RFR: 8292698: Improve performance of DataInputStream

[Сергей Цыпанов]

I found out that reading from `DataInputStream` wrapping `ByteArrayInputStream` (as well as `BufferedInputStream` or any `InputStream` relying on `byte[]`) can be significantly improved by accessing volatile `in` field only once per operation.

Current implementation does it for each call of `in.read()`, i.e. in `readInt()` method we do it 4 times:

public final int readInt() throws IOException {
    int ch1 = in.read();
    int ch2 = in.read();
    int ch3 = in.read();
    int ch4 = in.read();
    if ((ch1 | ch2 | ch3 | ch4) < 0)
        throw new EOFException();
    return ((ch1 << 24) + (ch2 << 16) + (ch3 << 8) + (ch4 << 0));
}

Apparently accessing volatile reference with underlying `byte[]` prevents runtime from doing some optimizations, so dereferencing local variable should be more efficient.

Benchmarking:

baseline:

Benchmark                     Mode  Cnt   Score   Error  Units
DataInputStreamTest.readChar  avgt   20  22,889 ± 0,648  us/op
DataInputStreamTest.readInt   avgt   20  21,804 ± 0,197  us/op

patch:

Benchmark                     Mode  Cnt   Score   Error  Units
DataInputStreamTest.readChar  avgt   20  11,018 ± 0,089  us/op
DataInputStreamTest.readInt   avgt   20   5,608 ± 0,087  us/op

-------------

Commit messages:
 - 8292698: Improve performance of reading from DataInputStream

Changes: https://git.openjdk.org/jdk/pull/9956/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9956&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292698
  Stats: 85 lines in 2 files changed: 76 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/9956.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9956/head:pull/9956

PR: https://git.openjdk.org/jdk/pull/9956