perf: eliminate strings.Split allocation in validateExpressionSizes by Copilot · Pull Request #45282 · github/gh-aw

Copilot · 2026-07-13T15:41:02Z

validateExpressionSizes was calling strings.Split(yamlContent, "\n") on every compilation, allocating a ~21KB []string slice for the compiled YAML and then scanning it twice (per-line length check + block-scalar check).

Changes

runtime_validation.go: Replaces the two-pass approach with validateExpressionSizesSinglePass, a single strings.SplitSeq pass that combines per-line and block-scalar validation, eliminating the slice allocation entirely.

// Before: allocates 1273-element []string, then loops twice
lines := strings.Split(yamlContent, "\n")
for lineNum, line := range lines { /* per-line check */ }
return validateBlockScalarExpressionSizes(lines, maxSize)

// After: one pass, zero slice allocation
for line := range strings.SplitSeq(yamlContent, "\n") { /* both checks */ }

validateBlockScalarExpressionSizes([]string, int) is preserved unchanged for its unit tests.

Benchmark (benchstat, 6+ runs)

metric	change	p-value
B/op	−2.10%	0.004
allocs/op	−0.05%	0.043
sec/op	−1.2%	0.177 (ns)

Memory reduction is statistically significant; timing is within noise. A walkRunBlockLines fast-path was also evaluated (adding an early strings.Contains skip for non-run-block lines) but reverted after profiling showed the Contains overhead on ~913 lines/compilation exceeded the savings from skipping TrimLeft+findRunValue.

Replace the two-pass approach (strings.Split + per-line loop + validateBlockScalarExpressionSizes loop) with a single strings.SplitSeq pass in validateExpressionSizesSinglePass. This removes the 1273-element []string allocation (~21KB/op) that strings.Split was creating for each compiled workflow YAML. Benchstat with 6+ runs shows a statistically significant -2.10% reduction in B/op (p=0.004) with no regression in throughput (p=0.177). The separate validateBlockScalarExpressionSizes([]string, int) function is preserved for backward compatibility with its unit tests. Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

github-actions · 2026-07-13T18:58:48Z

PR Triage (run 29275650494)\n\nCategory: perf | Risk: low | Score: 45/100 | Action: batch_review | Batch: draft-quality\n\nScore: Impact 20/50 + Urgency 12/30 + Quality 13/20\n\nEliminates strings.Split allocation in validateExpressionSizes via SplitSeq. Single file, +83/-24. DRAFT. Can be reviewed alongside #45280 and #45298.

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Copilot

Pull request overview

Optimizes expression-size validation by replacing an allocating, two-pass scan with a single strings.SplitSeq pass.

Changes:

Combines per-line and block-scalar validation.
Avoids allocating a YAML line slice.
Retains the legacy block-scalar helper for existing tests.

Show a summary per file

File	Description
`pkg/workflow/runtime_validation.go`	Implements single-pass expression-size validation.

Review details

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Files reviewed: 1/1 changed files
Comments generated: 2
Review effort level: Medium

Copilot · 2026-07-13T22:41:56Z

+		if colonIdx := strings.Index(trimmed, ":"); colonIdx > 0 {
+			afterColon := strings.TrimSpace(trimmed[colonIdx+1:])
+			if afterColon == "|" || afterColon == ">" || strings.HasPrefix(afterColon, "|-") || strings.HasPrefix(afterColon, ">-") {
+				inBlock = true


Addressed in commit 40d9367: extracted isBlockScalarHeader that covers all YAML-spec block scalar variants — |, >, |-, >-, |+, >+, |2–|9, |2-, |-2, >+3, etc. Both validateExpressionSizesSinglePass and validateBlockScalarExpressionSizes now use it. New test cases verify the previously-missed chomping/indent-indicator variants are caught.

gh-aw-bot · 2026-07-13T22:06:33Z

@copilot please run the pr-finisher skill, address any unresolved review feedback, and rerun checks once the branch is up to date.

Generated by 👨‍🍳 PR Sous Chef · 6.38 AIC · ⌖ 9.66 AIC · ⊞ 4.2K · ◷
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- Add isBlockScalarHeader helper to cover |+, >+, |2, |2-, |-2 etc. in both validateExpressionSizesSinglePass and validateBlockScalarExpressionSizes (previously only |- and >- were caught; keep-chomping and explicit-indent indicators were silently missed, letting oversized expression blocks bypass validation) - Add TestValidateExpressionSizesSinglePass with parity cases against the legacy helper plus per-line size check and EOF-block coverage - Add chomping/indent-indicator cases to TestValidateBlockScalarExpressionSizes Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>

Copilot · 2026-07-13T22:21:15Z

@copilot please run the pr-finisher skill, address any unresolved review feedback, and rerun checks once the branch is up to date.

