rtc_base/trace_event.h

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file under third_party_mods/chromium or at:
// http://src.chromium.org/svn/trunk/src/LICENSE

#ifndef RTC_BASE_TRACE_EVENT_H_
#define RTC_BASE_TRACE_EVENT_H_

#include <string>

#include "rtc_base/event_tracer.h"

#if defined(TRACE_EVENT0)
#error "Another copy of trace_event.h has already been included."
#endif

// Extracted from Chromium's src/base/debug/trace_event.h.

// This header is designed to give you trace_event macros without specifying
// how the events actually get collected and stored. If you need to expose trace
// event to some other universe, you can copy-and-paste this file,
// implement the TRACE_EVENT_API macros, and do any other necessary fixup for
// the target platform. The end result is that multiple libraries can funnel
// events through to a shared trace event collector.

// Trace events are for tracking application performance and resource usage.
// Macros are provided to track:
//    Begin and end of function calls
//    Counters
//
// Events are issued against categories. Whereas LOG's
// categories are statically defined, TRACE categories are created
// implicitly with a string. For example:
//   TRACE_EVENT_INSTANT0("MY_SUBSYSTEM", "SomeImportantEvent")
//
// Events can be INSTANT, or can be pairs of BEGIN and END in the same scope:
//   TRACE_EVENT_BEGIN0("MY_SUBSYSTEM", "SomethingCostly")
//   doSomethingCostly()
//   TRACE_EVENT_END0("MY_SUBSYSTEM", "SomethingCostly")
// Note: our tools can't always determine the correct BEGIN/END pairs unless
// these are used in the same scope. Use ASYNC_BEGIN/ASYNC_END macros if you
// need them to be in separate scopes.
//
// A common use case is to trace entire function scopes. This
// issues a trace BEGIN and END automatically:
//   void doSomethingCostly() {
//     TRACE_EVENT0("MY_SUBSYSTEM", "doSomethingCostly");
//     ...
//   }
//
// Additional parameters can be associated with an event:
//   void doSomethingCostly2(int howMuch) {
//     TRACE_EVENT1("MY_SUBSYSTEM", "doSomethingCostly",
//         "howMuch", howMuch);
//     ...
//   }
//
// The trace system will automatically add to this information the
// current process id, thread id, and a timestamp in microseconds.
//
// To trace an asynchronous procedure such as an IPC send/receive, use
// ASYNC_BEGIN and ASYNC_END:
//   [single threaded sender code]
//     static int send_count = 0;
//     ++send_count;
//     TRACE_EVENT_ASYNC_BEGIN0("ipc", "message", send_count);
//     Send(new MyMessage(send_count));
//   [receive code]
//     void OnMyMessage(send_count) {
//       TRACE_EVENT_ASYNC_END0("ipc", "message", send_count);
//     }
// The third parameter is a unique ID to match ASYNC_BEGIN/ASYNC_END pairs.
// ASYNC_BEGIN and ASYNC_END can occur on any thread of any traced process.
// Pointers can be used for the ID parameter, and they will be mangled
// internally so that the same pointer on two different processes will not
// match. For example:
//   class MyTracedClass {
//    public:
//     MyTracedClass() {
//       TRACE_EVENT_ASYNC_BEGIN0("category", "MyTracedClass", this);
//     }
//     ~MyTracedClass() {
//       TRACE_EVENT_ASYNC_END0("category", "MyTracedClass", this);
//     }
//   }
//
// Trace event also supports counters, which is a way to track a quantity
// as it varies over time. Counters are created with the following macro:
//   TRACE_COUNTER1("MY_SUBSYSTEM", "myCounter", g_myCounterValue);
//
// Counters are process-specific. The macro itself can be issued from any
// thread, however.
//
// Sometimes, you want to track two counters at once. You can do this with two
// counter macros:
//   TRACE_COUNTER1("MY_SUBSYSTEM", "myCounter0", g_myCounterValue[0]);
//   TRACE_COUNTER1("MY_SUBSYSTEM", "myCounter1", g_myCounterValue[1]);
// Or you can do it with a combined macro:
//   TRACE_COUNTER2("MY_SUBSYSTEM", "myCounter",
//       "bytesPinned", g_myCounterValue[0],
//       "bytesAllocated", g_myCounterValue[1]);
// This indicates to the tracing UI that these counters should be displayed
// in a single graph, as a summed area chart.
//
// Since counters are in a global namespace, you may want to disembiguate with a
// unique ID, by using the TRACE_COUNTER_ID* variations.
//
// By default, trace collection is compiled in, but turned off at runtime.
// Collecting trace data is the responsibility of the embedding
// application. In Chrome's case, navigating to about:tracing will turn on
// tracing and display data collected across all active processes.
//
//
// Memory scoping note:
// Tracing copies the pointers, not the string content, of the strings passed
// in for category, name, and arg_names.  Thus, the following code will
// cause problems:
//     char* str = strdup("impprtantName");
//     TRACE_EVENT_INSTANT0("SUBSYSTEM", str);  // BAD!
//     free(str);                   // Trace system now has dangling pointer
//
// To avoid this issue with the |name| and |arg_name| parameters, use the
// TRACE_EVENT_COPY_XXX overloads of the macros at additional runtime overhead.
// Notes: The category must always be in a long-lived char* (i.e. static const).
//        The |arg_values|, when used, are always deep copied with the _COPY
//        macros.
//
// When are string argument values copied:
// const char* arg_values are only referenced by default:
//     TRACE_EVENT1("category", "name",
//                  "arg1", "literal string is only referenced");
// Use TRACE_STR_COPY to force copying of a const char*:
//     TRACE_EVENT1("category", "name",
//                  "arg1", TRACE_STR_COPY("string will be copied"));
// std::string arg_values are always copied:
//     TRACE_EVENT1("category", "name",
//                  "arg1", std::string("string will be copied"));
//
//
// Thread Safety:
// Thread safety is provided by methods defined in event_tracer.h. See the file
// for details.


