[Tracing] Remove unnecessary memory allocation in runtime call stats.

Previously we didn't implement TRACE_STR_COPY when we write trace events to
file, which causes us to allocate a growing independent memory chunk for dumped
runtime call stats table. Since we now have a fully functional TRACE_STR_COPY,
this memory allocation can be avoided, this patch removes it.

BUG=v8:5089

Committed: https://crrev.com/e1997bb7d780d12e3a89078e8dd652dcf1d90039
Review-Url: https://codereview.chromium.org/2342643004
Cr-Original-Commit-Position: refs/heads/master@{#39462}
Cr-Commit-Position: refs/heads/master@{#39510}

src/counters.cc

@@ -364,7 +364,7 @@ void RuntimeCallStats::Reset() {
   in_use_ = true;
 }
 
-const char* RuntimeCallStats::Dump() {
+std::string RuntimeCallStats::Dump() {
   buffer_.str(std::string());
   buffer_.clear();
   buffer_ << "{";
@@ -393,15 +393,8 @@ const char* RuntimeCallStats::Dump() {
   FOR_EACH_HANDLER_COUNTER(DUMP_COUNTER)
 #undef DUMP_COUNTER
   buffer_ << "\"END\":[]}";
-  const std::string& buffer_str = buffer_.str();
-  size_t length = buffer_str.size();
-  if (length > len_) {
-    buffer_c_str_.reset(new char[length + 1]);
-    len_ = length;
-  }
-  strncpy(buffer_c_str_.get(), buffer_str.c_str(), length + 1);
   in_use_ = false;
-  return buffer_c_str_.get();
+  return buffer_.str();
 }
 
 }  // namespace internal

src/counters.h

@@ -791,7 +791,7 @@ class RuntimeCallStats {
 
   void Reset();
   V8_NOINLINE void Print(std::ostream& os);
-  V8_NOINLINE const char* Dump();
+  V8_NOINLINE std::string Dump();
 
   RuntimeCallStats() {
     Reset();
@@ -803,8 +803,6 @@ class RuntimeCallStats {
 
  private:
   std::stringstream buffer_;
-  std::unique_ptr<char[]> buffer_c_str_;
-  size_t len_ = 0;
   // Counter to track recursive time events.
   RuntimeCallTimer* current_timer_ = NULL;
   // Used to track nested tracing scopes.

src/tracing/trace-event.cc

@@ -28,9 +28,10 @@ void CallStatsScopedTracer::AddEndTraceEvent() {
         TRACE_EVENT_PHASE_END, p_data_->category_group_enabled, p_data_->name,
         v8::internal::tracing::kGlobalScope, v8::internal::tracing::kNoId,
         v8::internal::tracing::kNoId, TRACE_EVENT_FLAG_NONE,
-        "runtime-call-stats",
-        TRACE_STR_COPY(
-            p_data_->isolate->counters()->runtime_call_stats()->Dump()));
+        "runtime-call-stats", TRACE_STR_COPY(p_data_->isolate->counters()
+                                                 ->runtime_call_stats()
+                                                 ->Dump()
+                                                 .c_str()));
   } else {
     v8::internal::tracing::AddTraceEvent(
         TRACE_EVENT_PHASE_END, p_data_->category_group_enabled, p_data_->name,