Fix 0 getContractionSize() in MemorySubSpaceGenerational

getContractionSize() in MemorySubSpaceGenerational seems to always be 0.
MM_MemorySubSpace::getContractionSize() returns _contractionSize.
MM_MemorySubSpaceGenerational::checkResize() calls getMemorySubSpaceOld()->checkResize()
which sets the _contractionSize in its _memorySubSpaceOld but not its own.

env->_cycleState->_activeSubSpace->getContractionSize() != 0 is checked before calling
MM_ParallelGlobalGC::compactRequiredBeforeHeapContraction() in MM_ParallelGlobalGC::sweep().

Signed-off-by: Jason Hall <jasonhal@ca.ibm.com>

gc/base/MemorySubSpace.hpp

@@ -180,7 +180,7 @@ friend class GC_MemorySubSpaceRegionIterator;
 	MMINLINE void isAllocatable(bool isAllocatable) {_isAllocatable = isAllocatable; }
 	MMINLINE bool isAllocatable() { return _isAllocatable; }
 
-	MMINLINE uintptr_t getContractionSize() const { return _contractionSize; }
+	virtual MMINLINE uintptr_t getContractionSize() const { return _contractionSize; }
 	MMINLINE uintptr_t getExpansionSize() const { return _expansionSize; }
 	MMINLINE void setContractionSize(uintptr_t size) { _contractionSize = size; }
 	MMINLINE void setExpansionSize(uintptr_t size) { _expansionSize = size; }

gc/base/MemorySubSpaceGenerational.hpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 1991, 2015 IBM Corp. and others
+ * Copyright (c) 1991, 2020 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -109,6 +109,8 @@ class MM_MemorySubSpaceGenerational : public MM_MemorySubSpace
 	virtual uintptr_t releaseFreeMemoryPages(MM_EnvironmentBase* env);
 #endif
 
+	virtual MMINLINE uintptr_t getContractionSize() const { return _memorySubSpaceOld->getContractionSize(); }
+
 	/**
 	 * Create a MemorySubSpaceGenerational object.
 	 */