[GitHub] [lucene-solr] mcimadamore commented on a change in pull request #2176: Initial rewrite of MMapDirectory for JDK-16 preview (incubating) PANAMA APIs

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mcimadamore commented on a change in pull request #2176:
URL: https://github.com/apache/lucene-solr/pull/2176#discussion_r552026775



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File path: lucene/core/src/java/org/apache/lucene/store/MemorySegmentIndexInput.java
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@@ -0,0 +1,524 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.store;
+
+import java.io.EOFException;
+import java.io.IOException;
+import java.lang.invoke.VarHandle;
+import java.nio.ByteOrder;
+import java.util.Arrays;
+import java.util.Objects;
+
+import org.apache.lucene.util.IOUtils;
+
+import jdk.incubator.foreign.MemoryHandles;
+import jdk.incubator.foreign.MemorySegment;
+
+/**
+ * Base IndexInput implementation that uses an array of MemorySegments to represent a file.
+ *
+ * <p>For efficiency, this class requires that the segment size are a power-of-two
+ * (<code>chunkSizePower</code>).
+ */
+public abstract class MemorySegmentIndexInput extends IndexInput implements RandomAccessInput {
+  // We pass 1L as alignment, because currently Lucene file formats are heavy unaligned: :(
+  static final VarHandle VH_getByte = MemoryHandles.varHandle(byte.class, 1L, ByteOrder.BIG_ENDIAN).withInvokeExactBehavior();
+  static final VarHandle VH_getShort = MemoryHandles.varHandle(short.class, 1L, ByteOrder.BIG_ENDIAN).withInvokeExactBehavior();

Review comment:
       Just to be sure here - the offset accepted by this var handle is a _byte_ offset not a short offset. E.g. if you want to read consecutive short elements you need to do:
   ```
   VH_getShort(segment, 0);
   VH_getShort(segment, 2);
   VH_getShort(segment, 4);
   ```
   Alternatively, if you need array-like indexing, but don't want to mess with layouts, you can use the pre-baked statics inside the MemoryAccess class (e.g. `getShortAtIndex` vs. `getShortAtOffset` - the former take a logical short index, the latter takes a raw byte offset).




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