Move static flag modifications to V8::InitializeOncePerProcessImpl().

src/v8.cc

Index: src/v8.cc
diff --git a/src/v8.cc b/src/v8.cc
index 80b12deea62ca80773964baa70115d53389f497f..f9d543d16c8ea6e433c3755b0e18105e80a7eaf2 100644
--- a/src/v8.cc
+++ b/src/v8.cc
@@ -271,6 +271,34 @@ void V8::InitializeOncePerProcessImpl() {
     FLAG_max_new_space_size = (1 << (kPageSizeBits - 10)) * 2;
   }
   if (FLAG_trace_hydrogen) FLAG_parallel_recompilation = false;
+  if (FLAG_sweeper_threads == 0) {
+    if (FLAG_concurrent_sweeping) {
+      FLAG_sweeper_threads = SystemThreadManager::
+          NumberOfParallelSystemThreads(
+              SystemThreadManager::CONCURRENT_SWEEPING);
+    } else if (FLAG_parallel_sweeping) {
+      FLAG_sweeper_threads = SystemThreadManager::
+          NumberOfParallelSystemThreads(
+              SystemThreadManager::PARALLEL_SWEEPING);
+    }
+    if (FLAG_sweeper_threads == 0) {
+      FLAG_concurrent_sweeping = false;
+      FLAG_parallel_sweeping = false;
+    }
+  }
+  if (FLAG_parallel_marking && FLAG_marking_threads == 0) {
+    FLAG_marking_threads = SystemThreadManager::
+        NumberOfParallelSystemThreads(
+            SystemThreadManager::PARALLEL_MARKING);
+    if (FLAG_marking_threads == 0) {
+      FLAG_parallel_marking = false;
+    }
+  }
+  if (FLAG_parallel_recompilation &&
+      SystemThreadManager::NumberOfParallelSystemThreads(
+          SystemThreadManager::PARALLEL_RECOMPILATION) == 0) {
+    FLAG_parallel_recompilation = false;
+  }
   OS::SetUp();
   Sampler::SetUp();
   CPU::SetUp();