[v8::heap] Include main thread in num_tasks computations.

The num_tasks computation has long been based on NumberOfAvailableBackgroundThreads()

We used to have one background worker per core, stealing cycles from
the main thread. I fixed that @ crrev.com/534414. But now this
computation is wrong and generates one less task than it should (one
per worker but the main thread takes task #0 in practice).

Other usage of NumberOfAvailableBackgroundThreads() in V8 seem correct
already so this is the only tweak required.

R=mlippautz@chromium.org

Bug: chromium:808028
Change-Id: I784ed9b764017f146931547d30be4a3b180b5a2c
Reviewed-on: https://chromium-review.googlesource.com/904662
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51121}

src/heap/heap.cc

@@ -2010,11 +2010,10 @@ int Heap::NumberOfScavengeTasks() {
   if (!FLAG_parallel_scavenge) return 1;
   const int num_scavenge_tasks =
       static_cast<int>(new_space()->TotalCapacity()) / MB;
-  return Max(
-      1,
-      Min(Min(num_scavenge_tasks, kMaxScavengerTasks),
-          static_cast<int>(
-              V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads())));
+  static int num_cores =
+      1 + static_cast<int>(
+              V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads());
+  return Max(1, Min(Min(num_scavenge_tasks, kMaxScavengerTasks), num_cores));
 }
 
 void Heap::Scavenge() {

src/heap/mark-compact.cc

@@ -404,14 +404,22 @@ class RootMarkingVisitorSeedOnly : public RootVisitor {
   std::vector<Object*> buffered_objects_;
 };
 
-}  // namespace
-
-static int NumberOfAvailableCores() {
-  return Max(
-      1, static_cast<int>(
-             V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads()));
+int NumberOfAvailableCores() {
+  static int num_cores =
+      static_cast<int>(
+          V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads()) +
+      1;
+  // This number of cores should be greater than zero and never change.
+  DCHECK_GE(num_cores, 1);
+  DCHECK_EQ(
+      num_cores,
+      1 + static_cast<int>(
+              V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads()));
+  return num_cores;
 }
 
+}  // namespace
+
 int MarkCompactCollectorBase::NumberOfParallelCompactionTasks(int pages) {
   DCHECK_GT(pages, 0);
   return FLAG_parallel_compaction ? Min(NumberOfAvailableCores(), pages) : 1;
@@ -3253,15 +3261,16 @@ void MarkCompactCollectorBase::CreateAndExecuteEvacuationTasks(
   delete[] evacuators;
 
   if (FLAG_trace_evacuation) {
-    PrintIsolate(isolate(),
-                 "%8.0f ms: evacuation-summary: parallel=%s pages=%d "
-                 "wanted_tasks=%d tasks=%d cores=%" PRIuS
-                 " live_bytes=%" V8PRIdPTR " compaction_speed=%.f\n",
-                 isolate()->time_millis_since_init(),
-                 FLAG_parallel_compaction ? "yes" : "no", job->NumberOfItems(),
-                 wanted_num_tasks, job->NumberOfTasks(),
-                 V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads(),
-                 live_bytes, compaction_speed);
+    PrintIsolate(
+        isolate(),
+        "%8.0f ms: evacuation-summary: parallel=%s pages=%d "
+        "wanted_tasks=%d tasks=%d cores=%" PRIuS " live_bytes=%" V8PRIdPTR
+        " compaction_speed=%.f\n",
+        isolate()->time_millis_since_init(),
+        FLAG_parallel_compaction ? "yes" : "no", job->NumberOfItems(),
+        wanted_num_tasks, job->NumberOfTasks(),
+        V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads() + 1,
+        live_bytes, compaction_speed);
   }
 }