Reland "[heap] Disable the young generation in shared heaps"

This is a reland of 1186fc50

This reland fixes NewSpaceAllocationTopAddress() and
NewSpaceAllocationLimitAddress() by returning nullptr if no new space
is available. This is okay since those are never used later on.

We can't make this a build-time flag because we may only want to disable
the new space for the shared heap.

Original change's description:
> [heap] Disable the young generation in shared heaps
>
> A shared heap will not have a young generation in the beginning.
>
> Bug: v8:11708
> Change-Id: I947ddb91a23a72a8cee3aa3e554723dda8146011
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2891569
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74697}

Bug: v8:11708
Change-Id: I254b919f7076ce624d15c924e63cbde5eb4df749
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2912731Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74735}

src/heap/heap-inl.h

@@ -143,11 +143,11 @@ PagedSpace* Heap::paged_space(int idx) {
 Space* Heap::space(int idx) { return space_[idx]; }
 
 Address* Heap::NewSpaceAllocationTopAddress() {
-  return new_space_->allocation_top_address();
+  return new_space_ ? new_space_->allocation_top_address() : nullptr;
 }
 
 Address* Heap::NewSpaceAllocationLimitAddress() {
-  return new_space_->allocation_limit_address();
+  return new_space_ ? new_space_->allocation_limit_address() : nullptr;
 }
 
 Address* Heap::OldSpaceAllocationTopAddress() {

src/heap/heap.cc

@@ -449,7 +449,7 @@ GarbageCollector Heap::SelectGarbageCollector(AllocationSpace space,
     return MARK_COMPACTOR;
   }
 
-  if (FLAG_gc_global || ShouldStressCompaction() || FLAG_single_generation) {
+  if (FLAG_gc_global || ShouldStressCompaction() || !new_space()) {
     *reason = "GC in old space forced by flags";
     return MARK_COMPACTOR;
   }
@@ -2410,7 +2410,8 @@ void Heap::MarkCompact() {
 
 void Heap::MinorMarkCompact() {
 #ifdef ENABLE_MINOR_MC
-  DCHECK(FLAG_minor_mc && !FLAG_single_generation);
+  DCHECK(FLAG_minor_mc);
+  DCHECK(new_space());
 
   PauseAllocationObserversScope pause_observers(this);
   SetGCState(MINOR_MARK_COMPACT);
@@ -2517,7 +2518,7 @@ void Heap::EvacuateYoungGeneration() {
 }
 
 void Heap::Scavenge() {
-  DCHECK(!FLAG_single_generation);
+  DCHECK_NOT_NULL(new_space());
 
   if (fast_promotion_mode_ && CanPromoteYoungAndExpandOldGeneration(0)) {
     tracer()->NotifyYoungGenerationHandling(
@@ -5435,7 +5436,8 @@ class StressConcurrentAllocationObserver : public AllocationObserver {
 void Heap::SetUpSpaces() {
   // Ensure SetUpFromReadOnlySpace has been ran.
   DCHECK_NOT_NULL(read_only_space_);
-  if (!FLAG_single_generation) {
+  const bool has_young_gen = !FLAG_single_generation && !IsShared();
+  if (has_young_gen) {
     space_[NEW_SPACE] = new_space_ =
         new NewSpace(this, memory_allocator_->data_page_allocator(),
                      initial_semispace_size_, max_semi_space_size_);
@@ -5444,7 +5446,7 @@ void Heap::SetUpSpaces() {
   space_[CODE_SPACE] = code_space_ = new CodeSpace(this);
   space_[MAP_SPACE] = map_space_ = new MapSpace(this);
   space_[LO_SPACE] = lo_space_ = new OldLargeObjectSpace(this);
-  if (!FLAG_single_generation) {
+  if (has_young_gen) {
     space_[NEW_LO_SPACE] = new_lo_space_ =
         new NewLargeObjectSpace(this, NewSpaceCapacity());
   }
@@ -5483,7 +5485,7 @@ void Heap::SetUpSpaces() {
   }
 #endif  // ENABLE_MINOR_MC
 
-  if (!FLAG_single_generation) {
+  if (new_space()) {
     scavenge_job_.reset(new ScavengeJob());
     scavenge_task_observer_.reset(new ScavengeTaskObserver(
         this, ScavengeJob::YoungGenerationTaskTriggerSize(this)));

test/cctest/heap/test-shared-heap.cc

@@ -83,6 +83,9 @@ UNINITIALIZED_TEST(SharedCollection) {
   create_params.array_buffer_allocator = allocator.get();
   Isolate* shared_isolate = Isolate::NewShared(create_params);
 
+  DCHECK_NULL(shared_isolate->heap()->new_space());
+  DCHECK_NULL(shared_isolate->heap()->new_lo_space());
+
   CcTest::CollectGarbage(OLD_SPACE, shared_isolate);
   Isolate::Delete(shared_isolate);
 }