From 776e1cf1dfefd7cb1a0190ab71f71ad5ff25d0e4 Mon Sep 17 00:00:00 2001
From: Dean Long <dlong@openjdk.org>
Date: Wed, 9 Apr 2025 20:06:49 +0000
Subject: [PATCH] 8353041: NeverBranchNode causes incorrect block frequency
 calculation

Reviewed-by: thartmann, rcastanedalo
---
 src/hotspot/share/opto/domgraph.cpp | 14 +++++++++++++-
 src/hotspot/share/opto/gcm.cpp      |  7 ++++++-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/hotspot/share/opto/domgraph.cpp b/src/hotspot/share/opto/domgraph.cpp
index a80358e1d0f..b52f4232c4e 100644
--- a/src/hotspot/share/opto/domgraph.cpp
+++ b/src/hotspot/share/opto/domgraph.cpp
@@ -233,9 +233,21 @@ uint Block_Stack::most_frequent_successor( Block *b ) {
   case Op_Jump:
   case Op_Root:
   case Op_Goto:
-  case Op_NeverBranch:
     freq_idx = 0;               // fall thru
     break;
+  case Op_NeverBranch: {
+    Node* succ = n->as_NeverBranch()->proj_out(0)->unique_ctrl_out();
+    int succ_idx = 0; // normal case
+    if (succ == b->_succs[1]->head()) {
+      // Edges swapped, rare case. May happen due to an unusual matcher
+      // traversal order for peeled infinite loops.
+      succ_idx = 1;
+    } else {
+      assert(succ == b->_succs[0]->head(), "succ not found");
+    }
+    freq_idx = succ_idx;
+    break;
+  }
   case Op_TailCall:
   case Op_TailJump:
   case Op_ForwardException:
diff --git a/src/hotspot/share/opto/gcm.cpp b/src/hotspot/share/opto/gcm.cpp
index eeab96de3fa..a8080d5adb9 100644
--- a/src/hotspot/share/opto/gcm.cpp
+++ b/src/hotspot/share/opto/gcm.cpp
@@ -2160,8 +2160,13 @@ float Block::succ_prob(uint i) {
     // Pass frequency straight thru to target
     return 1.0f;
 
-  case Op_NeverBranch:
+  case Op_NeverBranch: {
+    Node* succ = n->as_NeverBranch()->proj_out(0)->unique_ctrl_out();
+    if (_succs[i]->head() == succ) {
+      return 1.0f;
+    }
     return 0.0f;
+  }
 
   case Op_TailCall:
   case Op_TailJump: