8329134: Reconsider TLAB zapping

Reviewed-by: stefank, rkennke

src/hotspot/share/gc/shared/memAllocator.cpp

@@ -316,18 +316,15 @@ HeapWord* MemAllocator::mem_allocate_inside_tlab_slow(Allocation& allocation) co
          PTR_FORMAT " min: " SIZE_FORMAT ", desired: " SIZE_FORMAT,
          p2i(mem), min_tlab_size, new_tlab_size);
 
+  // ...and clear or zap just allocated TLAB, if needed.
   if (ZeroTLAB) {
-    // ..and clear it.
     Copy::zero_to_words(mem, allocation._allocated_tlab_size);
-  } else {
-    // ...and zap just allocated object.
-#ifdef ASSERT
+  } else if (ZapTLAB) {
     // Skip mangling the space corresponding to the object header to
     // ensure that the returned space is not considered parsable by
     // any concurrent GC thread.
     size_t hdr_size = oopDesc::header_size();
     Copy::fill_to_words(mem + hdr_size, allocation._allocated_tlab_size - hdr_size, badHeapWordVal);
-#endif // ASSERT
   }
 
   tlab.fill(mem, mem + _word_size, allocation._allocated_tlab_size);

src/hotspot/share/gc/shared/threadLocalAllocBuffer.inline.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -39,14 +39,8 @@ inline HeapWord* ThreadLocalAllocBuffer::allocate(size_t size) {
   invariants();
   HeapWord* obj = top();
   if (pointer_delta(end(), obj) >= size) {
-    // successful thread-local allocation
-#ifdef ASSERT
-    // Skip mangling the space corresponding to the object header to
-    // ensure that the returned space is not considered parsable by
-    // any concurrent GC thread.
-    size_t hdr_size = oopDesc::header_size();
-    Copy::fill_to_words(obj + hdr_size, size - hdr_size, badHeapWordVal);
-#endif // ASSERT
+    // Successful thread-local allocation.
+
     // This addition is safe because we know that top is
     // at least size below end, so the add can't wrap.
     set_top(obj + size);

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

@@ -878,18 +878,15 @@ HeapWord* ShenandoahHeap::allocate_from_gclab_slow(Thread* thread, size_t size)
 
   assert (size <= actual_size, "allocation should fit");
 
+  // ...and clear or zap just allocated TLAB, if needed.
   if (ZeroTLAB) {
-    // ..and clear it.
     Copy::zero_to_words(gclab_buf, actual_size);
-  } else {
-    // ...and zap just allocated object.
-#ifdef ASSERT
+  } else if (ZapTLAB) {
     // Skip mangling the space corresponding to the object header to
     // ensure that the returned space is not considered parsable by
     // any concurrent GC thread.
     size_t hdr_size = oopDesc::header_size();
     Copy::fill_to_words(gclab_buf + hdr_size, actual_size - hdr_size, badHeapWordVal);
-#endif // ASSERT
   }
   gclab->set_buf(gclab_buf, actual_size);
   return gclab->allocate(size);

src/hotspot/share/interpreter/zero/bytecodeInterpreter.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2002, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2002, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -1991,11 +1991,9 @@ run:
             size_t obj_size = ik->size_helper();
             HeapWord* result = THREAD->tlab().allocate(obj_size);
             if (result != nullptr) {
-              // Initialize object field block:
-              //   - if TLAB is pre-zeroed, we can skip this path
-              //   - in debug mode, ThreadLocalAllocBuffer::allocate mangles
-              //     this area, and we still need to initialize it
-              if (DEBUG_ONLY(true ||) !ZeroTLAB) {
+              // Initialize object field block.
+              if (!ZeroTLAB) {
+                // The TLAB was not pre-zeroed, we need to clear the memory here.
                 size_t hdr_size = oopDesc::header_size();
                 Copy::fill_to_words(result + hdr_size, obj_size - hdr_size, 0);
               }

src/hotspot/share/runtime/globals.hpp

@@ -483,6 +483,9 @@ const int ObjectAlignmentInBytes = 8;
   develop(bool, ZapFillerObjects, trueInDebug,                              \
           "Zap filler objects")                                             \
                                                                             \
+  develop(bool, ZapTLAB, trueInDebug,                                       \
+          "Zap allocated TLABs")                                            \
+                                                                            \
   product(bool, ExecutingUnitTests, false,                                  \
           "Whether the JVM is running unit tests or not")                   \
                                                                             \