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/Config/config.h" 16 #include "llvm/Config/llvm-config.h" 17 18 #include <cassert> 19 #include <errno.h> 20 #include <optional> 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 #if LLVM_ENABLE_THREADS == 0 || \ 32 (!defined(_WIN32) && !defined(HAVE_PTHREAD_H)) 33 uint64_t llvm::get_threadid() { return 0; } 34 35 uint32_t llvm::get_max_thread_name_length() { return 0; } 36 37 void llvm::set_thread_name(const Twine &Name) {} 38 39 void llvm::get_thread_name(SmallVectorImpl<char> &Name) { Name.clear(); } 40 41 llvm::BitVector llvm::get_thread_affinity_mask() { return {}; } 42 43 unsigned llvm::ThreadPoolStrategy::compute_thread_count() const { 44 // When threads are disabled, ensure clients will loop at least once. 45 return 1; 46 } 47 48 // Unknown if threading turned off 49 int llvm::get_physical_cores() { return -1; } 50 51 #else 52 53 static int computeHostNumHardwareThreads(); 54 55 unsigned llvm::ThreadPoolStrategy::compute_thread_count() const { 56 int MaxThreadCount = 57 UseHyperThreads ? computeHostNumHardwareThreads() : get_physical_cores(); 58 if (MaxThreadCount <= 0) 59 MaxThreadCount = 1; 60 // Damage control threading. 61 // 62 // There are no heuristics to figure out how many threads makes sense to spawn, 63 // all while rolling with all available hw threads starts being detrimental to 64 // performance really early. 65 // 66 // Work around by putting a hard cap unless the user explicitly requested a certain amount. 67 // 68 // See https://discourse.llvm.org/t/avoidable-overhead-from-threading-by-default/69160 69 // for more details. 70 if (ThreadsRequested == 0) { 71 return std::min(MaxThreadCount, 4); 72 } 73 if (!Limit) 74 return ThreadsRequested; 75 return std::min((unsigned)MaxThreadCount, ThreadsRequested); 76 } 77 78 // Include the platform-specific parts of this class. 79 #ifdef LLVM_ON_UNIX 80 #include "Unix/Threading.inc" 81 #endif 82 #ifdef _WIN32 83 #include "Windows/Threading.inc" 84 #endif 85 86 // Must be included after Threading.inc to provide definition for llvm::thread 87 // because FreeBSD's condvar.h (included by user.h) misuses the "thread" 88 // keyword. 89 #include "llvm/Support/thread.h" 90 91 #if defined(__APPLE__) 92 // Darwin's default stack size for threads except the main one is only 512KB, 93 // which is not enough for some/many normal LLVM compilations. This implements 94 // the same interface as std::thread but requests the same stack size as the 95 // main thread (8MB) before creation. 96 const std::optional<unsigned> llvm::thread::DefaultStackSize = 8 * 1024 * 1024; 97 #else 98 const std::optional<unsigned> llvm::thread::DefaultStackSize; 99 #endif 100 101 102 #endif 103 104 std::optional<ThreadPoolStrategy> 105 llvm::get_threadpool_strategy(StringRef Num, ThreadPoolStrategy Default) { 106 if (Num == "all") 107 return llvm::hardware_concurrency(); 108 if (Num.empty()) 109 return Default; 110 unsigned V; 111 if (Num.getAsInteger(10, V)) 112 return std::nullopt; // malformed 'Num' value 113 if (V == 0) 114 return Default; 115 116 // Do not take the Default into account. This effectively disables 117 // heavyweight_hardware_concurrency() if the user asks for any number of 118 // threads on the cmd-line. 119 ThreadPoolStrategy S = llvm::hardware_concurrency(); 120 S.ThreadsRequested = V; 121 return S; 122 } 123