RFR: 4926314: Optimize Reader.read(CharBuffer) [v7]

[Alan Bateman]

On Fri, 12 Feb 2021 09:18:10 GMT, Philippe Marschall <github.com+471021+marschall at openjdk.org> wrote:

>> Implement three optimiztations for Reader.read(CharBuffer)
>> 
>> * Add a code path for heap buffers in Reader#read to use the backing array instead of allocating a new one.
>> * Change the code path for direct buffers in Reader#read to limit the intermediate allocation to `TRANSFER_BUFFER_SIZE`.
>> * Implement `InputStreamReader#read(CharBuffer)` and delegate to `StreamDecoder`.
>> * Implement `StreamDecoder#read(CharBuffer)` and avoid buffer allocation.
>
> Philippe Marschall has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Replace c-style array declarations
>  - Share work buffer between #skip and #read

src/java.base/share/classes/java/io/Reader.java line 221:

> 219:                     // if the last call to read returned -1 or the number of bytes
> 220:                     // requested have been read then break
> 221:                 } while (n >= 0 && remaining > 0);

The code for case that the char buffer has a backing array looks okay but I'm not sure about the direct buffer/other cases. One concern is that this is a read method, not a transferXXX method so we shouldn't be calling the underlying read several times. You'll see what I mean if you consider the scenario where you read < rem, then read again and the second read blocks or throws. I'l also concerned about "workBuffer" adding more per-stream footprint for cases where skip or read(CB) is used. Objects such as InputStreamReader are already a problem due to the underlying stream decoder.

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PR: https://git.openjdk.java.net/jdk/pull/1915