Use VarHandles to access int/long/short types in byte arrays (e.g. ByteArrayDataInput) [LUCENE-10113] #11150
Description
#11149 reminded me about something i wanted to do long ago: Basically for all IndexInputs/DataInputs we are able to natively read short, int, long using little endian with single CPU instructions (due to using ByteBuffer's methods that support primitive reads). Only ByteArrayDataInput still uses manual code beased on the the inherited byte-by-byte approach to read single bytes and combining the bytes using little endian.
The approach here is to use Java 9+ VarHandles to allow reading int/long/short as single cpu instructions and not manually recombining the bytes. The trick is to make a "view" var handle which allows to access the byte array using the same mechanisms as ByteBuffers or JDK 17 MemorySegments (under the hood it uses Unsafe to use CPU instructions and optionally swap bytes if platform endianness is BE).
In #11149 there were similar stuff done with LZ4 and a microbenchmark was written that showed a significant speed improvement when accessing the types with VarHandle.
P.S.: The same applies to FST.BytesReader and/or ByteSliceReader, but I am no sure if those use the int/short/long ones at all. At least this one does not override the methods to read ints, longs and shorts, so there is no optimization at all. FST seems to read bytes and byte[] only and ByteSliceReader mostly VInts.
Migrated from LUCENE-10113 by Uwe Schindler (@uschindler), resolved Sep 20 2021
Linked issues:
Pull requests: #308