Fix crbug/24815. Changes affect profiler "lazy" mode used for V8 in Chromium.

- don't engage the processing thread of CPU profiling until the first time profiling is resumed, this saves us a thread allocation for the majority of users;
- don't log shared libraries addresses: this is useless for JS-only profiling, and also consumes time on startup.

Review URL: http://codereview.chromium.org/340013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3154 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/log.cc

@@ -125,6 +125,9 @@ class Profiler: public Thread {
   bool overflow_;  // Tell whether a buffer overflow has occurred.
   Semaphore* buffer_semaphore_;  // Sempahore used for buffer synchronization.
 
+  // Tells whether profiler is engaged, that is, processing thread is stated.
+  bool engaged_;
+
   // Tells whether worker thread should continue running.
   bool running_;
 
@@ -243,17 +246,25 @@ void SlidingStateWindow::AddState(StateTag state) {
 //
 // Profiler implementation.
 //
-Profiler::Profiler() {
-  buffer_semaphore_ = OS::CreateSemaphore(0);
-  head_ = 0;
-  tail_ = 0;
-  overflow_ = false;
-  running_ = false;
+Profiler::Profiler()
+    : head_(0),
+      tail_(0),
+      overflow_(false),
+      buffer_semaphore_(OS::CreateSemaphore(0)),
+      engaged_(false),
+      running_(false) {
 }
 
 
 void Profiler::Engage() {
+  if (engaged_) return;
+  engaged_ = true;
+
+  // TODO(mnaganov): This is actually "Chromium" mode. Flags need to be revised.
+  // http://code.google.com/p/v8/issues/detail?id=487
+  if (!FLAG_prof_lazy) {
     OS::LogSharedLibraryAddresses();
+  }
 
   // Start thread processing the profiler buffer.
   running_ = true;
@@ -268,6 +279,8 @@ void Profiler::Engage() {
 
 
 void Profiler::Disengage() {
+  if (!engaged_) return;
+
   // Stop receiving ticks.
   Logger::ticker_->ClearProfiler();
 
@@ -1053,6 +1066,7 @@ void Logger::ResumeProfiler(int flags) {
   }
   if (modules_to_enable & PROFILER_MODULE_CPU) {
     if (FLAG_prof_lazy) {
+      profiler_->Engage();
       LOG(UncheckedStringEvent("profiler", "resume"));
       FLAG_log_code = true;
       LogCompiledFunctions();
@@ -1245,8 +1259,10 @@ bool Logger::Setup() {
     } else {
       is_logging_ = true;
     }
+    if (!FLAG_prof_lazy) {
       profiler_->Engage();
     }
+  }
 
   LogMessageBuilder::set_write_failure_handler(StopLoggingAndProfiling);
 

test/cctest/test-log.cc

@@ -256,11 +256,10 @@ TEST(ProfLazyMode) {
   // No sampling should happen prior to resuming profiler.
   CHECK(!LoggerTestHelper::IsSamplerActive());
 
-  // Read initial logged data (static libs map).
   EmbeddedVector<char, 102400> buffer;
+  // Nothing must be logged until profiling is resumed.
   int log_pos = GetLogLines(0, &buffer);
-  CHECK_GT(log_pos, 0);
-  CHECK_GT(buffer.length(), log_pos);
+  CHECK_EQ(0, log_pos);
 
   CompileAndRunScript("var a = (function(x) { return x + 1; })(10);");