RFR: 8352075: Perf regression accessing fields

[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 `FieldStreamBase`.

I think that the memory usage difference is negligible in the practice; if this matters, though, I have implemented two more optimizations in https://github.com/rvansa/jdk/tree/JDK-8352075-dev that should give the byte back, in average. However these optimizations have reduced performance in all of the benchmarks, hence I would prefer not to apply these.

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Commit messages:
 - 8352075: Perf regression accessing fields

Changes: https://git.openjdk.org/jdk/pull/24713/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24713&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352075
  Stats: 132 lines in 9 files changed: 74 ins; 9 del; 49 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