From 46e6b26bf8c3124bf4e0c5d26c3eb04e220bd0cb Mon Sep 17 00:00:00 2001
From: Saranya Natarajan <snatarajan@openjdk.org>
Date: Wed, 25 Mar 2026 12:47:41 +0000
Subject: [PATCH] 8372646: C2: Stress Counted Loop creation

Reviewed-by: rcastanedalo, chagedorn, dfenacci
---
 src/hotspot/share/opto/c2_globals.hpp |  3 +++
 src/hotspot/share/opto/compile.cpp    | 12 +++++++++---
 src/hotspot/share/opto/loopnode.cpp   | 15 +++++++++++++--
 3 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/src/hotspot/share/opto/c2_globals.hpp b/src/hotspot/share/opto/c2_globals.hpp
index e0833afa29d..a4f6d04f6a3 100644
--- a/src/hotspot/share/opto/c2_globals.hpp
+++ b/src/hotspot/share/opto/c2_globals.hpp
@@ -907,6 +907,9 @@
                                                                             \
   develop(bool, StressLoopPeeling, false,                                   \
           "Randomize loop peeling decision")                                \
+                                                                            \
+  develop(bool, StressCountedLoop, false,                                   \
+          "Randomly delay conversion to counted loops")                     \
 
 // end of C2_FLAGS
 
diff --git a/src/hotspot/share/opto/compile.cpp b/src/hotspot/share/opto/compile.cpp
index f1ea8231df9..f34c75656a4 100644
--- a/src/hotspot/share/opto/compile.cpp
+++ b/src/hotspot/share/opto/compile.cpp
@@ -741,7 +741,7 @@ Compile::Compile(ciEnv* ci_env, ciMethod* target, int osr_bci,
   if (StressLCM || StressGCM || StressIGVN || StressCCP ||
       StressIncrementalInlining || StressMacroExpansion ||
       StressMacroElimination || StressUnstableIfTraps ||
-      StressBailout || StressLoopPeeling) {
+      StressBailout || StressLoopPeeling || StressCountedLoop) {
     initialize_stress_seed(directive);
   }
 
@@ -2257,7 +2257,9 @@ bool Compile::optimize_loops(PhaseIterGVN& igvn, LoopOptsMode mode) {
       PhaseIdealLoop::optimize(igvn, mode);
       _loop_opts_cnt--;
       if (failing())  return false;
-      if (major_progress()) print_method(PHASE_PHASEIDEALLOOP_ITERATIONS, 2);
+      if (major_progress()) {
+        print_method(PHASE_PHASEIDEALLOOP_ITERATIONS, 2);
+      }
     }
   }
   return true;
@@ -3798,7 +3800,11 @@ void Compile::final_graph_reshaping_main_switch(Node* n, Final_Reshape_Counts& f
     }
     break;
   case Op_Loop:
-    assert(!n->as_Loop()->is_loop_nest_inner_loop() || _loop_opts_cnt == 0, "should have been turned into a counted loop");
+    // When StressCountedLoop is enabled, this loop may intentionally avoid a counted loop conversion.
+    // This is expected behavior for the stress mode, which exercises alternative compilation paths.
+    if (!StressCountedLoop) {
+      assert(!n->as_Loop()->is_loop_nest_inner_loop() || _loop_opts_cnt == 0, "should have been turned into a counted loop");
+    }
   case Op_CountedLoop:
   case Op_LongCountedLoop:
   case Op_OuterStripMinedLoop:
diff --git a/src/hotspot/share/opto/loopnode.cpp b/src/hotspot/share/opto/loopnode.cpp
index b2eb0c47458..686a0a05f66 100644
--- a/src/hotspot/share/opto/loopnode.cpp
+++ b/src/hotspot/share/opto/loopnode.cpp
@@ -2137,7 +2137,6 @@ bool CountedLoopConverter::is_counted_loop() {
   }
 
   assert(_head->Opcode() == Op_Loop || _head->Opcode() == Op_LongCountedLoop, "regular loops only");
-  _phase->C->print_method(PHASE_BEFORE_CLOOPS, 3, _head);
 
   // ===================================================
   // We can only convert this loop to a counted loop if we can guarantee that the iv phi will never overflow at runtime.
@@ -2411,6 +2410,12 @@ bool CountedLoopConverter::is_counted_loop() {
   _checked_for_counted_loop = true;
 #endif
 
+#ifndef PRODUCT
+  if (StressCountedLoop && (_phase->C->random() % 2 == 0)) {
+    return false;
+  }
+#endif
+
   return true;
 }
 
@@ -2553,6 +2558,8 @@ IdealLoopTree* CountedLoopConverter::convert() {
 
   PhaseIterGVN* igvn = &_phase->igvn();
 
+  _phase->C->print_method(PHASE_BEFORE_CLOOPS, 3, _head);
+
   if (_should_insert_stride_overflow_limit_check) {
     insert_stride_overflow_limit_check();
   }
@@ -4734,7 +4741,11 @@ void IdealLoopTree::counted_loop( PhaseIdealLoop *phase ) {
   } else if (_head->is_LongCountedLoop() || phase->try_convert_to_counted_loop(_head, loop, T_LONG)) {
     remove_safepoints(phase, true);
   } else {
-    assert(!_head->is_Loop() || !_head->as_Loop()->is_loop_nest_inner_loop(), "transformation to counted loop should not fail");
+    // When StressCountedLoop is enabled, this loop may intentionally avoid a counted loop conversion.
+    // This is expected behavior for the stress mode, which exercises alternative compilation paths.
+    if (!StressCountedLoop) {
+      assert(!_head->is_Loop() || !_head->as_Loop()->is_loop_nest_inner_loop(), "transformation to counted loop should not fail");
+    }
     if (_parent != nullptr && !_irreducible) {
       // Not a counted loop. Keep one safepoint.
       bool keep_one_sfpt = true;