From 0ba115c49f4d7033703d88ef0f027900bc6d5b40 Mon Sep 17 00:00:00 2001
From: Thomas Schatzl <tschatzl@openjdk.org>
Date: Mon, 27 Apr 2026 09:01:42 +0000
Subject: [PATCH] 8382635: G1: TAMSes overwritten by
 G1ConcurrentMark::fully_initialize() cause verification errors

Reviewed-by: iwalulya, ayang
---
 src/hotspot/share/gc/g1/g1ConcurrentMark.cpp   | 5 ++++-
 src/hotspot/share/gc/g1/g1_globals.hpp         | 2 +-
 test/hotspot/jtreg/gc/g1/TestVerifyGCType.java | 1 +
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp b/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp
index d5e3b9cc69d..9952fab5e2c 100644
--- a/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp
+++ b/src/hotspot/share/gc/g1/g1ConcurrentMark.cpp
@@ -524,7 +524,10 @@ void G1ConcurrentMark::fully_initialize() {
 
   uint max_num_regions = _g1h->max_num_regions();
   ::new (_region_mark_stats) G1RegionMarkStats[max_num_regions]{};
-  ::new (_top_at_mark_starts) Atomic<HeapWord*>[max_num_regions]{};
+  for (uint i = 0; i < max_num_regions; i++) {
+    ::new (&_top_at_mark_starts[i]) Atomic<HeapWord*>(_g1h->bottom_addr_for_region(i));
+  }
+  // Contrary to TAMS, the default value of _top_at_rebuild_starts needs to be null.
   ::new (_top_at_rebuild_starts) Atomic<HeapWord*>[max_num_regions]{};
 
   reset_at_marking_complete();
diff --git a/src/hotspot/share/gc/g1/g1_globals.hpp b/src/hotspot/share/gc/g1/g1_globals.hpp
index b338c11d5be..baed70b7088 100644
--- a/src/hotspot/share/gc/g1/g1_globals.hpp
+++ b/src/hotspot/share/gc/g1/g1_globals.hpp
@@ -316,7 +316,7 @@
   product(bool, G1VerifyHeapRegionCodeRoots, false, DIAGNOSTIC,             \
           "Verify the code root lists attached to each heap region.")       \
                                                                             \
-  develop(bool, G1VerifyBitmaps, false,                                     \
+  product(bool, G1VerifyBitmaps, false, DIAGNOSTIC,                         \
           "Verifies the consistency of the marking bitmaps")                \
                                                                             \
   product(uintx, G1PeriodicGCInterval, 0, MANAGEABLE,                       \
diff --git a/test/hotspot/jtreg/gc/g1/TestVerifyGCType.java b/test/hotspot/jtreg/gc/g1/TestVerifyGCType.java
index 710236e1aae..fb04f54f641 100644
--- a/test/hotspot/jtreg/gc/g1/TestVerifyGCType.java
+++ b/test/hotspot/jtreg/gc/g1/TestVerifyGCType.java
@@ -166,6 +166,7 @@ public class TestVerifyGCType {
                                        "-Xmx16m",
                                        "-XX:ParallelGCThreads=1",
                                        "-XX:G1HeapWastePercent=1",
+                                       "-XX:+G1VerifyBitmaps",
                                        "-XX:+VerifyBeforeGC",
                                        "-XX:+VerifyAfterGC",
                                        "-XX:+VerifyDuringGC"});