// By default, const char* argument values are assumed to have long-lived scope
// and will not be copied. Use this macro to force a const char* to be copied.
#define TRACE_STR_COPY(str) \
    webrtc::trace_event_internal::TraceStringWithCopy(str)

// This will mark the trace event as disabled by default. The user will need
// to explicitly enable the event.
#define TRACE_DISABLED_BY_DEFAULT(name) "disabled-by-default-" name

// By default, uint64 ID argument values are not mangled with the Process ID in
// TRACE_EVENT_ASYNC macros. Use this macro to force Process ID mangling.
#define TRACE_ID_MANGLE(id) \
    webrtc::trace_event_internal::TraceID::ForceMangle(id)

// Records a pair of begin and end events called "name" for the current
// scope, with 0, 1 or 2 associated arguments. If the category is not
// enabled, then this does nothing.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
#define TRACE_EVENT0(category, name) \
    INTERNAL_TRACE_EVENT_ADD_SCOPED(category, name)
#define TRACE_EVENT1(category, name, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_SCOPED(category, name, arg1_name, arg1_val)
#define TRACE_EVENT2(category, name, arg1_name, arg1_val, arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_SCOPED(category, name, arg1_name, arg1_val, \
        arg2_name, arg2_val)

// Same as TRACE_EVENT except that they are not included in official builds.
#ifdef OFFICIAL_BUILD
#define UNSHIPPED_TRACE_EVENT0(category, name) (void)0
#define UNSHIPPED_TRACE_EVENT1(category, name, arg1_name, arg1_val) (void)0
#define UNSHIPPED_TRACE_EVENT2(category, name, arg1_name, arg1_val, \
                               arg2_name, arg2_val) (void)0
#define UNSHIPPED_TRACE_EVENT_INSTANT0(category, name) (void)0
#define UNSHIPPED_TRACE_EVENT_INSTANT1(category, name, arg1_name, arg1_val) \
    (void)0
#define UNSHIPPED_TRACE_EVENT_INSTANT2(category, name, arg1_name, arg1_val, \
                                       arg2_name, arg2_val) (void)0
#else
#define UNSHIPPED_TRACE_EVENT0(category, name) \
    TRACE_EVENT0(category, name)
#define UNSHIPPED_TRACE_EVENT1(category, name, arg1_name, arg1_val) \
    TRACE_EVENT1(category, name, arg1_name, arg1_val)
#define UNSHIPPED_TRACE_EVENT2(category, name, arg1_name, arg1_val, \
                               arg2_name, arg2_val) \
    TRACE_EVENT2(category, name, arg1_name, arg1_val, arg2_name, arg2_val)
#define UNSHIPPED_TRACE_EVENT_INSTANT0(category, name) \
    TRACE_EVENT_INSTANT0(category, name)
#define UNSHIPPED_TRACE_EVENT_INSTANT1(category, name, arg1_name, arg1_val) \
    TRACE_EVENT_INSTANT1(category, name, arg1_name, arg1_val)
#define UNSHIPPED_TRACE_EVENT_INSTANT2(category, name, arg1_name, arg1_val, \
                                       arg2_name, arg2_val) \
    TRACE_EVENT_INSTANT2(category, name, arg1_name, arg1_val, \
                         arg2_name, arg2_val)
#endif

// Records a single event called "name" immediately, with 0, 1 or 2
// associated arguments. If the category is not enabled, then this
// does nothing.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
#define TRACE_EVENT_INSTANT0(category, name) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_INSTANT, \
        category, name, TRACE_EVENT_FLAG_NONE)
#define TRACE_EVENT_INSTANT1(category, name, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_INSTANT, \
        category, name, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
#define TRACE_EVENT_INSTANT2(category, name, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_INSTANT, \
        category, name, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val, \
        arg2_name, arg2_val)
#define TRACE_EVENT_COPY_INSTANT0(category, name) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_INSTANT, \
        category, name, TRACE_EVENT_FLAG_COPY)
#define TRACE_EVENT_COPY_INSTANT1(category, name, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_INSTANT, \
        category, name, TRACE_EVENT_FLAG_COPY, arg1_name, arg1_val)
#define TRACE_EVENT_COPY_INSTANT2(category, name, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_INSTANT, \
        category, name, TRACE_EVENT_FLAG_COPY, arg1_name, arg1_val, \
        arg2_name, arg2_val)

