8343944: C2: MinLNode::add_ring() computes _widen wrongly leading to an endless widening/compilation

Reviewed-by: thartmann, rcastanedalo

Author: chhagedorn

src/hotspot/share/opto/addnode.cpp

@@ -1546,7 +1546,7 @@ const Type* MinLNode::add_ring(const Type* t0, const Type* t1) const {
   const TypeLong* r0 = t0->is_long();
   const TypeLong* r1 = t1->is_long();
 
-  return TypeLong::make(MIN2(r0->_lo, r1->_lo), MIN2(r0->_hi, r1->_hi), MIN2(r0->_widen, r1->_widen));
+  return TypeLong::make(MIN2(r0->_lo, r1->_lo), MIN2(r0->_hi, r1->_hi), MAX2(r0->_widen, r1->_widen));
 }
 
 Node* MinLNode::Identity(PhaseGVN* phase) {

src/hotspot/test/hotspot/jtreg/compiler/ccp/TestWrongMinLWiden.java

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2024, 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 8343944
+ * @summary Test that _widen is set correctly in MinL::add_ring() to prevent an endless widening in CCP.
+ * @run main/othervm -Xcomp -XX:CompileCommand=compileonly,compiler.ccp.TestWrongMinLWiden::*
+ *                   compiler.ccp.TestWrongMinLWiden
+ */
+
+package compiler.ccp;
+
+public class TestWrongMinLWiden {
+    static long lFld;
+    static short sFld;
+
+    public static void main(String[] strArr) {
+        Math.min(3,3); // Make sure Math class is loaded.
+        test();
+        testWithMathMin();
+    }
+
+
+    static long test() {
+        long x = 50398;
+        for (int i = 1; i < 100; i++) {
+            long xMinus1 = x - 1; // x is a phi with type #long
+            // Long comparison:
+            //     ConvI2L(sFld) <= xMinus1
+            // First converted to
+            //     CMoveL(sFld <= xMinus1, sFld, xMinus1)
+            //     CMoveL(ConvI2L(sFld) <= long_phi, ConvI2L(sFld), long_phi)
+            // with types
+            //     CMoveL(#short <= #long, #short, #long)
+            // And then converted in CMoveNode::Ideal() to
+            //     MinL(sFld, xMinus1)
+            //     MinL(ConvI2L(sFld), long_phi)
+            //
+            // We wrongly set the _widen of the new type for MinL in MinL::add_ring() to the minimum of both types which
+            // is always 0 because the _widen of sFld is 0. As a result, we will endlessly widen the involved types
+            // because the new type for MinL keeps resetting _widen to 0.
+            // Instead, we should choose the maximum of both _widen fields for the new type for MinL which will then
+            // select the _widen of xMinus1 which will grow each time we set a new type for the long_phi. Eventually,
+            // we will saturate the type of long_phi to min_long to avoid an endless widening.
+            x = sFld <= xMinus1 ? sFld : xMinus1;
+        }
+        return x;
+    }
+
+    // Same as test() but with Math.min() which internally uses x <= y ? x : y which allows the CMoveL pattern to be
+    // replaced with MinL.
+    static long testWithMathMin() {
+        long x = 50398;
+        for (int i = 1; i < 100; i++) {
+            x = Math.min(sFld, x - 1);
+        }
+        return x;
+    }
+}