RFR: 8321620: Optimize JImage decompressors

[Glavo]

On Thu, 9 Nov 2023 02:07:57 GMT, Chen Liang <liach at openjdk.org> wrote:

>> This PR significantly speeds up decompressing resources in Jimage while significantly reducing temporary memory allocations in the process.
>> 
>> This will improve startup speed for runtime images generated using `jlink --compress 1` and `jlink --compress 2` .
>> 
>> I generated a runtime image containing javac using `jlink --compress 1 --add-modules jdk.compiler` and tested the time it took to compile a simple HelloWorld program 20 times using `perf stat -r20 javac /dev/shm/HelloWorld.java`, this PR reduces the total time taken from 17830ms to 13598ms (31.12% faster).
>
> src/java.base/share/classes/jdk/internal/jimage/decompressor/StringSharingDecompressor.java line 111:
> 
>> 109:         int count = Short.toUnsignedInt(bytesIn.getShort());
>> 110:         bytesOut[bytesOutOffset++] = (byte) ((count >> 8) & 0xff);
>> 111:         bytesOut[bytesOutOffset++] = (byte) (count & 0xff);
> 
> Probably use `ByteArray.setUnsignedShort`:
> Suggestion:
> 
>         ByteArray.setUnsignedShort(bytesOut, bytesOutOffset, count);
>         bytesOutOffset += 2;
> 
> Same remark elsewhere.

These classes need to be compiled with `--release 8`, so I can't use the `ByteArray`.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16556#discussion_r1387837310