// Records a single BEGIN event called "name" immediately, with 0, 1 or 2
// associated arguments. If the category is not enabled, then this
// does nothing.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
#define TRACE_EVENT_BEGIN0(category, name) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_BEGIN, \
        category, name, TRACE_EVENT_FLAG_NONE)
#define TRACE_EVENT_BEGIN1(category, name, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_BEGIN, \
        category, name, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
#define TRACE_EVENT_BEGIN2(category, name, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_BEGIN, \
        category, name, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val, \
        arg2_name, arg2_val)
#define TRACE_EVENT_COPY_BEGIN0(category, name) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_BEGIN, \
        category, name, TRACE_EVENT_FLAG_COPY)
#define TRACE_EVENT_COPY_BEGIN1(category, name, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_BEGIN, \
        category, name, TRACE_EVENT_FLAG_COPY, arg1_name, arg1_val)
#define TRACE_EVENT_COPY_BEGIN2(category, name, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_BEGIN, \
        category, name, TRACE_EVENT_FLAG_COPY, arg1_name, arg1_val, \
        arg2_name, arg2_val)

// Records a single END event for "name" immediately. If the category
// is not enabled, then this does nothing.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
#define TRACE_EVENT_END0(category, name) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_END, \
        category, name, TRACE_EVENT_FLAG_NONE)
#define TRACE_EVENT_END1(category, name, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_END, \
        category, name, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
#define TRACE_EVENT_END2(category, name, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_END, \
        category, name, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val, \
        arg2_name, arg2_val)
#define TRACE_EVENT_COPY_END0(category, name) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_END, \
        category, name, TRACE_EVENT_FLAG_COPY)
#define TRACE_EVENT_COPY_END1(category, name, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_END, \
        category, name, TRACE_EVENT_FLAG_COPY, arg1_name, arg1_val)
#define TRACE_EVENT_COPY_END2(category, name, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_END, \
        category, name, TRACE_EVENT_FLAG_COPY, arg1_name, arg1_val, \
        arg2_name, arg2_val)

// Records the value of a counter called "name" immediately. Value
// must be representable as a 32 bit integer.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
#define TRACE_COUNTER1(category, name, value) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_COUNTER, \
        category, name, TRACE_EVENT_FLAG_NONE, \
        "value", static_cast<int>(value))
#define TRACE_COPY_COUNTER1(category, name, value) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_COUNTER, \
        category, name, TRACE_EVENT_FLAG_COPY, \
        "value", static_cast<int>(value))

// Records the values of a multi-parted counter called "name" immediately.
// The UI will treat value1 and value2 as parts of a whole, displaying their
// values as a stacked-bar chart.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
#define TRACE_COUNTER2(category, name, value1_name, value1_val, \
        value2_name, value2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_COUNTER, \
        category, name, TRACE_EVENT_FLAG_NONE, \
        value1_name, static_cast<int>(value1_val), \
        value2_name, static_cast<int>(value2_val))
#define TRACE_COPY_COUNTER2(category, name, value1_name, value1_val, \
        value2_name, value2_val) \
    INTERNAL_TRACE_EVENT_ADD(TRACE_EVENT_PHASE_COUNTER, \
        category, name, TRACE_EVENT_FLAG_COPY, \
        value1_name, static_cast<int>(value1_val), \
        value2_name, static_cast<int>(value2_val))

// Records the value of a counter called "name" immediately. Value
// must be representable as a 32 bit integer.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
// - |id| is used to disambiguate counters with the same name. It must either
//   be a pointer or an integer value up to 64 bits. If it's a pointer, the bits
//   will be xored with a hash of the process ID so that the same pointer on
//   two different processes will not collide.
#define TRACE_COUNTER_ID1(category, name, id, value) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_COUNTER, \
        category, name, id, TRACE_EVENT_FLAG_NONE, \
        "value", static_cast<int>(value))
#define TRACE_COPY_COUNTER_ID1(category, name, id, value) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_COUNTER, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        "value", static_cast<int>(value))

// Records the values of a multi-parted counter called "name" immediately.
// The UI will treat value1 and value2 as parts of a whole, displaying their
// values as a stacked-bar chart.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
// - |id| is used to disambiguate counters with the same name. It must either
//   be a pointer or an integer value up to 64 bits. If it's a pointer, the bits
//   will be xored with a hash of the process ID so that the same pointer on
//   two different processes will not collide.
#define TRACE_COUNTER_ID2(category, name, id, value1_name, value1_val, \
        value2_name, value2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_COUNTER, \
        category, name, id, TRACE_EVENT_FLAG_NONE, \
        value1_name, static_cast<int>(value1_val), \
        value2_name, static_cast<int>(value2_val))
#define TRACE_COPY_COUNTER_ID2(category, name, id, value1_name, value1_val, \
        value2_name, value2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_COUNTER, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        value1_name, static_cast<int>(value1_val), \
        value2_name, static_cast<int>(value2_val))


