1 //===- llvm/Support/TimeProfiler.h - Hierarchical Time Profiler -*- 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 provides lightweight and dependency-free machinery to trace execution
10 // time around arbitrary code. Two API flavors are available.
11 //
12 // The primary API uses a RAII object to trigger tracing:
13 //
14 // \code
15 //   {
16 //     TimeTraceScope scope("my_event_name");
17 //     ...my code...
18 //   }
19 // \endcode
20 //
21 // If the code to be profiled does not have a natural lexical scope then
22 // it is also possible to start and end events with respect to an implicit
23 // per-thread stack of profiling entries:
24 //
25 // \code
26 //   timeTraceProfilerBegin("my_event_name");
27 //   ...my code...
28 //   timeTraceProfilerEnd();  // must be called on all control flow paths
29 // \endcode
30 //
31 // Time profiling entries can be given an arbitrary name and, optionally,
32 // an arbitrary 'detail' string. The resulting trace will include 'Total'
33 // entries summing the time spent for each name. Thus, it's best to choose
34 // names to be fairly generic, and rely on the detail field to capture
35 // everything else of interest.
36 //
37 // To avoid lifetime issues name and detail strings are copied into the event
38 // entries at their time of creation. Care should be taken to make string
39 // construction cheap to prevent 'Heisenperf' effects. In particular, the
40 // 'detail' argument may be a string-returning closure:
41 //
42 // \code
43 //   int n;
44 //   {
45 //     TimeTraceScope scope("my_event_name",
46 //                          [n]() { return (Twine("x=") + Twine(n)).str(); });
47 //     ...my code...
48 //   }
49 // \endcode
50 // The closure will not be called if tracing is disabled. Otherwise, the
51 // resulting string will be directly moved into the entry.
52 //
53 // The main process should begin with a timeTraceProfilerInitialize, and
54 // finish with timeTraceProfileWrite and timeTraceProfilerCleanup calls.
55 // Each new thread should begin with a timeTraceProfilerInitialize, and
56 // finish with a timeTraceProfilerFinishThread call.
57 //
58 // Timestamps come from std::chrono::stable_clock. Note that threads need
59 // not see the same time from that clock, and the resolution may not be
60 // the best available.
61 //
62 // Currently, there are a number of compatible viewers:
63 //  - chrome://tracing is the original chromium trace viewer.
64 //  - http://ui.perfetto.dev is the replacement for the above, under active
65 //    development by Google as part of the 'Perfetto' project.
66 //  - https://www.speedscope.app/ has also been reported as an option.
67 //
68 // Future work:
69 //  - Support akin to LLVM_DEBUG for runtime enable/disable of named tracing
70 //    families for non-debug builds which wish to support optional tracing.
71 //  - Evaluate the detail closures at profile write time to avoid
72 //    stringification costs interfering with tracing.
73 //
74 //===----------------------------------------------------------------------===//
75 
76 #ifndef LLVM_SUPPORT_TIMEPROFILER_H
77 #define LLVM_SUPPORT_TIMEPROFILER_H
78 
79 #include "llvm/ADT/STLFunctionalExtras.h"
80 #include "llvm/Support/Error.h"
81 
82 namespace llvm {
83 
84 class raw_pwrite_stream;
85 
86 struct TimeTraceProfiler;
87 TimeTraceProfiler *getTimeTraceProfilerInstance();
88 
89 /// Initialize the time trace profiler.
90 /// This sets up the global \p TimeTraceProfilerInstance
91 /// variable to be the profiler instance.
92 void timeTraceProfilerInitialize(unsigned TimeTraceGranularity,
93                                  StringRef ProcName);
94 
95 /// Cleanup the time trace profiler, if it was initialized.
96 void timeTraceProfilerCleanup();
97 
98 /// Finish a time trace profiler running on a worker thread.
99 void timeTraceProfilerFinishThread();
100 
101 /// Is the time trace profiler enabled, i.e. initialized?
102 inline bool timeTraceProfilerEnabled() {
103   return getTimeTraceProfilerInstance() != nullptr;
104 }
105 
106 /// Write profiling data to output stream.
107 /// Data produced is JSON, in Chrome "Trace Event" format, see
108 /// https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview
109 void timeTraceProfilerWrite(raw_pwrite_stream &OS);
110 
111 /// Write profiling data to a file.
112 /// The function will write to \p PreferredFileName if provided, if not
113 /// then will write to \p FallbackFileName appending .time-trace.
114 /// Returns a StringError indicating a failure if the function is
115 /// unable to open the file for writing.
116 Error timeTraceProfilerWrite(StringRef PreferredFileName,
117                              StringRef FallbackFileName);
118 
119 /// Manually begin a time section, with the given \p Name and \p Detail.
120 /// Profiler copies the string data, so the pointers can be given into
121 /// temporaries. Time sections can be hierarchical; every Begin must have a
122 /// matching End pair but they can nest.
123 void timeTraceProfilerBegin(StringRef Name, StringRef Detail);
124 void timeTraceProfilerBegin(StringRef Name,
125                             llvm::function_ref<std::string()> Detail);
126 
127 /// Manually end the last time section.
128 void timeTraceProfilerEnd();
129 
130 /// The TimeTraceScope is a helper class to call the begin and end functions
131 /// of the time trace profiler.  When the object is constructed, it begins
132 /// the section; and when it is destroyed, it stops it. If the time profiler
133 /// is not initialized, the overhead is a single branch.
134 struct TimeTraceScope {
135 
136   TimeTraceScope() = delete;
137   TimeTraceScope(const TimeTraceScope &) = delete;
138   TimeTraceScope &operator=(const TimeTraceScope &) = delete;
139   TimeTraceScope(TimeTraceScope &&) = delete;
140   TimeTraceScope &operator=(TimeTraceScope &&) = delete;
141 
142   TimeTraceScope(StringRef Name) {
143     if (getTimeTraceProfilerInstance() != nullptr)
144       timeTraceProfilerBegin(Name, StringRef(""));
145   }
146   TimeTraceScope(StringRef Name, StringRef Detail) {
147     if (getTimeTraceProfilerInstance() != nullptr)
148       timeTraceProfilerBegin(Name, Detail);
149   }
150   TimeTraceScope(StringRef Name, llvm::function_ref<std::string()> Detail) {
151     if (getTimeTraceProfilerInstance() != nullptr)
152       timeTraceProfilerBegin(Name, Detail);
153   }
154   ~TimeTraceScope() {
155     if (getTimeTraceProfilerInstance() != nullptr)
156       timeTraceProfilerEnd();
157   }
158 };
159 
160 } // end namespace llvm
161 
162 #endif
163