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//===-- llvm/Support/Threading.h - Control multithreading mode --*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file declares helper functions for running LLVM in a multi-threaded
// environment.
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/FunctionExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Config/llvm-config.h" // for LLVM_ON_UNIX
#include "llvm/Support/Compiler.h"
#include <ciso646> // So we can check the C++ standard lib macros.
#include <functional>
#if defined(_MSC_VER)
// MSVC's call_once implementation worked since VS 2015, which is the minimum
// supported version as of this writing.
#elif defined(LLVM_ON_UNIX) && \
(defined(_LIBCPP_VERSION) || \
!(defined(__NetBSD__) || defined(__OpenBSD__) || \
(defined(__ppc__) || defined(__PPC__))))
// std::call_once from libc++ is used on all Unix platforms. Other
// implementations like libstdc++ are known to have problems on NetBSD,
// OpenBSD and PowerPC.
#elif defined(LLVM_ON_UNIX) && \
((defined(__ppc__) || defined(__PPC__)) && defined(__LITTLE_ENDIAN__))
#include <mutex>
#include "llvm/Support/Atomic.h"
namespace llvm {
class Twine;
/// Returns true if LLVM is compiled with support for multi-threading, and
/// false otherwise.
bool llvm_is_multithreaded();
typedef std::once_flag once_flag;
enum InitStatus { Uninitialized = 0, Wait = 1, Done = 2 };
/// The llvm::once_flag structure
/// This type is modeled after std::once_flag to use with llvm::call_once.
/// This structure must be used as an opaque object. It is a struct to force
/// autoinitialization and behave like std::once_flag.
struct once_flag {
volatile sys::cas_flag status = Uninitialized;
/// Execute the function specified as a parameter once.
/// Typical usage:
/// \code
/// void foo() {...};
/// ...
/// static once_flag flag;
/// call_once(flag, foo);
/// \endcode
/// \param flag Flag used for tracking whether or not this has run.
/// \param F Function to call once.
template <typename Function, typename... Args>
void call_once(once_flag &flag, Function &&F, Args &&... ArgList) {
std::call_once(flag, std::forward<Function>(F),
// For other platforms we use a generic (if brittle) version based on our
// atomics.
sys::cas_flag old_val = sys::CompareAndSwap(&flag.status, Wait, Uninitialized);
if (old_val == Uninitialized) {
flag.status = Done;
} else {
// Wait until any thread doing the call has finished.
sys::cas_flag tmp = flag.status;
while (tmp != Done) {
tmp = flag.status;
/// This tells how a thread pool will be used
class ThreadPoolStrategy {
// The default value (0) means all available threads should be used,
// taking the affinity mask into account. If set, this value only represents
// a suggested high bound, the runtime might choose a lower value (not
// higher).
unsigned ThreadsRequested = 0;
// If SMT is active, use hyper threads. If false, there will be only one
// std::thread per core.
bool UseHyperThreads = true;
// If set, will constrain 'ThreadsRequested' to the number of hardware
// threads, or hardware cores.
bool Limit = false;
/// Retrieves the max available threads for the current strategy. This
/// accounts for affinity masks and takes advantage of all CPU sockets.
unsigned compute_thread_count() const;
/// Assign the current thread to an ideal hardware CPU or NUMA node. In a
/// multi-socket system, this ensures threads are assigned to all CPU
/// sockets. \p ThreadPoolNum represents a number bounded by [0,
/// compute_thread_count()).
void apply_thread_strategy(unsigned ThreadPoolNum) const;
/// Finds the CPU socket where a thread should go. Returns 'None' if the
/// thread shall remain on the actual CPU socket.
Optional<unsigned> compute_cpu_socket(unsigned ThreadPoolNum) const;
/// Build a strategy from a number of threads as a string provided in \p Num.
/// When Num is above the max number of threads specified by the \p Default
/// strategy, we attempt to equally allocate the threads on all CPU sockets.