// Records a single ASYNC_BEGIN event called "name" immediately, with 0, 1 or 2
// associated arguments. If the category is not enabled, then this
// does nothing.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
// - |id| is used to match the ASYNC_BEGIN event with the ASYNC_END event. ASYNC
//   events are considered to match if their category, name and id values all
//   match. |id| must either be a pointer or an integer value up to 64 bits. If
//   it's a pointer, the bits will be xored with a hash of the process ID so
//   that the same pointer on two different processes will not collide.
// An asynchronous operation can consist of multiple phases. The first phase is
// defined by the ASYNC_BEGIN calls. Additional phases can be defined using the
// ASYNC_STEP macros. When the operation completes, call ASYNC_END.
// An ASYNC trace typically occur on a single thread (if not, they will only be
// drawn on the thread defined in the ASYNC_BEGIN event), but all events in that
// operation must use the same |name| and |id|. Each event can have its own
// args.
#define TRACE_EVENT_ASYNC_BEGIN0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_NONE)
#define TRACE_EVENT_ASYNC_BEGIN1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
#define TRACE_EVENT_ASYNC_BEGIN2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_NONE, \
        arg1_name, arg1_val, arg2_name, arg2_val)
#define TRACE_EVENT_COPY_ASYNC_BEGIN0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_COPY)
#define TRACE_EVENT_COPY_ASYNC_BEGIN1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val)
#define TRACE_EVENT_COPY_ASYNC_BEGIN2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val, arg2_name, arg2_val)

// Records a single ASYNC_STEP event for |step| immediately. If the category
// is not enabled, then this does nothing. The |name| and |id| must match the
// ASYNC_BEGIN event above. The |step| param identifies this step within the
// async event. This should be called at the beginning of the next phase of an
// asynchronous operation.
#define TRACE_EVENT_ASYNC_STEP0(category, name, id, step) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_STEP, \
        category, name, id, TRACE_EVENT_FLAG_NONE, "step", step)
#define TRACE_EVENT_ASYNC_STEP1(category, name, id, step, \
                                      arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_STEP, \
        category, name, id, TRACE_EVENT_FLAG_NONE, "step", step, \
        arg1_name, arg1_val)
#define TRACE_EVENT_COPY_ASYNC_STEP0(category, name, id, step) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_STEP, \
        category, name, id, TRACE_EVENT_FLAG_COPY, "step", step)
#define TRACE_EVENT_COPY_ASYNC_STEP1(category, name, id, step, \
        arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_STEP, \
        category, name, id, TRACE_EVENT_FLAG_COPY, "step", step, \
        arg1_name, arg1_val)

// Records a single ASYNC_END event for "name" immediately. If the category
// is not enabled, then this does nothing.
#define TRACE_EVENT_ASYNC_END0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_END, \
        category, name, id, TRACE_EVENT_FLAG_NONE)
#define TRACE_EVENT_ASYNC_END1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_END, \
        category, name, id, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
#define TRACE_EVENT_ASYNC_END2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_END, \
        category, name, id, TRACE_EVENT_FLAG_NONE, \
        arg1_name, arg1_val, arg2_name, arg2_val)
#define TRACE_EVENT_COPY_ASYNC_END0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_END, \
        category, name, id, TRACE_EVENT_FLAG_COPY)
#define TRACE_EVENT_COPY_ASYNC_END1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_END, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val)
#define TRACE_EVENT_COPY_ASYNC_END2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_ASYNC_END, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val, arg2_name, arg2_val)


// Records a single FLOW_BEGIN event called "name" immediately, with 0, 1 or 2
// associated arguments. If the category is not enabled, then this
// does nothing.
// - category and name strings must have application lifetime (statics or
//   literals). They may not include " chars.
// - |id| is used to match the FLOW_BEGIN event with the FLOW_END event. FLOW
//   events are considered to match if their category, name and id values all
//   match. |id| must either be a pointer or an integer value up to 64 bits. If
//   it's a pointer, the bits will be xored with a hash of the process ID so
//   that the same pointer on two different processes will not collide.
// FLOW events are different from ASYNC events in how they are drawn by the
// tracing UI. A FLOW defines asynchronous data flow, such as posting a task
// (FLOW_BEGIN) and later executing that task (FLOW_END). Expect FLOWs to be
// drawn as lines or arrows from FLOW_BEGIN scopes to FLOW_END scopes. Similar
// to ASYNC, a FLOW can consist of multiple phases. The first phase is defined
// by the FLOW_BEGIN calls. Additional phases can be defined using the FLOW_STEP
// macros. When the operation completes, call FLOW_END. An async operation can
// span threads and processes, but all events in that operation must use the
// same |name| and |id|. Each event can have its own args.
#define TRACE_EVENT_FLOW_BEGIN0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_NONE)
#define TRACE_EVENT_FLOW_BEGIN1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
#define TRACE_EVENT_FLOW_BEGIN2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_NONE, \
        arg1_name, arg1_val, arg2_name, arg2_val)
#define TRACE_EVENT_COPY_FLOW_BEGIN0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_COPY)
#define TRACE_EVENT_COPY_FLOW_BEGIN1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val)
#define TRACE_EVENT_COPY_FLOW_BEGIN2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_BEGIN, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val, arg2_name, arg2_val)

