[test] Make %SimulateNewspaceFull more robust

GC stress flags (--gc-interval, --random-gc-interval) could trigger
unexpected fake allocation failures while trying to fill up a page.
An AlwaysAllocateScope suppresses that.

Drive-by fix: allocation requests with exactly kMaxRegularHeapObjectSize
can take the fast path.

Bug: v8:9700
Change-Id: I7fa35e56848e74f12d3606da453e1ae7254c268b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1834121Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64079}

src/heap/heap-inl.h

@@ -176,8 +176,9 @@ AllocationResult Heap::AllocateRaw(int size_in_bytes, AllocationType type,
   HeapObject object;
   AllocationResult allocation;
 
-  if (FLAG_single_generation && type == AllocationType::kYoung)
+  if (FLAG_single_generation && type == AllocationType::kYoung) {
     type = AllocationType::kOld;
+  }
 
   if (AllocationType::kYoung == type) {
     if (large_object) {
@@ -250,7 +251,7 @@ HeapObject Heap::AllocateRawWith(int size, AllocationType allocation,
   Address* limit = heap->NewSpaceAllocationLimitAddress();
   if (allocation == AllocationType::kYoung &&
       alignment == AllocationAlignment::kWordAligned &&
-      size < kMaxRegularHeapObjectSize &&
+      size <= kMaxRegularHeapObjectSize &&
       (*limit - *top >= static_cast<unsigned>(size)) &&
       V8_LIKELY(!FLAG_single_generation && FLAG_inline_new &&
                 FLAG_gc_interval == 0)) {

src/runtime/runtime-test.cc

@@ -711,6 +711,7 @@ RUNTIME_FUNCTION(Runtime_SimulateNewspaceFull) {
   Heap* heap = isolate->heap();
   NewSpace* space = heap->new_space();
   PauseAllocationObserversScope pause_observers(heap);
+  AlwaysAllocateScope always_allocate(heap);
   do {
     FillUpOneNewSpacePage(isolate, heap);
   } while (space->AddFreshPage());