList)...);
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#else
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// For other platforms we use a generic (if brittle) version based on our
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// atomics.
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sys::cas_flag old_val = sys::CompareAndSwap(&flag.status, Wait, Uninitialized);
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if (old_val == Uninitialized) {
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std::forward<Function>(F)(std::forward<Args>(ArgList)...);
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sys::MemoryFence();
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TsanIgnoreWritesBegin();
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TsanHappensBefore(&flag.status);
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flag.status = Done;
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TsanIgnoreWritesEnd();
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} else {
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// Wait until any thread doing the call has finished.
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sys::cas_flag tmp = flag.status;
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sys::MemoryFence();
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while (tmp != Done) {
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tmp = flag.status;
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sys::MemoryFence();
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}
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}
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TsanHappensAfter(&flag.status);
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#endif
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}
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/// Get the amount of currency to use for tasks requiring significant
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/// memory or other resources. Currently based on physical cores, if
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/// available for the host system, otherwise falls back to
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/// thread::hardware_concurrency().
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/// Returns 1 when LLVM is configured with LLVM_ENABLE_THREADS=OFF
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unsigned heavyweight_hardware_concurrency();
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}
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#endif
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