From e8ce930e28c26b12fb0eafab201ccedaa2650e5d Mon Sep 17 00:00:00 2001
From: William Kemper <wkemper@openjdk.org>
Date: Wed, 25 Mar 2026 00:23:41 +0000
Subject: [PATCH] 8379020: GenShen: Promote all objects when whitebox full GC
 is requested

Reviewed-by: ysr, kdnilsen
---
 .../gc/shenandoah/shenandoahAgeCensus.hpp     | 28 +++++++++++++++++++
 .../shenandoahGenerationalControlThread.cpp   |  7 +++++
 2 files changed, 35 insertions(+)

diff --git a/src/hotspot/share/gc/shenandoah/shenandoahAgeCensus.hpp b/src/hotspot/share/gc/shenandoah/shenandoahAgeCensus.hpp
index 5ccd0b21398..9c5baaedcd6 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahAgeCensus.hpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahAgeCensus.hpp
@@ -97,6 +97,8 @@ struct ShenandoahNoiseStats {
 // once the per-worker data is consolidated into the appropriate population vector
 // per minor collection. The _local_age_table is thus C x N, for N GC workers.
 class ShenandoahAgeCensus: public CHeapObj<mtGC> {
+  friend class ShenandoahTenuringOverride;
+
   AgeTable** _global_age_tables;      // Global age tables used for adapting tenuring threshold, one per snapshot
   AgeTable** _local_age_tables;       // Local scratch age tables to track object ages, one per worker
 
@@ -148,6 +150,10 @@ class ShenandoahAgeCensus: public CHeapObj<mtGC> {
     return _tenuring_threshold[prev];
   }
 
+  // Override the tenuring threshold for the current epoch. This is used to
+  // cause everything to be promoted for a whitebox full gc request.
+  void set_tenuring_threshold(uint threshold) { _tenuring_threshold[_epoch] = threshold; }
+
 #ifndef PRODUCT
   // Return the sum of size of objects of all ages recorded in the
   // census at snapshot indexed by snap.
@@ -232,4 +238,26 @@ class ShenandoahAgeCensus: public CHeapObj<mtGC> {
   void print();
 };
 
+// RAII object that temporarily overrides the tenuring threshold for the
+// duration of a scope, restoring the original value on destruction.
+// Used to force promotion of all young objects during whitebox full GCs.
+class ShenandoahTenuringOverride : public StackObj {
+  ShenandoahAgeCensus* _census;
+  uint _saved_threshold;
+  bool _active;
+public:
+  ShenandoahTenuringOverride(bool active, ShenandoahAgeCensus* census) :
+    _census(census), _saved_threshold(0), _active(active) {
+    if (_active) {
+      _saved_threshold = _census->tenuring_threshold();
+      _census->set_tenuring_threshold(0);
+    }
+  }
+  ~ShenandoahTenuringOverride() {
+    if (_active) {
+      _census->set_tenuring_threshold(_saved_threshold);
+    }
+  }
+};
+
 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHAGECENSUS_HPP
diff --git a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp
index 1edff443ded..67cc4d2f703 100644
--- a/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahGenerationalControlThread.cpp
@@ -24,6 +24,7 @@
  *
  */
 
+#include "gc/shenandoah/shenandoahAgeCensus.hpp"
 #include "gc/shenandoah/shenandoahAsserts.hpp"
 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
 #include "gc/shenandoah/shenandoahConcurrentGC.hpp"
@@ -271,6 +272,12 @@ void ShenandoahGenerationalControlThread::run_gc_cycle(const ShenandoahGCRequest
     // Cannot uncommit bitmap slices during concurrent reset
     ShenandoahNoUncommitMark forbid_region_uncommit(_heap);
 
+    // When a whitebox full GC is requested, set the tenuring threshold to zero
+    // so that all young objects are promoted to old. This ensures that tests
+    // using WB.fullGC() to promote objects to old gen will not loop forever.
+    ShenandoahTenuringOverride tenuring_override(request.cause == GCCause::_wb_full_gc,
+                                                 _heap->age_census());
+
     _heap->print_before_gc();
     switch (gc_mode()) {
       case concurrent_normal: {