// Records a single FLOW_STEP event for |step| immediately. If the category
// is not enabled, then this does nothing. The |name| and |id| must match the
// FLOW_BEGIN event above. The |step| param identifies this step within the
// async event. This should be called at the beginning of the next phase of an
// asynchronous operation.
#define TRACE_EVENT_FLOW_STEP0(category, name, id, step) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_STEP, \
        category, name, id, TRACE_EVENT_FLAG_NONE, "step", step)
#define TRACE_EVENT_FLOW_STEP1(category, name, id, step, \
        arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_STEP, \
        category, name, id, TRACE_EVENT_FLAG_NONE, "step", step, \
        arg1_name, arg1_val)
#define TRACE_EVENT_COPY_FLOW_STEP0(category, name, id, step) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_STEP, \
        category, name, id, TRACE_EVENT_FLAG_COPY, "step", step)
#define TRACE_EVENT_COPY_FLOW_STEP1(category, name, id, step, \
        arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_STEP, \
        category, name, id, TRACE_EVENT_FLAG_COPY, "step", step, \
        arg1_name, arg1_val)

// Records a single FLOW_END event for "name" immediately. If the category
// is not enabled, then this does nothing.
#define TRACE_EVENT_FLOW_END0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_END, \
        category, name, id, TRACE_EVENT_FLAG_NONE)
#define TRACE_EVENT_FLOW_END1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_END, \
        category, name, id, TRACE_EVENT_FLAG_NONE, arg1_name, arg1_val)
#define TRACE_EVENT_FLOW_END2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_END, \
        category, name, id, TRACE_EVENT_FLAG_NONE, \
        arg1_name, arg1_val, arg2_name, arg2_val)
#define TRACE_EVENT_COPY_FLOW_END0(category, name, id) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_END, \
        category, name, id, TRACE_EVENT_FLAG_COPY)
#define TRACE_EVENT_COPY_FLOW_END1(category, name, id, arg1_name, arg1_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_END, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val)
#define TRACE_EVENT_COPY_FLOW_END2(category, name, id, arg1_name, arg1_val, \
        arg2_name, arg2_val) \
    INTERNAL_TRACE_EVENT_ADD_WITH_ID(TRACE_EVENT_PHASE_FLOW_END, \
        category, name, id, TRACE_EVENT_FLAG_COPY, \
        arg1_name, arg1_val, arg2_name, arg2_val)


////////////////////////////////////////////////////////////////////////////////
// Implementation specific tracing API definitions.

// Get a pointer to the enabled state of the given trace category. Only
// long-lived literal strings should be given as the category name. The returned
// pointer can be held permanently in a local static for example. If the
// unsigned char is non-zero, tracing is enabled. If tracing is enabled,
// TRACE_EVENT_API_ADD_TRACE_EVENT can be called. It's OK if tracing is disabled
// between the load of the tracing state and the call to
// TRACE_EVENT_API_ADD_TRACE_EVENT, because this flag only provides an early out
// for best performance when tracing is disabled.
// const unsigned char*
//     TRACE_EVENT_API_GET_CATEGORY_ENABLED(const char* category_name)
#define TRACE_EVENT_API_GET_CATEGORY_ENABLED \
    webrtc::EventTracer::GetCategoryEnabled

// Add a trace event to the platform tracing system.
// void TRACE_EVENT_API_ADD_TRACE_EVENT(
//                    char phase,
//                    const unsigned char* category_enabled,
//                    const char* name,
//                    unsigned long long id,
//                    int num_args,
//                    const char** arg_names,
//                    const unsigned char* arg_types,
//                    const unsigned long long* arg_values,
//                    unsigned char flags)
#define TRACE_EVENT_API_ADD_TRACE_EVENT webrtc::EventTracer::AddTraceEvent

////////////////////////////////////////////////////////////////////////////////

// Implementation detail: trace event macros create temporary variables
// to keep instrumentation overhead low. These macros give each temporary
// variable a unique name based on the line number to prevent name collissions.
#define INTERNAL_TRACE_EVENT_UID3(a,b) \
    trace_event_unique_##a##b
#define INTERNAL_TRACE_EVENT_UID2(a,b) \
    INTERNAL_TRACE_EVENT_UID3(a,b)
#define INTERNAL_TRACE_EVENT_UID(name_prefix) \
    INTERNAL_TRACE_EVENT_UID2(name_prefix, __LINE__)

#if WEBRTC_NON_STATIC_TRACE_EVENT_HANDLERS
#define INTERNAL_TRACE_EVENT_INFO_TYPE const unsigned char*
#else
#define INTERNAL_TRACE_EVENT_INFO_TYPE static const unsigned char*
#endif  // WEBRTC_NON_STATIC_TRACE_EVENT_HANDLERS

