From fd55ba22fd0772ecbde98fef120f7a71562c4bb4 Mon Sep 17 00:00:00 2001
From: Vladimir Ivanov <vladimir.x.ivanov@oracle.com>
Date: Thu, 26 Mar 2026 14:53:30 -0700
Subject: [PATCH] Undo changes in allocation of PhaseIdealLoop internal
 structures

---
 src/hotspot/share/opto/loopnode.cpp     | 14 +++++++-------
 src/hotspot/share/opto/reachability.cpp |  2 +-
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/hotspot/share/opto/loopnode.cpp b/src/hotspot/share/opto/loopnode.cpp
index 6b521d0e63e..0ddaed31518 100644
--- a/src/hotspot/share/opto/loopnode.cpp
+++ b/src/hotspot/share/opto/loopnode.cpp
@@ -4344,7 +4344,7 @@ void IdealLoopTree::allpaths_check_safepts(VectorSet &visited, Node_List &stack)
       // Terminate this path
     } else if (n->Opcode() == Op_SafePoint) {
       if (_phase->get_loop(n) != this) {
-        if (_required_safept == nullptr) _required_safept = new Node_List(_phase->arena());
+        if (_required_safept == nullptr) _required_safept = new Node_List();
         // save the first we run into on that path: closest to the tail if the head has a single backedge
         _required_safept->push(n);
       }
@@ -4490,7 +4490,7 @@ void IdealLoopTree::check_safepts(VectorSet &visited, Node_List &stack) {
     // inner loop attempts to delete it's safepoints.
     if (_child != nullptr && !has_call && !has_local_ncsfpt) {
       if (nonlocal_ncsfpt != nullptr) {
-        if (_required_safept == nullptr) _required_safept = new Node_List(_phase->arena());
+        if (_required_safept == nullptr) _required_safept = new Node_List();
         _required_safept->push(nonlocal_ncsfpt);
       } else {
         // Failed to find a suitable safept on the dom-path.  Now use
@@ -4838,11 +4838,10 @@ uint IdealLoopTree::est_loop_flow_merge_sz() const {
 
 void IdealLoopTree::register_reachability_fence(ReachabilityFenceNode* rf) {
   if (_reachability_fences == nullptr) {
-    _reachability_fences = new Node_List(_phase->arena());
-  }
-  if (!_reachability_fences->contains(rf)) {
-    _reachability_fences->push(rf);
+    _reachability_fences = new Node_List();
   }
+  assert(!_reachability_fences->contains(rf), "already registered");
+  _reachability_fences->push(rf);
 }
 
 #ifndef PRODUCT
@@ -5909,6 +5908,7 @@ bool IdealLoopTree::verify_tree(IdealLoopTree* loop_verify) const {
 
 //------------------------------set_idom---------------------------------------
 void PhaseIdealLoop::set_idom(Node* d, Node* n, uint dom_depth) {
+  _nesting.check(); // Check if a potential reallocation in the resource arena is safe
   uint idx = d->_idx;
   if (idx >= _idom_size) {
     uint newsize = next_power_of_2(idx);
@@ -6301,7 +6301,7 @@ int PhaseIdealLoop::build_loop_tree_impl(Node* n, int pre_order) {
         innermost->_has_call = 1; // = true
       } else if (n->Opcode() == Op_SafePoint) {
         // Record all safepoints in this loop.
-        if (innermost->_safepts == nullptr) innermost->_safepts = new Node_List(&_arena);
+        if (innermost->_safepts == nullptr) innermost->_safepts = new Node_List();
         innermost->_safepts->push(n);
       } else if (n->is_ReachabilityFence()) {
         innermost->register_reachability_fence(n->as_ReachabilityFence());
diff --git a/src/hotspot/share/opto/reachability.cpp b/src/hotspot/share/opto/reachability.cpp
index 175a4be5bed..cee3f337d23 100644
--- a/src/hotspot/share/opto/reachability.cpp
+++ b/src/hotspot/share/opto/reachability.cpp
@@ -214,7 +214,7 @@ bool PhaseIdealLoop::optimize_reachability_fences() {
 
   assert(OptimizeReachabilityFences, "required");
 
-  ResourceMark rm;
+  // ResourceMark rm; // NB! not safe because insert_rf may trigger _idom reallocation
   Unique_Node_List redundant_rfs;
   GrowableArray<Pair<Node*,Node*>> worklist;
   for (int i = 0; i < C->reachability_fences_count(); i++) {