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 <optional>
20 #include <stdlib.h>
21
22 using namespace llvm;
23
24 //===----------------------------------------------------------------------===//
25 //=== WARNING: Implementation here must contain only TRULY operating system
26 //=== independent code.
27 //===----------------------------------------------------------------------===//
28
29 #if LLVM_ENABLE_THREADS == 0 || \
30 (!defined(_WIN32) && !defined(HAVE_PTHREAD_H))
get_threadid()31 uint64_t llvm::get_threadid() { return 0; }
32
get_max_thread_name_length()33 uint32_t llvm::get_max_thread_name_length() { return 0; }
34
set_thread_name(const Twine & Name)35 void llvm::set_thread_name(const Twine &Name) {}
36
get_thread_name(SmallVectorImpl<char> & Name)37 void llvm::get_thread_name(SmallVectorImpl<char> &Name) { Name.clear(); }
38
get_thread_affinity_mask()39 llvm::BitVector llvm::get_thread_affinity_mask() { return {}; }
40
compute_thread_count() const41 unsigned llvm::ThreadPoolStrategy::compute_thread_count() const {
42 // When threads are disabled, ensure clients will loop at least once.
43 return 1;
44 }
45
46 // Unknown if threading turned off
get_physical_cores()47 int llvm::get_physical_cores() { return -1; }
48
49 #else
50
51 static int computeHostNumHardwareThreads();
52
compute_thread_count() const53 unsigned llvm::ThreadPoolStrategy::compute_thread_count() const {
54 int MaxThreadCount =
55 UseHyperThreads ? computeHostNumHardwareThreads() : get_physical_cores();
56 if (MaxThreadCount <= 0)
57 MaxThreadCount = 1;
58 // Damage control threading.
59 //
60 // There are no heuristics to figure out how many threads makes sense to spawn,
61 // all while rolling with all available hw threads starts being detrimental to
62 // performance really early.
63 //
64 // Work around by putting a hard cap unless the user explicitly requested a certain amount.
65 //
66 // See https://discourse.llvm.org/t/avoidable-overhead-from-threading-by-default/69160
67 // for more details.
68 if (ThreadsRequested == 0) {
69 return std::min(MaxThreadCount, 4);
70 }
71 if (!Limit)
72 return ThreadsRequested;
73 return std::min((unsigned)MaxThreadCount, ThreadsRequested);
74 }
75
76 // Include the platform-specific parts of this class.
77 #ifdef LLVM_ON_UNIX
78 #include "Unix/Threading.inc"
79 #endif
80 #ifdef _WIN32
81 #include "Windows/Threading.inc"
82 #endif
83
84 // Must be included after Threading.inc to provide definition for llvm::thread
85 // because FreeBSD's condvar.h (included by user.h) misuses the "thread"
86 // keyword.
87 #include "llvm/Support/thread.h"
88
89 #if defined(__APPLE__)
90 // Darwin's default stack size for threads except the main one is only 512KB,
91 // which is not enough for some/many normal LLVM compilations. This implements
92 // the same interface as std::thread but requests the same stack size as the
93 // main thread (8MB) before creation.
94 const std::optional<unsigned> llvm::thread::DefaultStackSize = 8 * 1024 * 1024;
95 #elif defined(_AIX)
96 // On AIX, the default pthread stack size limit is ~192k for 64-bit programs.
97 // This limit is easily reached when doing link-time thinLTO. AIX library
98 // developers have used 4MB, so we'll do the same.
99 const std::optional<unsigned> llvm::thread::DefaultStackSize = 4 * 1024 * 1024;
100 #else
101 const std::optional<unsigned> llvm::thread::DefaultStackSize;
102 #endif
103
104
105 #endif
106
107 std::optional<ThreadPoolStrategy>
get_threadpool_strategy(StringRef Num,ThreadPoolStrategy Default)108 llvm::get_threadpool_strategy(StringRef Num, ThreadPoolStrategy Default) {
109 if (Num == "all")
110 return llvm::hardware_concurrency();
111 if (Num.empty())
112 return Default;
113 unsigned V;
114 if (Num.getAsInteger(10, V))
115 return std::nullopt; // malformed 'Num' value
116 if (V == 0)
117 return Default;
118
119 // Do not take the Default into account. This effectively disables
120 // heavyweight_hardware_concurrency() if the user asks for any number of
121 // threads on the cmd-line.
122 ThreadPoolStrategy S = llvm::hardware_concurrency();
123 S.ThreadsRequested = V;
124 return S;
125 }
126