// Implementation detail: internal macro to create static category.
#define INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category) \
    INTERNAL_TRACE_EVENT_INFO_TYPE INTERNAL_TRACE_EVENT_UID(catstatic) = \
        TRACE_EVENT_API_GET_CATEGORY_ENABLED(category);

// Implementation detail: internal macro to create static category and add
// event if the category is enabled.
#define INTERNAL_TRACE_EVENT_ADD(phase, category, name, flags, ...) \
    do { \
      INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category); \
      if (*INTERNAL_TRACE_EVENT_UID(catstatic)) { \
        webrtc::trace_event_internal::AddTraceEvent(          \
            phase, INTERNAL_TRACE_EVENT_UID(catstatic), name, \
            webrtc::trace_event_internal::kNoEventId, flags, ##__VA_ARGS__); \
      } \
    } while (0)

// Implementation detail: internal macro to create static category and add begin
// event if the category is enabled. Also adds the end event when the scope
// ends.
#define INTERNAL_TRACE_EVENT_ADD_SCOPED(category, name, ...) \
    INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category); \
    webrtc::trace_event_internal::TraceEndOnScopeClose  \
        INTERNAL_TRACE_EVENT_UID(profileScope); \
    if (*INTERNAL_TRACE_EVENT_UID(catstatic)) { \
      webrtc::trace_event_internal::AddTraceEvent(      \
          TRACE_EVENT_PHASE_BEGIN, \
          INTERNAL_TRACE_EVENT_UID(catstatic), \
          name, webrtc::trace_event_internal::kNoEventId,       \
          TRACE_EVENT_FLAG_NONE, ##__VA_ARGS__); \
      INTERNAL_TRACE_EVENT_UID(profileScope).Initialize( \
          INTERNAL_TRACE_EVENT_UID(catstatic), name); \
    }

// Implementation detail: internal macro to create static category and add
// event if the category is enabled.
#define INTERNAL_TRACE_EVENT_ADD_WITH_ID(phase, category, name, id, flags, \
                                         ...) \
    do { \
      INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category); \
      if (*INTERNAL_TRACE_EVENT_UID(catstatic)) { \
        unsigned char trace_event_flags = flags | TRACE_EVENT_FLAG_HAS_ID; \
        webrtc::trace_event_internal::TraceID trace_event_trace_id( \
            id, &trace_event_flags); \
        webrtc::trace_event_internal::AddTraceEvent( \
            phase, INTERNAL_TRACE_EVENT_UID(catstatic), \
            name, trace_event_trace_id.data(), trace_event_flags, \
            ##__VA_ARGS__); \
      } \
    } while (0)

// Notes regarding the following definitions:
// New values can be added and propagated to third party libraries, but existing
// definitions must never be changed, because third party libraries may use old
// definitions.

// Phase indicates the nature of an event entry. E.g. part of a begin/end pair.
#define TRACE_EVENT_PHASE_BEGIN    ('B')
#define TRACE_EVENT_PHASE_END      ('E')
#define TRACE_EVENT_PHASE_INSTANT  ('I')
#define TRACE_EVENT_PHASE_ASYNC_BEGIN ('S')
#define TRACE_EVENT_PHASE_ASYNC_STEP  ('T')
#define TRACE_EVENT_PHASE_ASYNC_END   ('F')
#define TRACE_EVENT_PHASE_FLOW_BEGIN ('s')
#define TRACE_EVENT_PHASE_FLOW_STEP  ('t')
#define TRACE_EVENT_PHASE_FLOW_END   ('f')
#define TRACE_EVENT_PHASE_METADATA ('M')
#define TRACE_EVENT_PHASE_COUNTER  ('C')

// Flags for changing the behavior of TRACE_EVENT_API_ADD_TRACE_EVENT.
#define TRACE_EVENT_FLAG_NONE        (static_cast<unsigned char>(0))
#define TRACE_EVENT_FLAG_COPY        (static_cast<unsigned char>(1 << 0))
#define TRACE_EVENT_FLAG_HAS_ID      (static_cast<unsigned char>(1 << 1))
#define TRACE_EVENT_FLAG_MANGLE_ID   (static_cast<unsigned char>(1 << 2))

// Type values for identifying types in the TraceValue union.
#define TRACE_VALUE_TYPE_BOOL         (static_cast<unsigned char>(1))
#define TRACE_VALUE_TYPE_UINT         (static_cast<unsigned char>(2))
#define TRACE_VALUE_TYPE_INT          (static_cast<unsigned char>(3))
#define TRACE_VALUE_TYPE_DOUBLE       (static_cast<unsigned char>(4))
#define TRACE_VALUE_TYPE_POINTER      (static_cast<unsigned char>(5))
#define TRACE_VALUE_TYPE_STRING       (static_cast<unsigned char>(6))
#define TRACE_VALUE_TYPE_COPY_STRING  (static_cast<unsigned char>(7))

namespace webrtc {
namespace trace_event_internal {

// Specify these values when the corresponding argument of AddTraceEvent is not
// used.
const int kZeroNumArgs = 0;
const unsigned long long kNoEventId = 0;

