[VM/Runtime] Inline first zone in a StackZone when we are not using

fibers.

TEST=ci

Change-Id: I1308b2450d77ac83c89208acb7d18ad1be5bdcbf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243856
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

runtime/vm/zone.cc

@@ -320,19 +320,25 @@ char* Zone::VPrint(const char* format, va_list args) {
 }
 
 StackZone::StackZone(ThreadState* thread)
+#if defined(DART_USE_ABSL)
+    // DART_USE_ABSL encodes the use of fibers in the Dart VM for threading.
     : StackResource(thread), zone_(new Zone()) {
+#else
+    : StackResource(thread), zone_() {
+#endif  // defined(DART_USE_ABSL)
   if (FLAG_trace_zones) {
     OS::PrintErr("*** Starting a new Stack zone 0x%" Px "(0x%" Px ")\n",
                  reinterpret_cast<intptr_t>(this),
-                 reinterpret_cast<intptr_t>(zone_));
+                 reinterpret_cast<intptr_t>(GetZone()));
   }
 
   // This thread must be preventing safepoints or the GC could be visiting the
   // chain of handle blocks we're about the mutate.
   ASSERT(Thread::Current()->MayAllocateHandles());
 
-  zone_->Link(thread->zone());
-  thread->set_zone(zone_);
+  Zone* lzone = GetZone();
+  lzone->Link(thread->zone());
+  thread->set_zone(lzone);
 }
 
 StackZone::~StackZone() {
@@ -340,15 +346,19 @@ StackZone::~StackZone() {
   // chain of handle blocks we're about the mutate.
   ASSERT(Thread::Current()->MayAllocateHandles());
 
-  ASSERT(thread()->zone() == zone_);
-  thread()->set_zone(zone_->previous_);
+  Zone* lzone = GetZone();
+  ASSERT(thread()->zone() == lzone);
+  thread()->set_zone(lzone->previous_);
   if (FLAG_trace_zones) {
     OS::PrintErr("*** Deleting Stack zone 0x%" Px "(0x%" Px ")\n",
                  reinterpret_cast<intptr_t>(this),
-                 reinterpret_cast<intptr_t>(zone_));
+                 reinterpret_cast<intptr_t>(lzone));
   }
 
+#if defined(DART_USE_ABSL)
+  // DART_USE_ABSL encodes the use of fibers in the Dart VM for threading.
   delete zone_;
+#endif  // defined(DART_USE_ABSL)
 }
 
 }  // namespace dart

runtime/vm/zone.h

@@ -192,6 +192,8 @@ class StackZone : public StackResource {
   // Delete all memory associated with the zone.
   virtual ~StackZone();
 
+  // DART_USE_ABSL encodes the use of fibers in the Dart VM for threading,
+#if defined(DART_USE_ABSL)
   // Compute the total size of this zone. This includes wasted space that is
   // due to internal fragmentation in the segments.
   uintptr_t SizeInBytes() const { return zone_->SizeInBytes(); }
@@ -200,9 +202,34 @@ class StackZone : public StackResource {
   intptr_t CapacityInBytes() const { return zone_->CapacityInBytes(); }
 
   Zone* GetZone() { return zone_; }
+#else
+  // Compute the total size of this zone. This includes wasted space that is
+  // due to internal fragmentation in the segments.
+  uintptr_t SizeInBytes() const {
+    return zone_.SizeInBytes();
+  }
+
+  // Computes the used space in the zone.
+  intptr_t CapacityInBytes() const {
+    return zone_.CapacityInBytes();
+  }
+
+  Zone* GetZone() {
+    return &zone_;
+  }
+#endif  // defined(DART_USE_ABSL)
 
  private:
+#if defined(DART_USE_ABSL)
+  // When fibers are used we have to make do with a smaller stack size and hence
+  // the first zone is allocated instead of being a stack resource.
   Zone* zone_;
+#else
+  // For regular configurations that have larger stack sizes it is ok to
+  // have the first zone be a stack resource, avoids the overhead of a malloc
+  // call for every stack zone creation.
+  Zone zone_;
+#endif  // defined(DART_USE_ABSL)
 
   template <typename T>
   friend class GrowableArray;