Revert of Remove default value for number of threads in Isolate::Init.

src/isolate.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stdlib.h>
 
 #include <fstream>  // NOLINT(readability/streams)
 #include <sstream>
 
 #include "src/v8.h"
@@ skipping 2036 matching lines @@
 
   InitializeThreadLocal();
 
   bootstrapper_->Initialize(create_heap_objects);
   builtins_.SetUp(this, create_heap_objects);
 
   if (FLAG_log_internal_timer_events) {
     set_event_logger(Logger::DefaultEventLoggerSentinel);
   }
 
+  // Set default value if not yet set.
+  // TODO(yangguo): move this to ResourceConstraints::ConfigureDefaults
+  // once ResourceConstraints becomes an argument to the Isolate constructor.
+  if (max_available_threads_ < 1) {
+    // Choose the default between 1 and 4.
+    max_available_threads_ =
+        Max(Min(base::SysInfo::NumberOfProcessors(), 4), 1);
+  }
+
   if (FLAG_trace_hydrogen || FLAG_trace_hydrogen_stubs) {
     PrintF("Concurrent recompilation has been disabled for tracing.\n");
   } else if (OptimizingCompilerThread::Enabled(max_available_threads_)) {
     optimizing_compiler_thread_ = new OptimizingCompilerThread(this);
     optimizing_compiler_thread_->Start();
   }
 
   // Initialize runtime profiler before deserialization, because collections may
   // occur, clearing/updating ICs.
   runtime_profiler_ = new RuntimeProfiler(this);
@@ skipping 513 matching lines @@
   if (prev_ && prev_->Intercept(flag)) return true;
   // Then check whether this scope intercepts.
   if ((flag & intercept_mask_)) {
     intercepted_flags_ |= flag;
     return true;
   }
   return false;
 }
 
 } }  // namespace v8::internal