[SPARK-12181] Check Cached unaligned-access capability before using Unsafe

## What changes were proposed in this pull request?

For MemoryMode.OFF_HEAP, Unsafe.getInt etc. are used with no restriction.

However, the Oracle implementation uses these methods only if the class variable unaligned (commented as "Cached unaligned-access capability") is true, which seems to be calculated whether the architecture is i386, x86, amd64, or x86_64.

I think we should perform similar check for the use of Unsafe.

Reference: https://github.com/netty/netty/blob/4.1/common/src/main/java/io/netty/util/internal/PlatformDependent0.java#L112

## How was this patch tested?

Unit test suite

Author: tedyu <yuzhihong@gmail.com>

Closes #11943 from tedyu/master.

common/unsafe/src/main/java/org/apache/spark/unsafe/Platform.java

@@ -18,6 +18,7 @@
 package org.apache.spark.unsafe;
 
 import java.lang.reflect.Field;
+import java.lang.reflect.Method;
 
 import sun.misc.Unsafe;
 
@@ -37,6 +38,33 @@ public final class Platform {
 
   public static final int DOUBLE_ARRAY_OFFSET;
 
+  private static final boolean unaligned;
+  static {
+    boolean _unaligned;
+    // use reflection to access unaligned field
+    try {
+      Class<?> bitsClass =
+        Class.forName("java.nio.Bits", false, ClassLoader.getSystemClassLoader());
+      Method unalignedMethod = bitsClass.getDeclaredMethod("unaligned");
+      unalignedMethod.setAccessible(true);
+      _unaligned = Boolean.TRUE.equals(unalignedMethod.invoke(null));
+    } catch (Throwable t) {
+      // We at least know x86 and x64 support unaligned access.
+      String arch = System.getProperty("os.arch", "");
+      //noinspection DynamicRegexReplaceableByCompiledPattern
+      _unaligned = arch.matches("^(i[3-6]86|x86(_64)?|x64|amd64)$");
+    }
+    unaligned = _unaligned;
+  }
+
+  /**
+   * @return true when running JVM is having sun's Unsafe package available in it and underlying
+   *         system having unaligned-access capability.
+   */
+  public static boolean unaligned() {
+    return unaligned;
+  }
+
   public static int getInt(Object object, long offset) {
     return _UNSAFE.getInt(object, offset);
   }

core/src/main/scala/org/apache/spark/memory/MemoryManager.scala

@@ -23,6 +23,7 @@ import org.apache.spark.SparkConf
 import org.apache.spark.internal.Logging
 import org.apache.spark.storage.BlockId
 import org.apache.spark.storage.memory.MemoryStore
+import org.apache.spark.unsafe.Platform
 import org.apache.spark.unsafe.array.ByteArrayMethods
 import org.apache.spark.unsafe.memory.MemoryAllocator
 
@@ -190,6 +191,8 @@ private[spark] abstract class MemoryManager(
     if (conf.getBoolean("spark.memory.offHeap.enabled", false)) {
       require(conf.getSizeAsBytes("spark.memory.offHeap.size", 0) > 0,
         "spark.memory.offHeap.size must be > 0 when spark.memory.offHeap.enabled == true")
+      require(Platform.unaligned(),
+        "No support for unaligned Unsafe. Set spark.memory.offHeap.enabled to false.")
       MemoryMode.OFF_HEAP
     } else {
       MemoryMode.ON_HEAP