RFR: 8292698: Improve performance of DataInputStream

[Alan Bateman]

On Sun, 21 Aug 2022 06:29:43 GMT, Сергей Цыпанов <duke at openjdk.org> wrote:

> 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

'in' is a protected field so it requires thinking about subclasses that might change 'in', say when the input stream asynchronously closed. BufferedInputStream is an example of a  FilterInputStream that sets 'in' to null when asynchronously closed. There are other examples that change the underlying input stream to one that returns EOF when closed. So it might be that changing readChar/readInt/readLong is okay but it would have a bigger effect on readFully, skip and the other methods. So I think this is case of proceeding with caution as there may be more gong on.

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