RFR: 8321620: Optimize JImage decompressors

[Chen Liang]

On Wed, 8 Nov 2023 11:55:22 GMT, Glavo <duke 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.

src/java.base/share/classes/jdk/internal/jimage/decompressor/StringSharingDecompressor.java line 113:

> 111:         bytesOut[bytesOutOffset++] = (byte) (count & 0xff);
> 112:         for (int i = 1; i < count; i++) {
> 113:             int tag = bytesIn.get() & 0xff;

Suggestion:

            int tag = Byte.toUnsignedInt(bytesIn.get());

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PR Review Comment: https://git.openjdk.org/jdk/pull/16556#discussion_r1387400142
PR Review Comment: https://git.openjdk.org/jdk/pull/16556#discussion_r1387397234