8262096: Vector API fails to work due to VectorShape initialization exception

src/hotspot/share/prims/vectorSupport.cpp

@@ -362,13 +362,21 @@ int VectorSupport::vop2ideal(jint id, BasicType bt) {
  */
 
 JVM_ENTRY(jint, VectorSupport_GetMaxLaneCount(JNIEnv *env, jclass vsclazz, jobject clazz)) {
-#ifdef COMPILER2
   oop mirror = JNIHandles::resolve_non_null(clazz);
   if (java_lang_Class::is_primitive(mirror)) {
     BasicType bt = java_lang_Class::primitive_type(mirror);
-    return Matcher::max_vector_size(bt);
-  }
+    int min_lane_count = 64 / type2aelembytes(bt);
+    // The return value should not be less than min_lane_count since the minimal vector size
+    // supported by the Vector API is 64-bit (see VectorShape.preferredShape() for more details).
+    // This is fine because the vector size is unlikely to be less than 64-bit on modern hardware.
+    // And for the worst case, where the hardware doesn't support 64-bit long, the non-vector
+    // implementation will be called, which is still fine.
+#ifdef COMPILER2
+    return MAX2(Matcher::max_vector_size(bt), min_lane_count);
+#else
+    return min_lane_count;
 #endif // COMPILER2
+  }
   return -1;
 } JVM_END
 

src/jdk.incubator.vector/share/classes/jdk/incubator/vector/VectorShape.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2017, 2021, 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
@@ -246,11 +246,6 @@ private static VectorShape computePreferredShape() {
             Class<?> etype = type.elementType;
             int maxLaneCount = VectorSupport.getMaxLaneCount(etype);
             int maxSize = type.elementSize * maxLaneCount;
-            // FIXME: Consider removing, since unlikely to occur on modern hardware
-            if (maxSize < Double.SIZE) {
-                String msg = "shape unavailable for lane type: " + etype.getName();
-                throw new UnsupportedOperationException(msg);
-            }
             prefBitSize = Math.min(prefBitSize, maxSize);
         }
         // If these assertions fail, we must reconsider our API portability assumptions.

test/hotspot/jtreg/compiler/vectorapi/VectorShapeInitTest.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2021 THL A29 Limited, a Tencent company. 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.vectorapi;
+
+import jdk.incubator.vector.DoubleVector;
+import jdk.incubator.vector.VectorOperators;
+import jdk.incubator.vector.VectorSpecies;
+
+/*
+ * @test
+ * @bug 8262096
+ * @summary Test the initialization of vector shapes
+ * @modules jdk.incubator.vector
+ * @run main/othervm -XX:MaxVectorSize=8 compiler.vectorapi.VectorShapeInitTest
+ * @run main/othervm -XX:MaxVectorSize=4 compiler.vectorapi.VectorShapeInitTest
+ */
+
+public class VectorShapeInitTest {
+    static final VectorSpecies<Double> SPECIES = DoubleVector.SPECIES_64;
+    static double[] a = new double[64];
+    static double[] r = new double[64];
+
+    public static void main(String[] args) {
+        DoubleVector av = DoubleVector.fromArray(SPECIES, a, 0);
+        av.lanewise(VectorOperators.ABS).intoArray(r, 0);
+    }
+}