perf(string): fuse accumulator concat chains

[proggeramlug]

Summary

• recognize s = s + a + b + ... for proven string accumulators in local, captured, and module-global slots
• lower the accumulator and suffix operands left-to-right into one rooted runtime call
• reuse a unique accumulator's existing capacity, or allocate the complete result exactly once
• preserve the existing dynamic-add fallback when the string proof is incomplete

Refs #8410.

Profile

The macOS sample utility hung before collecting a sample on this host, including when reproduced against yes. I used the same symbol-preserving build (PERRY_KEEP_SYMBOLS=1 PERRY_DEBUG_SYMBOLS=1, about 17.7k symbols in the generated fixture) with Instruments Time Profiler over a private 10x iso_miss fixture instead.

Baseline sampled CPU was 8,422 ms. The largest leaves were evalNode (38.90%), js_string_concat_chain (14.15%), lookup (12.19%), memmove (4.19%), arena allocation (3.72%), and the separate accumulator append/storage/rooting envelope (6.80% combined).

Candidate sampled CPU was 7,716 ms (-8.38%). The suffix concat plus separate append envelope becomes one js_string_append_chain leaf (15.15%); evalNode remained dominant (42.69%) and lookup remained essentially unchanged (12.08%).

Performance

Five shuffled, interleaved repeats on macOS, comparing a locally built 8d1cca158 baseline with this branch. Both arms used the identical -p perry -p perry-runtime-static -p perry-stdlib-static release build set, and archive mtimes were verified after each build. Medians are shown.

iso_miss	before	after	delta
wall	3.36 s	2.39 s	-28.87%
instructions	12,484,326,208	11,497,214,518	-7.91%
cycles	2,501,750,408	2,267,980,449	-9.34%
peak RSS	32,817,152 B	32,866,304 B	+49,152 B (+0.15%)

The wall ranges overlap widely (1.41-5.00 s before, 1.24-7.07 s after), so the wall median is not treated as a real 28.9% result. Instructions retired are the primary signal. The observed RSS cost is 48 KiB; the implementation does not add a cache or retained reserve and allocates the complete result once, but the measured peak is reported rather than rounded away.

Whole-corpus medians:

program	wall	instructions	cycles	peak RSS
asyncpipe	-27.78%	-0.13%	-4.31%	+0.04%
churn	+0.00%	-0.05%	+2.56%	+0.07%
churn_alloc	-27.27%	-0.08%	-2.87%	+0.00%
churn_read	+0.00%	-0.13%	-1.21%	+0.28%
cycles	+0.00%	+0.10%	+3.90%	+0.00%
deeplist	+0.00%	-0.06%	-0.69%	+0.03%
fib40	+1.59%	-0.03%	-0.89%	+0.34%
interp	+22.22%	-0.01%	-1.02%	+0.05%
iso_miss	-28.87%	-7.91%	-9.34%	+0.15%
pipeline	+0.00%	+0.08%	+0.79%	-0.07%
push_cls	+9.09%	-0.02%	-2.75%	+0.07%
push_num	-11.11%	-0.15%	-5.42%	-0.06%
retain	-22.22%	-0.09%	+1.84%	+0.01%
retain1	-11.11%	-0.05%	-2.04%	+0.00%
retain_wide	+13.51%	+0.41%	-0.50%	+0.00%
retain_wide1	-11.11%	-0.20%	-1.85%	+0.00%
shapes	+33.33%	-0.03%	-2.24%	+0.15%
tree	-22.73%	-0.11%	-4.88%	-0.06%
tree_wide	+21.54%	+0.02%	-0.41%	+0.00%

No non-target row moved more than 0.41% in instructions. All non-target wall ranges overlapped; the large wall/cycle swings track host contention rather than instruction count.

Validation

• all 19 corpus programs byte-exact against expected output
• cargo test --release -p perry-runtime --lib: 2,608 passed, 4 ignored
• cargo test --release -p perry-codegen --lib: 1,121 passed
• cargo test --release -p perry --bin perry: 1,008 passed
• lint: initial full run passed 50/52; the two branch-specific ratchets were fixed and pass individually, but the final full 52-gate rerun is blocked by the host disk floor
• gap suite: interrupted after 25/575 when the documented skip-build path reached a nested HTTP-extension Cargo build; a complete retry is blocked by the host disk floor

Host status at handoff: 6 GiB free after deleting this worktree's generated target/ and reducing the reserved external target to about 832 MiB. The task requires stopping below 15 GiB, so this PR remains draft until those two acceptance runs can complete.

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 5b055ef1-07c8-4680-9f33-100a11fbb9b4

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}