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