RFR: 8346239: Improve memory efficiency of JimageDiffGenerator

[Severin Gehwolf]

On Thu, 19 Dec 2024 18:14:39 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

> Please review this fairly simple change to improve how the `JimageDiffGenerator` works. The original implementation is pretty naive and read all bytes into memory and then compared them. This improved version only reads bytes on a bound buffer into memory compares those bytes and if equal continues on to reading the next bytes (`2k` at most) at a time. Previously it was `2*N` (where `N` is the file size of a file in bytes) part of the JDK. Ouch.
> 
> There is still the off-chance of reading a full file into memory when the generator detects a change, but this shouldn't happen for large files since the total diff should be around `600K` as of today.
> 
> This also reverts changes from [JDK-8344036](https://bugs.openjdk.org/browse/JDK-8344036) other than the `/timeout` change to the test, which is preserved as I think this bump is no longer needed.
> 
> Testing:
> - [x] GHA
> - [x] jlink tests on a fastdebug build with `--with-native-debug-symbols=internal` (so as to have a large libjvm.so).
> - [x] Manual reproducer from the bug which fails with OOM before the patch and passes after
> 
> Thoughts?

@slowhog @AlanBateman @archiecobbs Clean ups done in https://github.com/openjdk/jdk/pull/23014 PTAL.

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PR Comment: https://git.openjdk.org/jdk/pull/22835#issuecomment-2580889692