...

Done. Two issues addressed in commits fix: extend block-scalar header detection and fix: correct test name:

Thread 1 (line 77 — missing tests for validateExpressionSizesSinglePass): Added TestValidateExpressionSizesSinglePass in block_scalar_expression_size_test.go with full parity coverage against the legacy helper, plus two cases unique to the single-pass path: a per-line oversized value check and an EOF-open-block check.

Thread 2 (lines 149-152 — chomping/indent indicators missed): Extracted an isBlockScalarHeader helper that recognises all YAML-spec block scalar header variants (|, >, |-, >-, |+, >+, |2–|9, |2-, |-2, >+3, etc.). Both validateExpressionSizesSinglePass and validateBlockScalarExpressionSizes now use it. New test cases (three for each function) confirm the previously-missed indicators are caught.

All make fmt, make lint, and make test-unit pass locally.

gh-aw-bot · 2026-07-13T22:36:43Z

pr-sous-chef
@copilot please run the pr-finisher skill, address these unresolved review comments, and rerun checks once the branch is up to date:

copilot-pull-request-reviewer: perf: eliminate strings.Split allocation in validateExpressionSizes #45282 (comment)
copilot-pull-request-reviewer: perf: eliminate strings.Split allocation in validateExpressionSizes #45282 (comment)

Run: https://github.com/github/gh-aw/actions/runs/29289965782

Generated by 👨‍🍳 PR Sous Chef · 10.9 AIC · ⌖ 3.94 AIC · ⊞ 7.5K · ◷
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Initial plan

bc23b2f

Copilot AI assigned Copilot and pelikhan Jul 13, 2026

Copilot started work on behalf of pelikhan July 13, 2026 15:41 View session

Copilot AI linked an issue Jul 13, 2026 that may be closed by this pull request

[performance] Regression in CompileSimpleWorkflow: 28.6% slower #45097

Open

Copilot AI and others added 2 commits July 13, 2026 16:26

fix: clarify blockStartLine comment in validateExpressionSizesSinglePass

7787b6b

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix regression in CompileSimpleWorkflow performance~~ perf: eliminate strings.Split allocation in validateExpressionSizes Jul 13, 2026

Copilot AI requested a review from pelikhan July 13, 2026 16:31

Copilot finished work on behalf of pelikhan July 13, 2026 16:31

github-actions Bot added pr-agent:copilot pr-priority:medium pr-risk:low pr-type:refactor pr-batch:draft-quality pr-action:batch-review labels Jul 13, 2026

This was referenced Jul 13, 2026

Refactor token env lookup and schedule helpers to remove semantic drift #45298

Merged

fix: CLI consistency — rename --no-staged, add -e to logs, grammar/style fixes #45280

Merged

pelikhan marked this pull request as ready for review July 13, 2026 21:17

Copilot AI review requested due to automatic review settings July 13, 2026 21:17

Copilot started reviewing on behalf of pelikhan July 13, 2026 21:17 View session

Copilot AI reviewed Jul 13, 2026

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github-actions Bot mentioned this pull request Jul 13, 2026

[pr-sous-chef] Run report — 4 nudged, 0 skipped #45140

Closed

Copilot started work on behalf of gh-aw-bot July 13, 2026 22:07 View session

Copilot AI and others added 2 commits July 13, 2026 22:20

fix: correct test name keep chomping (|-) -> (|+)

f3c8e86

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>

Copilot finished work on behalf of gh-aw-bot July 13, 2026 22:21

Copilot AI requested a review from gh-aw-bot July 13, 2026 22:21

Copilot started work on behalf of gh-aw-bot July 13, 2026 22:37 View session

Copilot finished work on behalf of gh-aw-bot July 13, 2026 22:43

github-actions Bot mentioned this pull request Jul 14, 2026

[pr-sous-chef] Run report — 4 nudged, 3 skipped #45345

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pelikhan closed this Jul 14, 2026

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perf: eliminate strings.Split allocation in validateExpressionSizes#45282

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Copilot AI commented Jul 13, 2026 •

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