8286638: C2: CmpU needs to do more precise over/underflow analysis

src/hotspot/share/opto/subnode.cpp

@@ -815,11 +815,10 @@ const Type* CmpUNode::Value(PhaseGVN* phase) const {
         int w = MAX2(r0->_widen, r1->_widen); // _widen does not matter here
         const TypeInt* tr1 = TypeInt::make(lo_tr1, hi_tr1, w);
         const TypeInt* tr2 = TypeInt::make(lo_tr2, hi_tr2, w);
-        const Type* cmp1 = sub(tr1, t2);
-        const Type* cmp2 = sub(tr2, t2);
-        if (cmp1 == cmp2) {
-          return cmp1; // Hit!
-        }
+        const TypeInt* cmp1 = sub(tr1, t2)->is_int();
+        const TypeInt* cmp2 = sub(tr2, t2)->is_int();
+        // compute union, so that cmp handles all possible results from the two cases
+        return cmp1->meet(cmp2);
       }
     }
   }

test/hotspot/jtreg/compiler/rangechecks/TestRangeCheckCmpUUnderflow.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2022, 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 8286638
+ * @summary Dominator failure because ConvL2I node becomes TOP, kills data-flow, but range-check does not collapse
+ *          due to insufficient overflow/underflow handling in CmpUNode::Value.
+ *
+ * @run main/othervm -XX:+UnlockDiagnosticVMOptions -XX:+StressIGVN -Xcomp -XX:-TieredCompilation
+ *                   -XX:CompileCommand=compileonly,compiler.rangechecks.TestRangeCheckCmpUUnderflow::*
+ *                   -XX:RepeatCompilation=300
+ *                   compiler.rangechecks.TestRangeCheckCmpUUnderflow
+*/
+
+package compiler.rangechecks;
+
+public class TestRangeCheckCmpUUnderflow {
+    volatile int a;
+    int b[];
+    float c[];
+    void e() {
+        int g, f, i;
+        for (g = 2;; g++) {
+            for (i = g; i < 1; i++) {
+                f = a;
+                c[i - 1] -= b[i];
+            }
+        }
+    }
+    public static void main(String[] args) {
+        try {
+            TestRangeCheckCmpUUnderflow j = new TestRangeCheckCmpUUnderflow();
+            j.e();
+        } catch (Exception ex) {}
+    }
+}