1 //===-- llvm/Support/Threading.cpp- Control multithreading mode --*- C++ -*-==// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file defines helper functions for running LLVM in a multi-threaded 10 // environment. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Support/Threading.h" 15 #include "llvm/ADT/Optional.h" 16 #include "llvm/Config/config.h" 17 #include "llvm/Support/Host.h" 18 19 #include <cassert> 20 #include <errno.h> 21 #include <stdlib.h> 22 #include <string.h> 23 24 using namespace llvm; 25 26 //===----------------------------------------------------------------------===// 27 //=== WARNING: Implementation here must contain only TRULY operating system 28 //=== independent code. 29 //===----------------------------------------------------------------------===// 30 31 bool llvm::llvm_is_multithreaded() { 32 #if LLVM_ENABLE_THREADS != 0 33 return true; 34 #else 35 return false; 36 #endif 37 } 38 39 #if LLVM_ENABLE_THREADS == 0 || \ 40 (!defined(_WIN32) && !defined(HAVE_PTHREAD_H)) 41 uint64_t llvm::get_threadid() { return 0; } 42 43 uint32_t llvm::get_max_thread_name_length() { return 0; } 44 45 void llvm::set_thread_name(const Twine &Name) {} 46 47 void llvm::get_thread_name(SmallVectorImpl<char> &Name) { Name.clear(); } 48 49 llvm::BitVector llvm::get_thread_affinity_mask() { return {}; } 50 51 unsigned llvm::ThreadPoolStrategy::compute_thread_count() const { 52 // When threads are disabled, ensure clients will loop at least once. 53 return 1; 54 } 55 56 #else 57 58 int computeHostNumHardwareThreads(); 59 60 unsigned llvm::ThreadPoolStrategy::compute_thread_count() const { 61 int MaxThreadCount = UseHyperThreads ? computeHostNumHardwareThreads() 62 : sys::getHostNumPhysicalCores(); 63 if (MaxThreadCount <= 0) 64 MaxThreadCount = 1; 65 // Damage control threading. 66 // 67 // There are no heuristics to figure out how many threads makes sense to spawn, 68 // all while rolling with all available hw threads starts being detrimental to 69 // performance really early. 70 // 71 // Work around by putting a hard cap unless the user explicitly requested a certain amount. 72 // 73 // See https://discourse.llvm.org/t/avoidable-overhead-from-threading-by-default/69160 74 // for more details. 75 if (ThreadsRequested == 0) { 76 return std::min(MaxThreadCount, 4); 77 } 78 if (!Limit) 79 return ThreadsRequested; 80 return std::min((unsigned)MaxThreadCount, ThreadsRequested); 81 } 82 83 // Include the platform-specific parts of this class. 84 #ifdef LLVM_ON_UNIX 85 #include "Unix/Threading.inc" 86 #endif 87 #ifdef _WIN32 88 #include "Windows/Threading.inc" 89 #endif 90 91 // Must be included after Threading.inc to provide definition for llvm::thread 92 // because FreeBSD's condvar.h (included by user.h) misuses the "thread" 93 // keyword. 94 #include "llvm/Support/thread.h" 95 96 #if defined(__APPLE__) 97 // Darwin's default stack size for threads except the main one is only 512KB, 98 // which is not enough for some/many normal LLVM compilations. This implements 99 // the same interface as std::thread but requests the same stack size as the 100 // main thread (8MB) before creation. 101 const llvm::Optional<unsigned> llvm::thread::DefaultStackSize = 8 * 1024 * 1024; 102 #else 103 const llvm::Optional<unsigned> llvm::thread::DefaultStackSize = None; 104 #endif 105 106 107 #endif 108 109 Optional<ThreadPoolStrategy> 110 llvm::get_threadpool_strategy(StringRef Num, ThreadPoolStrategy Default) { 111 if (Num == "all") 112 return llvm::hardware_concurrency(); 113 if (Num.empty()) 114 return Default; 115 unsigned V; 116 if (Num.getAsInteger(10, V)) 117 return None; // malformed 'Num' value 118 if (V == 0) 119 return Default; 120 121 // Do not take the Default into account. This effectively disables 122 // heavyweight_hardware_concurrency() if the user asks for any number of 123 // threads on the cmd-line. 124 ThreadPoolStrategy S = llvm::hardware_concurrency(); 125 S.ThreadsRequested = V; 126 return S; 127 } 128