RFR: 8352075: Perf regression accessing fields [v2]

[Radim Vansa]

> This optimization is a followup to https://github.com/openjdk/jdk/pull/24290 trying to reduce the performance regression in some scenarios introduced in https://bugs.openjdk.org/browse/JDK-8292818
> 
> Iteration through the field stream of several variable-length-encoded items is limited by inherent dependency of fully decoding the field before knowing the position of next field. On the critical codepath addressed by this PR, only the name and signature indices from are used though.
> Inspired by the Varint-GB and Varint-G81U encodings, the idea of this change is in adding a control stream to the stream of field infos; each field uses one byte of this control stream, and currently the lower 6 bits encode the length of the encoded field, allowing more efficient skipping through the stream. The most significant bit duplicates the injected field flag (name/signature lookup requires that), one bit is currently unused.
> 
> My measurements on the attached reproducer
> 
> hyperfine -w 50 -r 100 '/path/to/jdk-17/bin/java -cp /tmp CCC'
> Benchmark 1: /path/to/jdk-17/bin/java -cp /tmp CCC
>   Time (mean ± σ):      51.3 ms ±   2.8 ms    [User: 44.7 ms, System: 13.7 ms]
>   Range (min … max):    45.1 ms …  53.9 ms    100 runs
> 
> 
> hyperfine -w 50 -r 100 '/path/to/jdk25-master/bin/java -cp /tmp CCC'
> Benchmark 1: /path/to/jdk25-master/bin/java -cp /tmp CCC
>   Time (mean ± σ):      78.2 ms ±   1.0 ms    [User: 74.6 ms, System: 17.3 ms]
>   Range (min … max):    73.8 ms …  79.7 ms    100 runs
> 
> (the `jdk25-master` above already contains JDK-8353175)
> 
> hyperfine -w 50 -r 100 '/path/to/jdk25-this-pr/bin/java -cp /tmp CCC'
> Benchmark 1: /path/to/jdk25-this-pr/bin/java -cp /tmp CCC
>   Time (mean ± σ):      51.8 ms ±   2.1 ms    [User: 48.9 ms, System: 16.8 ms]
>   Range (min … max):    47.4 ms …  55.1 ms    100 runs
> 
> So in case of the synthetic reproducer we're already on the level of JDK 17. However, the undisclosed production-grade reproducer still shows regression, so there is still space for optimization.
> 
> JDK 17: 1.6 s
> JDK 21 (no patches): 22 s
> JDK25-master: 12.3 s
> JDK25-this-pr: 3.1 s
> 
> 
> About the downsides: This PR increases the consumption by 1 byte for each field in loaded classes. I've executed some tests on a simple Spring Boot application with 6800 instance classes loaded -> about 16k fields in total, with NMT on; the memory usage (Class.Metadata.used) seems to have grown by 32kB; this discrepancy could be investigated later on.
> 
> Architecturally, I had to remove `const`ness from some methods on ...

Radim Vansa has updated the pull request incrementally with one additional commit since the last revision:

  Make skip_bytes private

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24713/files
  - new: https://git.openjdk.org/jdk/pull/24713/files/69a50374..930e47fd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24713&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24713&range=00-01

  Stats: 5 lines in 1 file changed: 1 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24713.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24713/head:pull/24713

PR: https://git.openjdk.org/jdk/pull/24713