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 . Based both on performance and memory consumption it is a (better) alternative to https://github.com/openjdk/jdk/pull/24713 .

This PR optimizes local field lookup in classes with more than 16 fields; rather than sequentially iterating through all fields during lookup we sort the fields based on the field name. The stream includes extra table after the field information: for field at position 16, 32 ... we record the (variable-length-encoded) offset of the field info in this stream. On field lookup, rather than iterating through all fields, we iterate through this table, resolve names for given fields and continue field-by-field iteration only after the last record (hence at most 16 fields).

In classes with <= 16 fields this PR reduces the memory consumption by 1 byte that was left with value 0 at the end of stream. In classes with > 16 fields we add extra 4 bytes with offset of the table, and the table contains one varint for each 16 fields. The terminal byte is not used either.

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/jdk/bin/java -cp /tmp CCC
  Time (mean ± σ):      38.5 ms ±   0.5 ms    [User: 34.4 ms, System: 17.3 ms]
  Range (min … max):    37.7 ms …  42.1 ms    100 runs

While https://github.com/openjdk/jdk/pull/24713 returned the performance to previous levels, this PR improves it by 25% compared to JDK 17 (which does not contain the regression)! This time, the undisclosed production-grade reproducer shows even higher improvement:

JDK 17: 1.6 s
JDK 21 (no patches): 22 s
JDK25-master: 12.3 s
JDK25-this-pr: 0.5 s

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Commit messages:
 - 8352075: Perf regression accessing fields

Changes: https://git.openjdk.org/jdk/pull/24847/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24847&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352075
  Stats: 208 lines in 14 files changed: 151 ins; 13 del; 44 mod
  Patch: https://git.openjdk.org/jdk/pull/24847.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24847/head:pull/24847

PR: https://git.openjdk.org/jdk/pull/24847