Bug ID: JDK-8138610 add assert to ThreadLocalAllocBuffer::clear_before_allocation to check the storage above top

Type: Enhancement
Component: hotspot
Sub-Component: gc
Affected Version: 9

Priority: P3
Status: Open
Resolution: Unresolved

Submitted: 2015-09-29
Updated: 2020-09-24

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tbdUnresolved

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
 }