This is a patch that came out of JVMCI.

--- old/src/share/vm/gc/shared/threadLocalAllocBuffer.cpp 2015-09-24 08:37:21.000000000 -1000
+++ new/src/share/vm/gc/shared/threadLocalAllocBuffer.cpp 2015-09-24 08:37:21.000000000 -1000
@@ -40,6 +40,9 @@
 
 void ThreadLocalAllocBuffer::clear_before_allocation() {
   _slow_refill_waste += (unsigned)remaining();
+ // In debug mode we expect the storage above top to be uninitialized
+ // or filled with a padding object.
+ assert(!ZapUnusedHeapArea || VM_Version::reserve_for_allocation_prefetch() > 0 || top() == NULL || *(intptr_t*)top() != 0, "overzeroing detected");
   make_parsable(true); // also retire the TLAB
 }