8367627: C2: Missed Ideal() optimization opportunity with MemBar

src/hotspot/share/opto/graphKit.cpp

@@ -3417,6 +3417,7 @@ Node* GraphKit::insert_mem_bar(int opcode, Node* precedent) {
   mb->init_req(TypeFunc::Control, control());
   mb->init_req(TypeFunc::Memory,  reset_memory());
   Node* membar = _gvn.transform(mb);
+  record_for_igvn(membar);
   set_control(_gvn.transform(new ProjNode(membar, TypeFunc::Control)));
   set_all_memory_call(membar);
   return membar;
@@ -3446,6 +3447,7 @@ Node* GraphKit::insert_mem_bar_volatile(int opcode, int alias_idx, Node* precede
     mb->set_req(TypeFunc::Memory, memory(alias_idx));
   }
   Node* membar = _gvn.transform(mb);
+  record_for_igvn(membar);
   set_control(_gvn.transform(new ProjNode(membar, TypeFunc::Control)));
   if (alias_idx == Compile::AliasIdxBot) {
     merged_memory()->set_base_memory(_gvn.transform(new ProjNode(membar, TypeFunc::Memory)));

test/hotspot/jtreg/compiler/c2/TestMissingOptMemBarRemovePrecedentEdge.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug JDK-8367627
+ * @summary When we load from a volatile field, a MemBarAcquire node gets inserted
+ *          after the load. It happens that the load gets converted to a constant
+ *          during the initial _gvn.transform(...) call. The MemBarAcquire node
+ *          ends up with a constant node as its Precedent input, and this should
+ *          trigger MemBar::Ideal if the MemBar is the only use of the constant.
+ *          This test ensures that this optimization is not missed.
+ * @run main/othervm -XX:+IgnoreUnrecognizedVMOptions -XX:+UnlockDiagnosticVMOptions
+ *      -Xcomp -XX:-TieredCompilation
+ *      -XX:CompileCommand=compileonly,*TestMissingOptMemBarRemovePrecedentEdge::test*
+ *      -XX:VerifyIterativeGVN=1110
+ *      -XX:-EliminateAutoBox
+ *      -XX:-DoEscapeAnalysis
+ *      compiler.c2.TestMissingOptMemBarRemovePrecedentEdge
+ * @run main compiler.c2.TestMissingOptMemBarRemovePrecedentEdge
+ *
+ */
+
+package compiler.c2;
+
+public class TestMissingOptMemBarRemovePrecedentEdge {
+    static class MyClass {
+        volatile long l;
+    }
+
+    static class MyClass2 {
+        volatile MyClass o;
+    }
+
+    static int test0() {
+        var c = new MyClass();
+        // the conversion ensures that the ConL node only has one use
+        // in the end, which triggers the optimization
+        return (int) c.l;
+    }
+
+    static int test1() {
+        var c = new MyClass2();
+        // the Load gets folded to a ConP, and gets added to the worklist
+        // because the basic type is T_OBJECT and there is specific logic
+        // in GraphKit::make_load.
+        // However this only happens if escape analysis is enabled
+        if (c.o == null) return 0;
+        else return 1;
+    }
+
+    public static void main(String[] args) {
+        new MyClass();
+        new MyClass2();
+        test0();
+        test1();
+    }
+}