8333750: GenShen: Only instantiate young/old generations in generational mode

src/hotspot/share/gc/shenandoah/shenandoahConcurrentGC.cpp

@@ -1224,7 +1224,9 @@ void ShenandoahConcurrentGC::op_final_roots() {
       heap->old_generation()->transfer_pointers_from_satb();
     }
 
-    ShenandoahGenerationalHeap::heap()->update_region_ages();
+    if (!_generation->is_old()) {
+      ShenandoahGenerationalHeap::heap()->update_region_ages(_generation->complete_marking_context());
+    }
   }
 }
 

src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.cpp

@@ -1116,8 +1116,7 @@ void ShenandoahGenerationalHeap::entry_global_coalesce_and_fill() {
   coalesce_and_fill_old_regions(true);
 }
 
-void ShenandoahGenerationalHeap::update_region_ages() {
-  ShenandoahMarkingContext* ctx = active_generation()->complete_marking_context();
+void ShenandoahGenerationalHeap::update_region_ages(ShenandoahMarkingContext* ctx) {
   ShenandoahUpdateRegionAges cl(ctx);
   parallel_heap_region_iterate(&cl);
 }

src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.hpp

@@ -68,7 +68,7 @@ class ShenandoahGenerationalHeap : public ShenandoahHeap {
 
   // Ages regions that haven't been used for allocations in the current cycle.
   // Resets ages for regions that have been used for allocations.
-  void update_region_ages();
+  void update_region_ages(ShenandoahMarkingContext* ctx);
 
   oop evacuate_object(oop p, Thread* thread) override;
   oop try_evacuate_object(oop p, Thread* thread, ShenandoahHeapRegion* from_region, ShenandoahAffiliation target_gen);

src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp

@@ -453,8 +453,8 @@ class ShenandoahHeapRegion {
   void set_affiliation(ShenandoahAffiliation new_affiliation);
 
   // Region ageing and rejuvenation
-  uint age() { return _age; }
-  CENSUS_NOISE(uint youth() { return _youth; })
+  uint age() const { return _age; }
+  CENSUS_NOISE(uint youth() const { return _youth; })
 
   void increment_age() {
     const uint max_age = markWord::max_age;

src/hotspot/share/gc/shenandoah/shenandoahOldGC.cpp

@@ -111,6 +111,12 @@ bool ShenandoahOldGC::collect(GCCause::Cause cause) {
   // Complete marking under STW
   vmop_entry_final_mark();
 
+  if (_generation->is_concurrent_mark_in_progress()) {
+    assert(heap->cancelled_gc(), "Safepoint operation observed gc cancellation");
+    // GC may have been cancelled before final mark, but after the preceding cancellation check.
+    return false;
+  }
+
   // We aren't dealing with old generation evacuation yet. Our heuristic
   // should not have built a cset in final mark.
   assert(!heap->is_evacuation_in_progress(), "Old gen evacuations are not supported");