8371674: C2 fails with Missed optimization opportunity in PhaseIterGVN for MoveL2D

Reviewed-by: epeter, chagedorn

Author: benoitmaillard

src/hotspot/share/opto/node.cpp

@@ -1209,9 +1209,12 @@ bool Node::has_special_unique_user() const {
   if (this->is_Store()) {
     // Condition for back-to-back stores folding.
     return n->Opcode() == op && n->in(MemNode::Memory) == this;
-  } else if (this->is_Load() || this->is_DecodeN() || this->is_Phi()) {
+  } else if ((this->is_Load() || this->is_DecodeN() || this->is_Phi()) && n->Opcode() == Op_MemBarAcquire) {
     // Condition for removing an unused LoadNode or DecodeNNode from the MemBarAcquire precedence input
-    return n->Opcode() == Op_MemBarAcquire;
+    return true;
+  } else if (this->is_Load() && n->is_Move()) {
+    // Condition for MoveX2Y (LoadX mem) => LoadY mem
+    return true;
   } else if (op == Op_AddL) {
     // Condition for convL2I(addL(x,y)) ==> addI(convL2I(x),convL2I(y))
     return n->Opcode() == Op_ConvL2I && n->in(1) == this;

test/hotspot/jtreg/compiler/c2/TestMissingOptMoveX2YLoadX.java

@@ -0,0 +1,61 @@
+/*
+ * 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 8371674
+ * @summary An expression of the form "MoveX2Y (LoadX mem)" should be
+ *          transformed into "LoadY mem". This test ensures that changes
+ *          to the number of users of the Load node propagate as expected
+ *          and that the optimization is not missed.
+ * @run main/othervm -XX:+IgnoreUnrecognizedVMOptions -XX:-TieredCompilation -Xcomp
+ *      -XX:CompileCommand=compileonly,compiler.c2.TestMissingOptMoveX2YLoadX::test*
+ *      -XX:VerifyIterativeGVN=1110 compiler.c2.TestMissingOptMoveX2YLoadX
+ * @run main compiler.c2.TestMissingOptMoveX2YLoadX
+ *
+ */
+
+package compiler.c2;
+
+public class TestMissingOptMoveX2YLoadX {
+    static final int N = 400;
+    static volatile long b;
+
+    public static void main(String[] strArr) {
+        // could theoretically happen with other variants of MoveNode
+        // but there is no known reproducer for the other cases
+        Double.longBitsToDouble(0l);
+        testMoveL2D();
+    }
+
+    static void testMoveL2D() {
+        int e = 8, f, g = 9, h = 2, i[] = new int[N];
+        long j[] = new long[N];
+        while (++e < 37) {
+            for (f = 1; f < 7; f++) {
+                h >>>= (int)(--g - Double.longBitsToDouble(j[e]));
+                b -= b;
+            }
+        }
+    }
+}