/// "0" or an empty string will return the \p Default strategy.
/// "all" for using all hardware threads.
get_threadpool_strategy(StringRef Num, ThreadPoolStrategy Default = {});
/// Returns a thread strategy for tasks requiring significant memory or other
/// resources. To be used for workloads where hardware_concurrency() proves to
/// be less efficient. Avoid this strategy if doing lots of I/O. Currently
/// based on physical cores, if available for the host system, otherwise falls
/// back to hardware_concurrency(). Returns 1 when LLVM is configured with
inline ThreadPoolStrategy
heavyweight_hardware_concurrency(unsigned ThreadCount = 0) {
ThreadPoolStrategy S;
S.UseHyperThreads = false;
S.ThreadsRequested = ThreadCount;
return S;
/// Like heavyweight_hardware_concurrency() above, but builds a strategy
/// based on the rules described for get_threadpool_strategy().
/// If \p Num is invalid, returns a default strategy where one thread per
/// hardware core is used.
inline ThreadPoolStrategy heavyweight_hardware_concurrency(StringRef Num) {
Optional<ThreadPoolStrategy> S =
get_threadpool_strategy(Num, heavyweight_hardware_concurrency());
if (S)
return *S;
return heavyweight_hardware_concurrency();
/// Returns a default thread strategy where all available hardware resources
/// are to be used, except for those initially excluded by an affinity mask.
/// This function takes affinity into consideration. Returns 1 when LLVM is
/// configured with LLVM_ENABLE_THREADS=OFF.
inline ThreadPoolStrategy hardware_concurrency(unsigned ThreadCount = 0) {
ThreadPoolStrategy S;
S.ThreadsRequested = ThreadCount;
return S;
/// Returns an optimal thread strategy to execute specified amount of tasks.
/// This strategy should prevent us from creating too many threads if we
/// occasionaly have an unexpectedly small amount of tasks.
inline ThreadPoolStrategy optimal_concurrency(unsigned TaskCount = 0) {
ThreadPoolStrategy S;
S.Limit = true;
S.ThreadsRequested = TaskCount;
return S;
/// Return the current thread id, as used in various OS system calls.
/// Note that not all platforms guarantee that the value returned will be
/// unique across the entire system, so portable code should not assume
/// this.
uint64_t get_threadid();
/// Get the maximum length of a thread name on this platform.
/// A value of 0 means there is no limit.
uint32_t get_max_thread_name_length();
/// Set the name of the current thread. Setting a thread's name can
/// be helpful for enabling useful diagnostics under a debugger or when
/// logging. The level of support for setting a thread's name varies
/// wildly across operating systems, and we only make a best effort to
/// perform the operation on supported platforms. No indication of success
/// or failure is returned.
void set_thread_name(const Twine &Name);
/// Get the name of the current thread. The level of support for
/// getting a thread's name varies wildly across operating systems, and it
/// is not even guaranteed that if you can successfully set a thread's name
/// that you can later get it back. This function is intended for diagnostic
/// purposes, and as with setting a thread's name no indication of whether
/// the operation succeeded or failed is returned.
void get_thread_name(SmallVectorImpl<char> &Name);
/// Returns a mask that represents on which hardware thread, core, CPU, NUMA
/// group, the calling thread can be executed. On Windows, threads cannot
/// cross CPU sockets boundaries.
llvm::BitVector get_thread_affinity_mask();
/// Returns how many physical CPUs or NUMA groups the system has.
unsigned get_cpus();
enum class ThreadPriority {
Background = 0,
Default = 1,
/// If priority is Background tries to lower current threads priority such
/// that it does not affect foreground tasks significantly. Can be used for
/// long-running, latency-insensitive tasks to make sure cpu is not hogged by
/// this task.
/// If the priority is default tries to restore current threads priority to
/// default scheduling priority.
enum class SetThreadPriorityResult { FAILURE, SUCCESS };
SetThreadPriorityResult set_thread_priority(ThreadPriority Priority);