// TraceID encapsulates an ID that can either be an integer or pointer. Pointers
// are mangled with the Process ID so that they are unlikely to collide when the
// same pointer is used on different processes.
class TraceID {
 public:
  class ForceMangle {
    public:
     explicit ForceMangle(unsigned long long id) : data_(id) {}
     explicit ForceMangle(unsigned long id) : data_(id) {}
     explicit ForceMangle(unsigned int id) : data_(id) {}
     explicit ForceMangle(unsigned short id) : data_(id) {}
     explicit ForceMangle(unsigned char id) : data_(id) {}
     explicit ForceMangle(long long id)
         : data_(static_cast<unsigned long long>(id)) {}
     explicit ForceMangle(long id)
         : data_(static_cast<unsigned long long>(id)) {}
     explicit ForceMangle(int id)
         : data_(static_cast<unsigned long long>(id)) {}
     explicit ForceMangle(short id)
         : data_(static_cast<unsigned long long>(id)) {}
     explicit ForceMangle(signed char id)
         : data_(static_cast<unsigned long long>(id)) {}

     unsigned long long data() const { return data_; }

    private:
     unsigned long long data_;
  };

  explicit TraceID(const void* id, unsigned char* flags)
      : data_(static_cast<unsigned long long>(
              reinterpret_cast<uintptr_t>(id))) {
    *flags |= TRACE_EVENT_FLAG_MANGLE_ID;
  }
  explicit TraceID(ForceMangle id, unsigned char* flags) : data_(id.data()) {
    *flags |= TRACE_EVENT_FLAG_MANGLE_ID;
  }
  explicit TraceID(unsigned long long id, unsigned char* flags)
      : data_(id) { (void)flags; }
  explicit TraceID(unsigned long id, unsigned char* flags)
      : data_(id) { (void)flags; }
  explicit TraceID(unsigned int id, unsigned char* flags)
      : data_(id) { (void)flags; }
  explicit TraceID(unsigned short id, unsigned char* flags)
      : data_(id) { (void)flags; }
  explicit TraceID(unsigned char id, unsigned char* flags)
      : data_(id) { (void)flags; }
  explicit TraceID(long long id, unsigned char* flags)
      : data_(static_cast<unsigned long long>(id)) { (void)flags; }
  explicit TraceID(long id, unsigned char* flags)
      : data_(static_cast<unsigned long long>(id)) { (void)flags; }
  explicit TraceID(int id, unsigned char* flags)
      : data_(static_cast<unsigned long long>(id)) { (void)flags; }
  explicit TraceID(short id, unsigned char* flags)
      : data_(static_cast<unsigned long long>(id)) { (void)flags; }
  explicit TraceID(signed char id, unsigned char* flags)
      : data_(static_cast<unsigned long long>(id)) { (void)flags; }

  unsigned long long data() const { return data_; }

 private:
  unsigned long long data_;
};

// Simple union to store various types as unsigned long long.
union TraceValueUnion {
  bool as_bool;
  unsigned long long as_uint;
  long long as_int;
  double as_double;
  const void* as_pointer;
  const char* as_string;
};

// Simple container for const char* that should be copied instead of retained.
class TraceStringWithCopy {
 public:
  explicit TraceStringWithCopy(const char* str) : str_(str) {}
  operator const char* () const { return str_; }
 private:
  const char* str_;
};

// Define SetTraceValue for each allowed type. It stores the type and
// value in the return arguments. This allows this API to avoid declaring any
// structures so that it is portable to third_party libraries.
#define INTERNAL_DECLARE_SET_TRACE_VALUE(actual_type, \
                                         union_member, \
                                         value_type_id) \
    static inline void SetTraceValue(actual_type arg, \
                                     unsigned char* type, \
                                     unsigned long long* value) { \
      TraceValueUnion type_value; \
      type_value.union_member = arg; \
      *type = value_type_id; \
      *value = type_value.as_uint; \
    }
// Simpler form for int types that can be safely casted.
#define INTERNAL_DECLARE_SET_TRACE_VALUE_INT(actual_type, \
                                             value_type_id) \
    static inline void SetTraceValue(actual_type arg, \
                                     unsigned char* type, \
                                     unsigned long long* value) { \
      *type = value_type_id; \
      *value = static_cast<unsigned long long>(arg); \
    }

