Added support to trace_event for passing static string arguments without copy

base/debug/trace_event.h

Index: base/debug/trace_event.h
diff --git a/base/debug/trace_event.h b/base/debug/trace_event.h
index 0fcdaff165927c405a33d1dd012cc8d2680bf86f..2b6b9e15f4ec50bb586e290acaad6efdde659652 100644
--- a/base/debug/trace_event.h
+++ b/base/debug/trace_event.h
@@ -96,6 +96,11 @@
 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
 #include "base/timer.h"
 
+// Short hand convenience macro for specifying unscoped strings as argument
+// values to avoid allocating/copying. The string pointer must remain valid
+// until tracing is disabled.
+#define TRACE_STATIC_STR(str) base::debug::TraceValue::StaticString(str)
+
 // Older style trace macros with explicit id and extra data
 // Only these macros result in publishing data to ETW as currently implemented.
 #define TRACE_EVENT_BEGIN_ETW(name, id, extra) \
@@ -313,7 +318,8 @@ class BASE_EXPORT TraceValue {
     TRACE_TYPE_INT,
     TRACE_TYPE_DOUBLE,
     TRACE_TYPE_POINTER,
-    TRACE_TYPE_STRING
+    TRACE_TYPE_STRING,
+    TRACE_TYPE_STATIC_STRING
   };
 
   TraceValue() : type_(TRACE_TYPE_UNDEFINED) {
@@ -355,6 +361,12 @@ class BASE_EXPORT TraceValue {
   TraceValue(const char* rhs) : type_(TRACE_TYPE_STRING) {
     value_.as_string = rhs;
   }
+  // Use this if your string is already global and does not need to be copied.
+  static TraceValue StaticString(const char* rhs) {
+    TraceValue tv(rhs);
+    tv.type_ = TRACE_TYPE_STATIC_STRING;
+    return tv;
+  }
 
   void AppendAsJSON(std::string* out) const;
 
@@ -382,7 +394,7 @@ class BASE_EXPORT TraceValue {
     return value_.as_pointer;
   }
   const char* as_string() const {
-    DCHECK_EQ(TRACE_TYPE_STRING, type_);
+    DCHECK(type_ == TRACE_TYPE_STRING || type_ == TRACE_TYPE_STATIC_STRING);
     return value_.as_string;
   }
   const char** as_assignable_string() {
@@ -431,6 +443,14 @@ class TraceEvent {
 
   TimeTicks timestamp() const { return timestamp_; }
 
+  // Exposed for unittesting:
+
+  const base::RefCountedString* parameter_copy_storage() const {
+    return parameter_copy_storage_.get();
+  }
+
+  const char* name() const { return name_; }
+
  private:
   unsigned long process_id_;
   unsigned long thread_id_;
@@ -502,10 +522,18 @@ class BASE_EXPORT TraceLog {
     AddTraceEventEtw(phase, name, id, extra.c_str());
   }
 
-  // Exposed for unittesting only, allows resurrecting our
-  // singleton instance post-AtExit processing.
+  // Exposed for unittesting:
+
+  // Allows resurrecting our singleton instance post-AtExit processing.
   static void Resurrect();
 
+  // Allow tests to inspect TraceEvents.
+  size_t GetEventsSize() const { return logged_events_.size(); }
+  const TraceEvent& GetEventAt(size_t index) const {
+    DCHECK(index < logged_events_.size());
+    return logged_events_[index];
+  }
+
  private:
   // This allows constructor and destructor to be private and usable only
   // by the Singleton class.