[tracing] OPTIONAL_TRACE_EVENT

Add OPTIONAL_TRACE_EVENT{0,1,2} macros. They are equivalent to regular
TRACE_EVENTs, but they are disabled by default on the platforms where
binary size is a significant factor.

R=eseckler@chromium.org
BUG=1043616

Change-Id: Ifb4265856826423c5079c5d412fc1234db19e820
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359132
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800770}

base/BUILD.gn

@@ -18,6 +18,7 @@
 # huge sequence of random-looking conditionals.
 
 import("//base/allocator/allocator.gni")
+import("//base/trace_event/tracing.gni")
 import("//build/buildflag_header.gni")
 import("//build/config/allocator.gni")
 import("//build/config/arm.gni")
@@ -2106,6 +2107,7 @@ component("base") {
       "trace_event/memory_infra_background_allowlist.h",
       "trace_event/memory_usage_estimator.cc",
       "trace_event/memory_usage_estimator.h",
+      "trace_event/optional_trace_event.h",
       "trace_event/process_memory_dump.cc",
       "trace_event/process_memory_dump.h",
       "trace_event/thread_instruction_count.cc",
@@ -2298,6 +2300,7 @@ buildflag_header("tracing_buildflags") {
   flags = [
     "ENABLE_BASE_TRACING=$enable_base_tracing",
     "USE_PERFETTO_CLIENT_LIBRARY=$use_perfetto_client_library",
+    "OPTIONAL_TRACE_EVENTS_ENABLED=$optional_trace_events_enabled",
   ]
 }
 

base/trace_event/optional_trace_event.h

@@ -0,0 +1,31 @@
+// Copyright 2020 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.
+
+#ifndef BASE_TRACE_EVENT_OPTIONAL_TRACE_EVENT_H_
+#define BASE_TRACE_EVENT_OPTIONAL_TRACE_EVENT_H_
+
+#include "base/trace_event/trace_event.h"
+#include "base/tracing_buildflags.h"
+#include "build/buildflag.h"
+
+// These macros are functionally equivalent to TRACE_EVENTX macros,
+// but they are disabled if optional_trace_events_enabled gn flag
+// defined in tracing.gni is false.
+
+#if BUILDFLAG(OPTIONAL_TRACE_EVENTS_ENABLED)
+
+#define OPTIONAL_TRACE_EVENT0(...) TRACE_EVENT0(__VA_ARGS__)
+#define OPTIONAL_TRACE_EVENT1(...) TRACE_EVENT1(__VA_ARGS__)
+#define OPTIONAL_TRACE_EVENT2(...) TRACE_EVENT2(__VA_ARGS__)
+
+#else  // BUILDFLAG(OPTIONAL_TRACE_EVENTS_ENABLED)
+
+#define OPTIONAL_TRACE_EVENT0(category, name)
+#define OPTIONAL_TRACE_EVENT1(category, name, arg1_name, arg1_val)
+#define OPTIONAL_TRACE_EVENT2(category, name, arg1_name, arg1_val, arg2_name, \
+                              arg2_val)
+
+#endif  // BUILDFLAG(OPTIONAL_TRACE_EVENTS_ENABLED)
+
+#endif  // BASE_TRACE_EVENT_OPTIONAL_TRACE_EVENT_H_

base/trace_event/tracing.gni

@@ -0,0 +1,20 @@
+# Copyright 2020 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.
+
+import("//build/config/chrome_build.gni")
+
+declare_args() {
+  # Enable more trace events. Disabled by default due to binary size impact,
+  # but highly recommended for local development.
+  extended_tracing_enabled = false
+}
+
+# Separate block so that we can refer to extended_tracing_enabled's value.
+declare_args() {
+  # Whether OPTIONAL_TRACE_EVENT macros are included in the build or not.
+  # Disabled by default on Android and ChromeOS due to binary size impact,
+  # enabled everywhere else.
+  optional_trace_events_enabled =
+      (!is_android && !is_chromeos) || extended_tracing_enabled
+}