8202948: C2: assert(init_offset >= 0) failed: positive offset from object start

Summary: convert the assert into compilation check which will skip superword optimization
Reviewed-by: roland, thartmann

src/share/vm/opto/superword.cpp

@@ -482,7 +482,9 @@ bool SuperWord::ref_is_alignable(SWPointer& p) {
   if (init_nd->is_Con() && p.invar() == NULL) {
     int init = init_nd->bottom_type()->is_int()->get_con();
     int init_offset = init * p.scale_in_bytes() + offset;
-    assert(init_offset >= 0, "positive offset from object start");
+    if (init_offset < 0) { // negative offset from object start?
+      return false;        // may happen in dead loop
+    }
     if (vw % span == 0) {
       // If vm is a multiple of span, we use formula (1).
       if (span > 0) {

test/compiler/loopopts/superword/TestNegBaseOffset.java

+/*
+ * Copyright (c) 2018, 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 8202948
+ * @summary Test skipping vector packs with negative base offset.
+ * @comment Test fails only with -Xcomp when profiling data is not present.
+ * @run main/othervm -XX:+IgnoreUnrecognizedVMOptions -XX:+UnlockExperimentalVMOptions
+ *                   -Xcomp -XX:-TieredCompilation -XX:CICompilerCount=1
+ *                   -XX:CompileOnly=compiler/loopopts/superword/TestNegBaseOffset
+ *                   compiler.loopopts.superword.TestNegBaseOffset
+ */
+
+package compiler.loopopts.superword;
+
+public class TestNegBaseOffset {
+    public static final int N = 400;
+    public static int iFld=10;
+    public static int iArr[]=new int[N];
+
+    public static void mainTest() {
+        int i0=1, i2;
+        while (++i0 < 339) {
+            if ((i0 % 2) == 0) {
+                for (i2 = 2; i2 > i0; i2 -= 3) {
+                    iArr[i2 - 1] &= iFld;
+                }
+            }
+        }
+    }
+
+    public static void main(String[] strArr) {
+        for (int i = 0; i < 10; i++) {
+            mainTest();
+        }
+    }
+}
+