Reland "Disable the CompilerDispatcher if we don't have idle time"

Original issue's description:
> Disable the CompilerDispatcher if we don't have idle time
>
> Since we can't do all steps on background threads, we need idle time to
> work
>
> BUG=v8:5215
> R=danno@chromium.org
>
> Review-Url: https://codereview.chromium.org/2600743002
> Cr-Commit-Position: refs/heads/master@{#41944}
> Committed: https://chromium.googlesource.com/v8/v8/+/a0d9eb346bba90aa0b32a
2d3184cbbfd6adb243e

BUG=v8:5215

Review-Url: https://codereview.chromium.org/2606233002
Cr-Commit-Position: refs/heads/master@{#42009}

src/compiler-dispatcher/compiler-dispatcher.cc

@@ -110,6 +110,8 @@ CompilerDispatcher::~CompilerDispatcher() {
 }
 
 bool CompilerDispatcher::Enqueue(Handle<SharedFunctionInfo> function) {
+  if (!IsEnabled()) return false;
+
   // We only handle functions (no eval / top-level code / wasm) that are
   // attached to a script.
   if (!function->script()->IsScript() || !function->is_function() ||
@@ -127,6 +129,11 @@ bool CompilerDispatcher::Enqueue(Handle<SharedFunctionInfo> function) {
   return true;
 }
 
+bool CompilerDispatcher::IsEnabled() const {
+  v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
+  return FLAG_compiler_dispatcher && platform_->IdleTasksEnabled(v8_isolate);
+}
+
 bool CompilerDispatcher::IsEnqueued(Handle<SharedFunctionInfo> function) const {
   return GetJobFor(function) != jobs_.end();
 }
@@ -183,7 +190,7 @@ CompilerDispatcher::JobMap::const_iterator CompilerDispatcher::GetJobFor(
 
 void CompilerDispatcher::ScheduleIdleTaskIfNeeded() {
   v8::Isolate* v8_isolate = reinterpret_cast<v8::Isolate*>(isolate_);
-  if (!platform_->IdleTasksEnabled(v8_isolate)) return;
+  DCHECK(platform_->IdleTasksEnabled(v8_isolate));
   if (idle_task_scheduled_) return;
   if (jobs_.empty()) return;
   idle_task_scheduled_ = true;

src/compiler-dispatcher/compiler-dispatcher.h

@@ -61,6 +61,7 @@ class V8_EXPORT_PRIVATE CompilerDispatcher {
       JobMap;
   class IdleTask;
 
+  bool IsEnabled() const;
   JobMap::const_iterator GetJobFor(Handle<SharedFunctionInfo> shared) const;
   void ScheduleIdleTaskIfNeeded();
   void DoIdleWork(double deadline_in_seconds);

src/flag-definitions.h

@@ -300,6 +300,7 @@ DEFINE_BOOL(string_slices, true, "use string slices")
 DEFINE_BOOL(ignition, false, "use ignition interpreter")
 DEFINE_BOOL(ignition_staging, false, "use ignition with all staged features")
 DEFINE_IMPLICATION(ignition_staging, ignition)
+DEFINE_IMPLICATION(ignition_staging, compiler_dispatcher)
 DEFINE_STRING(ignition_filter, "*", "filter for ignition interpreter")
 DEFINE_BOOL(ignition_deadcode, true,
             "use ignition dead code elimination optimizer")
@@ -666,6 +667,9 @@ DEFINE_BOOL(compilation_cache, true, "enable compilation cache")
 
 DEFINE_BOOL(cache_prototype_transitions, true, "cache prototype transitions")
 
+// compiler-dispatcher.cc
+DEFINE_BOOL(compiler_dispatcher, false, "enable compiler dispatcher")
+
 // cpu-profiler.cc
 DEFINE_INT(cpu_profiler_sampling_interval, 1000,
            "CPU profiler sampling interval in microseconds")

test/unittests/compiler-dispatcher/compiler-dispatcher-unittest.cc

@@ -16,7 +16,29 @@
 namespace v8 {
 namespace internal {
 
-typedef TestWithContext CompilerDispatcherTest;
+class CompilerDispatcherTest : public TestWithContext {
+ public:
+  CompilerDispatcherTest() = default;
+  ~CompilerDispatcherTest() override = default;
+
+  static void SetUpTestCase() {
+    old_flag_ = i::FLAG_ignition;
+    i::FLAG_compiler_dispatcher = true;
+    TestWithContext::SetUpTestCase();
+  }
+
+  static void TearDownTestCase() {
+    TestWithContext::TearDownTestCase();
+    i::FLAG_compiler_dispatcher = old_flag_;
+  }
+
+ private:
+  static bool old_flag_;
+
+  DISALLOW_COPY_AND_ASSIGN(CompilerDispatcherTest);
+};
+
+bool CompilerDispatcherTest::old_flag_;
 
 namespace {