perf: Replace regex captures with hand-written parsers in berry lockfile identifiers

[vercel-gh-bot-2]

Summary

Replaces the two hot regexes in crates/turborepo-lockfiles/src/berry/identifiers.rs (^(?:@([^/]+?)/)?([^@/]+)$ for Ident and ^(?:@([^/]+?)/)?([^@/]+?)(?:@(.+))$ for Descriptor/Locator) with exact hand-written parsers. On a yarn-berry monorepo these regexes run ~40–50k times per turbo run invocation: once per descriptor in every lockfile entry key during BerryLockfile construction, once per resolve_package call from the transitive-closure DP, and once per dependency inside every all_dependencies call. Cold-path regexes (from_patch_reference, patch_file, is_patch_builtin) are unchanged. Public API, error messages, and Cow::Borrowed lifetimes are unchanged.

• Author model: anthropic/claude-fable-5
• Reviewer model: openai/gpt-5.6-sol (gpt_performance_reviewer)

Hypothesis

Berry identifier parsing is a materially hot path (per-call regex capture extraction and its allocation) on large yarn-berry monorepos; replacing it with allocation-free manual parsing reduces end-to-end turbo run --dry=json wall time with zero behavior change.

Diagnostics that motivated it: instrumented span profile on cal.com showed resolve_package 45.5 ms/11,520 calls, all_dependencies 19.3 ms/3,812 calls, parse_lockfile 40.4 ms of a ~210 ms run; perf on a symbolized build showed regex_automata::dfa::onepass::DFA::try_search_slots_imp (8.0%) and Regex::captures_at (2.4%) as top leaves of a lockfile-only workload.

Methodology

• Environment: Linux 6.18.40 x86_64, Intel Xeon @ 2.90 GHz, 2 vCPU, 4.2 GB RAM, THP=madvise, rustc 1.97.0-nightly (pinned nightly-2026-05-22), hyperfine 1.20.0 (official release, SHA-256 verified). Shared/noisy VM disclosed; mitigated with paired balanced blocks, medians, and bootstrap CIs.
• Binaries: both built with cargo build --profile release-turborepo -p turbo. Baseline from clean checkout 37be819 (SHA-256 992fbd03…, 50,632,544 B); candidate from this diff (SHA-256 77d54a01…, 50,630,752 B; −1,792 B). Both preserved outside the checkout; all evidence fingerprints these exact binaries.
• Workload: cal.com @ 176037d0afbe572f870a3c702985e7cd83fe6c0c (yarn berry, 1.5 MB yarn.lock, 3,935 entries), detached and immutable. Command: turbo run build --dry=json --no-daemon (daemon disabled; warm page cache; identical preparation both sides).
• Recorded phases: clean-checkout baseline and same-command after (20 runs each, stable binary path), 8 paired AB/BA comparison hyperfine blocks (--warmup 3 --runs 8, order alternating, 64 samples per binary), and correctness validation, all via run_performance_command with binary and corpus fingerprints.

Results (recorded evidence)

End-to-end, cal.com dry run (8 balanced blocks, block speedup = 1 − candidate_median/baseline_median):

block	order	baseline median	candidate median	speedup
1	AB	151.9 ms	138.0 ms	+9.19%
2	BA	145.3 ms	149.4 ms	−2.85%
3	AB	148.1 ms	143.9 ms	+2.83%
4	BA	145.7 ms	145.3 ms	+0.31%
5	AB	157.3 ms	149.1 ms	+5.20%
6	BA	151.4 ms	144.6 ms	+4.53%
7	AB	153.4 ms	144.4 ms	+5.90%
8	BA	163.3 ms	142.4 ms	+12.85%

• Mean block speedup +4.74% (median +4.87%); both orders improve (AB +5.78%, BA +3.71%); bootstrap (20k resamples) 95% CI of mean block speedup [+1.59%, +7.91%]; pooled medians 150.8 → 144.4 ms (−6.5 ms, −4.28%, n=64/64).
• Recorded baseline vs after phases (same command text, 20 runs): 153.0 ± 8.9 ms → 142.0 ± 6.0 ms.
• Earlier independent 12-block set against a bit-different (pre-rustfmt, semantically identical) candidate build agreed: mean +3.30%, both orders positive, 95% CI [+2.34%, +4.23%].

Mechanism corroboration (targeted benchmark; full parse + all_dependencies + resolve_package sweep over all 3,935 entries ×20 iterations per process; baseline binary built from a pristine worktree of 37be819, candidate from this diff; identical bench source, outputs identical keys=3935 deps=4247 resolved=4107): 6 balanced AB/BA hyperfine blocks, 60 samples per binary, mean block speedup +11.62%, both orders positive (AB +12.76%, BA +10.47%), bootstrap 95% CI [+9.27%, +13.92%], pooled medians 676.9 → 594.2 ms (−12.2%).

Non-regression sanity (recorded): payload (pnpm; berry path not executed) 90.2 ± 3.7 ms vs 88.1 ± 4.6 ms — unchanged within noise.

Correctness

• cargo test -p turborepo-lockfiles: 314/314 pass, including a new exhaustive differential test (all 55,987 strings up to length 6 over {@, /, a, b, :, \n} compared against the original regexes, kept in-test as oracles) and realistic/edge cases (Unicode, patch locators, @a@b/c@1.0.0, trailing @, newline-in-range, empty string).
• cargo clippy -p turborepo-lockfiles --all-targets and cargo fmt --check: clean.
• End-to-end equality with both preserved binaries on cal.com: --dry=json output byte-equal modulo the random run id (115 tasks; all task hashes and global hash identical); turbo prune @calcom/web outputs recursively identical, pruned yarn.lock byte-identical.

Review

gpt_performance_reviewer (openai/gpt-5.6-sol) approved the exact final diff after one round of blocking findings (the initial mechanism benchmark lacked reproducible source, paired blocks, and CIs; it was replaced with the fully provenance-tracked paired benchmark above). Reviewer summary: parsers preserve the original regex semantics with exhaustive differential coverage and unchanged API/error behavior; targeted benchmark and end-to-end paired evidence are consistent with the regex-elimination mechanism; approval limited to the stated cal.com workload claims.

Limitations

• The change only affects yarn-berry repositories; the measured claim is specific to the cal.com workload. Other berry repos are expected to benefit in proportion to lockfile size, but no cross-repository number is claimed.
• Measurements were taken on a shared 2-vCPU VM with documented noise; mitigations: balanced AB/BA blocks, medians, bootstrap CIs, and an independent earlier block set that agrees.

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