INTERNAL_DECLARE_SET_TRACE_VALUE_INT(unsigned long long, TRACE_VALUE_TYPE_UINT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(unsigned long, TRACE_VALUE_TYPE_UINT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(unsigned int, TRACE_VALUE_TYPE_UINT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(unsigned short, TRACE_VALUE_TYPE_UINT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(unsigned char, TRACE_VALUE_TYPE_UINT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(long long, TRACE_VALUE_TYPE_INT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(long, TRACE_VALUE_TYPE_INT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(int, TRACE_VALUE_TYPE_INT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(short, TRACE_VALUE_TYPE_INT)
INTERNAL_DECLARE_SET_TRACE_VALUE_INT(signed char, TRACE_VALUE_TYPE_INT)
INTERNAL_DECLARE_SET_TRACE_VALUE(bool, as_bool, TRACE_VALUE_TYPE_BOOL)
INTERNAL_DECLARE_SET_TRACE_VALUE(double, as_double, TRACE_VALUE_TYPE_DOUBLE)
INTERNAL_DECLARE_SET_TRACE_VALUE(const void*, as_pointer,
                                 TRACE_VALUE_TYPE_POINTER)
INTERNAL_DECLARE_SET_TRACE_VALUE(const char*, as_string,
                                 TRACE_VALUE_TYPE_STRING)
INTERNAL_DECLARE_SET_TRACE_VALUE(const TraceStringWithCopy&, as_string,
                                 TRACE_VALUE_TYPE_COPY_STRING)

#undef INTERNAL_DECLARE_SET_TRACE_VALUE
#undef INTERNAL_DECLARE_SET_TRACE_VALUE_INT

// std::string version of SetTraceValue so that trace arguments can be strings.
static inline void SetTraceValue(const std::string& arg,
                                 unsigned char* type,
                                 unsigned long long* value) {
  TraceValueUnion type_value;
  type_value.as_string = arg.c_str();
  *type = TRACE_VALUE_TYPE_COPY_STRING;
  *value = type_value.as_uint;
}

// These AddTraceEvent template functions are defined here instead of in the
// macro, because the arg_values could be temporary objects, such as
// std::string. In order to store pointers to the internal c_str and pass
// through to the tracing API, the arg_values must live throughout
// these procedures.

static inline void AddTraceEvent(char phase,
                                const unsigned char* category_enabled,
                                const char* name,
                                unsigned long long id,
                                unsigned char flags) {
  TRACE_EVENT_API_ADD_TRACE_EVENT(phase, category_enabled, name, id,
                                  kZeroNumArgs, nullptr, nullptr, nullptr,
                                  flags);
}

template<class ARG1_TYPE>
static inline void AddTraceEvent(char phase,
                                const unsigned char* category_enabled,
                                const char* name,
                                unsigned long long id,
                                unsigned char flags,
                                const char* arg1_name,
                                const ARG1_TYPE& arg1_val) {
  const int num_args = 1;
  unsigned char arg_types[1];
  unsigned long long arg_values[1];
  SetTraceValue(arg1_val, &arg_types[0], &arg_values[0]);
  TRACE_EVENT_API_ADD_TRACE_EVENT(
      phase, category_enabled, name, id,
      num_args, &arg1_name, arg_types, arg_values,
      flags);
}

template<class ARG1_TYPE, class ARG2_TYPE>
static inline void AddTraceEvent(char phase,
                                const unsigned char* category_enabled,
                                const char* name,
                                unsigned long long id,
                                unsigned char flags,
                                const char* arg1_name,
                                const ARG1_TYPE& arg1_val,
                                const char* arg2_name,
                                const ARG2_TYPE& arg2_val) {
  const int num_args = 2;
  const char* arg_names[2] = { arg1_name, arg2_name };
  unsigned char arg_types[2];
  unsigned long long arg_values[2];
  SetTraceValue(arg1_val, &arg_types[0], &arg_values[0]);
  SetTraceValue(arg2_val, &arg_types[1], &arg_values[1]);
  TRACE_EVENT_API_ADD_TRACE_EVENT(
      phase, category_enabled, name, id,
      num_args, arg_names, arg_types, arg_values,
      flags);
}

// Used by TRACE_EVENTx macro. Do not use directly.
class TraceEndOnScopeClose {
 public:
  // Note: members of data_ intentionally left uninitialized. See Initialize.
  TraceEndOnScopeClose() : p_data_(nullptr) {}
  ~TraceEndOnScopeClose() {
    if (p_data_)
      AddEventIfEnabled();
  }

  void Initialize(const unsigned char* category_enabled,
                  const char* name) {
    data_.category_enabled = category_enabled;
    data_.name = name;
    p_data_ = &data_;
  }

 private:
  // Add the end event if the category is still enabled.
  void AddEventIfEnabled() {
    // Only called when p_data_ is non-null.
    if (*p_data_->category_enabled) {
      TRACE_EVENT_API_ADD_TRACE_EVENT(TRACE_EVENT_PHASE_END,
                                      p_data_->category_enabled, p_data_->name,
                                      kNoEventId, kZeroNumArgs, nullptr,
                                      nullptr, nullptr, TRACE_EVENT_FLAG_NONE);
    }
  }

  // This Data struct workaround is to avoid initializing all the members
  // in Data during construction of this object, since this object is always
  // constructed, even when tracing is disabled. If the members of Data were
  // members of this class instead, compiler warnings occur about potential
  // uninitialized accesses.
  struct Data {
    const unsigned char* category_enabled;
    const char* name;
  };
  Data* p_data_;
  Data data_;
};

}  // namespace trace_event_internal
}  // namespace webrtc

#endif  // RTC_BASE_TRACE_EVENT_H_