8310524: C2: record parser-generated LoadN nodes for IGVN

Reviewed-by: chagedorn, rcastanedalo, thartmann

src/hotspot/share/opto/graphKit.cpp

@@ -1563,6 +1563,11 @@ Node* GraphKit::make_load(Node* ctl, Node* adr, const Type* t, BasicType bt,
   if (((bt == T_OBJECT) && C->do_escape_analysis()) || C->eliminate_boxing()) {
     // Improve graph before escape analysis and boxing elimination.
     record_for_igvn(ld);
+    if (ld->is_DecodeN()) {
+      // Also record the actual load (LoadN) in case ld is DecodeN
+      assert(ld->in(1)->Opcode() == Op_LoadN, "Assumption invalid: input to DecodeN is not LoadN");
+      record_for_igvn(ld->in(1));
+    }
   }
   return ld;
 }

test/hotspot/jtreg/compiler/c2/irTests/igvn/TestLoadNIdeal.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright (c) 2023, 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.
+ */
+
+package compiler.c2.irTests.igvn;
+
+import compiler.lib.ir_framework.*;
+
+/*
+ * @test
+ * @bug 8310524
+ * @summary Test that IGVN optimizes away one of two identical LoadNs.
+ * @library /test/lib /
+ * @run driver compiler.c2.irTests.igvn.TestLoadNIdeal
+ */
+
+public class TestLoadNIdeal {
+
+    public static void main(String[] args) {
+        // Ensure that we run with compressed oops
+        TestFramework.runWithFlags("-XX:+UseCompressedOops");
+    }
+
+    static class A { int x; }
+
+    @DontInline
+    void dummy(A p[]) { }
+
+    @Test
+    @IR(applyIf = { "UseCompressedOops", "true" }, counts = { IRNode.LOAD_N, "1" })
+    int test() {
+        A p[] = new A[1];
+        p[0] = new A();
+
+        // The dummy method is not inlined => Escape analysis
+        // cannot ensure that p[0] is unmodified after the call.
+        dummy(p);
+
+        // We should only need to load p[0] once here. Storing A within an
+        // array adds range checks for the first load and ensures the second
+        // load is not optimized already at bytecode parsing.
+        return p[0].x + p[0].x;
+    }
+}