Try to simplify the semantics of the profiling code by making...

src/platform-macos.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 531 matching lines @@
   Sampler* sampler_;
   // Note: for profiled_thread_ Mach primitives are used instead of PThread's
   // because the latter doesn't provide thread manipulation primitives required.
   // For details, consult "Mac OS X Internals" book, Section 7.3.
   mach_port_t task_self_;
   thread_act_t profiled_thread_;
   pthread_t sampler_thread_;
 
   // Sampler thread handler.
   void Runner() {
-    // Loop until the sampler is disengaged, keeping the specified samling freq.
+    // Loop until the sampler is disengaged, keeping the specified
+    // sampling frequency.
     for ( ; sampler_->IsActive(); OS::Sleep(sampler_->interval_)) {
       TickSample sample_obj;
       TickSample* sample = CpuProfiler::TickSampleEvent();
       if (sample == NULL) sample = &sample_obj;
 
+      // If the sampler runs in sync with the JS thread, we try to
+      // suspend it. If we fail, we skip the current sample.
+      if (sampler_->IsSynchronous()) {
+        if (KERN_SUCCESS != thread_suspend(profiled_thread_)) continue;
+      }
+
       // We always sample the VM state.
       sample->state = VMState::current_state();
+
       // If profiling, we record the pc and sp of the profiled thread.
-      if (sampler_->IsProfiling()
-          && KERN_SUCCESS == thread_suspend(profiled_thread_)) {
+      if (sampler_->IsProfiling()) {
 #if V8_HOST_ARCH_X64
         thread_state_flavor_t flavor = x86_THREAD_STATE64;
         x86_thread_state64_t state;
         mach_msg_type_number_t count = x86_THREAD_STATE64_COUNT;
 #if __DARWIN_UNIX03
 #define REGISTER_FIELD(name) __r ## name
 #else
 #define REGISTER_FIELD(name) r ## name
 #endif  // __DARWIN_UNIX03
 #elif V8_HOST_ARCH_IA32
@@ skipping 11 matching lines @@
 
         if (thread_get_state(profiled_thread_,
                              flavor,
                              reinterpret_cast<natural_t*>(&state),
                              &count) == KERN_SUCCESS) {
           sample->pc = reinterpret_cast<Address>(state.REGISTER_FIELD(ip));
           sample->sp = reinterpret_cast<Address>(state.REGISTER_FIELD(sp));
           sample->fp = reinterpret_cast<Address>(state.REGISTER_FIELD(bp));
           sampler_->SampleStack(sample);
         }
-        thread_resume(profiled_thread_);
       }
 
       // Invoke tick handler with program counter and stack pointer.
       sampler_->Tick(sample);
+
+      // If the sampler runs in sync with the JS thread, we have to
+      // remember to resume it.
+      if (sampler_->IsSynchronous()) thread_resume(profiled_thread_);
     }
   }
 };
 
 #undef REGISTER_FIELD
 
 
 // Entry point for sampler thread.
 static void* SamplerEntry(void* arg) {
   Sampler::PlatformData* data =
       reinterpret_cast<Sampler::PlatformData*>(arg);
   data->Runner();
   return 0;
 }
 
 
 Sampler::Sampler(int interval, bool profiling)
-    : interval_(interval), profiling_(profiling), active_(false) {
+    : interval_(interval),
+      profiling_(profiling),
+      synchronous_(profiling),
+      active_(false) {
   data_ = new PlatformData(this);
 }
 
 
 Sampler::~Sampler() {
   delete data_;
 }
 
 
 void Sampler::Start() {
-  // If we are profiling, we need to be able to access the calling
-  // thread.
-  if (IsProfiling()) {
+  // If we are starting a synchronous sampler, we need to be able to
+  // access the calling thread.
+  if (IsSynchronous()) {
     data_->profiled_thread_ = mach_thread_self();
   }
 
   // Create sampler thread with high priority.
   // According to POSIX spec, when SCHED_FIFO policy is used, a thread
   // runs until it exits or blocks.
   pthread_attr_t sched_attr;
   sched_param fifo_param;
   pthread_attr_init(&sched_attr);
   pthread_attr_setinheritsched(&sched_attr, PTHREAD_EXPLICIT_SCHED);
   pthread_attr_setschedpolicy(&sched_attr, SCHED_FIFO);
   fifo_param.sched_priority = sched_get_priority_max(SCHED_FIFO);
   pthread_attr_setschedparam(&sched_attr, &fifo_param);
 
   active_ = true;
   pthread_create(&data_->sampler_thread_, &sched_attr, SamplerEntry, data_);
 }
 
 
 void Sampler::Stop() {
   // Seting active to false triggers termination of the sampler
   // thread.
   active_ = false;
 
   // Wait for sampler thread to terminate.
   pthread_join(data_->sampler_thread_, NULL);
 
   // Deallocate Mach port for thread.
-  if (IsProfiling()) {
+  if (IsSynchronous()) {
     mach_port_deallocate(data_->task_self_, data_->profiled_thread_);
   }
 }
 
 #endif  // ENABLE_LOGGING_AND_PROFILING
 
 } }  // namespace v8::internal