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perf: fix CompileSimpleWorkflow regression (+177.5% slowdown)#28408

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copilot/fix-compile-simple-workflow-performance
Apr 25, 2026
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perf: fix CompileSimpleWorkflow regression (+177.5% slowdown)#28408
pelikhan merged 4 commits intomainfrom
copilot/fix-compile-simple-workflow-performance

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Copilot AI commented Apr 25, 2026

BenchmarkCompileSimpleWorkflow regressed to 4.80ms/op (from 1.73ms historical avg) with a 37.7% increase in allocations, caused by three hot-path inefficiencies in the Claude engine compilation path.

Changes

  • claude_engine.gocomputeAllowedClaudeToolsString() was called twice per compilation in GetExecutionSteps: once to build the --allowed-tools arg, and again redundantly to generate a YAML comment. Second call eliminated by reusing the already-computed allowedTools string.

  • claude_tools.gogenerateAllowedToolsComment() used fmt.Fprintf in a loop and indent + "..." concatenations, allocating on every iteration. Replaced with strings.Builder.WriteString and a pre-sized buffer:

    const avgToolNameLen = 16 // e.g. "Bash(echo)"=10, "mcp__github__issue_read"=23
    comment.Grow(len(indent)*len(tools) + len("# Allowed tools (sorted):\n") + len(tools)*(len("# - \n")+avgToolNameLen))
  • features.goisFeatureEnabled() is called ~34 times per compilation. Its featuresLog.Printf(...) calls allocate interface values for variadic boxing at the call site even when logging is disabled. All log calls guarded with if featuresLog.Enabled().

Net result: ~6,217 → ~5,893 allocs/op (-5.2%) on BenchmarkCompileSimpleWorkflow.

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Copilot AI and others added 2 commits April 25, 2026 06:30
- Eliminate redundant computeAllowedClaudeToolsString call in GetExecutionSteps
  (was called twice: once for --allowed-tools arg, once for comment generation)
- Optimize generateAllowedToolsComment to avoid fmt.Fprintf allocations and
  string concatenations; pre-size builder to eliminate buffer growth reallocations
- Guard featuresLog.Printf calls with Enabled() checks in features.go to avoid
  variadic interface boxing overhead when debug logging is disabled

Reduces allocs/op from 6217 to ~5893 (-5.2%) in BenchmarkCompileSimpleWorkflow

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/5ea3e761-b229-49fb-87bf-7075c089bd2c

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>
Address code review: replace magic number 16 with named constant avgToolNameLen
with a comment explaining the rationale (typical Claude tool name lengths).

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/5ea3e761-b229-49fb-87bf-7075c089bd2c

Co-authored-by: gh-aw-bot <259018956+gh-aw-bot@users.noreply.github.com>
Copilot AI changed the title [WIP] Fix regression in CompileSimpleWorkflow performance perf: fix CompileSimpleWorkflow regression (+177.5% slowdown) Apr 25, 2026
Copilot AI requested a review from gh-aw-bot April 25, 2026 06:33
@pelikhan pelikhan marked this pull request as ready for review April 25, 2026 10:13
Copilot AI review requested due to automatic review settings April 25, 2026 10:13
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Pull request overview

Addresses a performance regression in BenchmarkCompileSimpleWorkflow by reducing redundant work and allocations in the Claude engine compilation path.

Changes:

  • Reuse the precomputed Claude --allowed-tools string when generating the YAML allowed-tools comment.
  • Rewrite allowed-tools comment generation to use strings.Builder (vs fmt.Fprintf in a loop) with an attempted pre-sizing strategy.
  • Guard featuresLog.Printf(...) calls with featuresLog.Enabled() to avoid variadic boxing/allocation when logging is disabled.
Show a summary per file
File Description
pkg/workflow/claude_engine.go Removes a redundant computeAllowedClaudeToolsString() call by reusing allowedTools.
pkg/workflow/claude_tools.go Switches comment construction to strings.Builder and adds Grow(...) to reduce allocations.
pkg/workflow/features.go Adds Enabled() guards around feature logging to avoid allocations when logs are off.

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  • Files reviewed: 3/3 changed files
  • Comments generated: 2

Comment thread pkg/workflow/claude_tools.go Outdated
Comment on lines +466 to +474
// Pre-size the builder to avoid reallocations: header line + one line per tool.
// Each tool line is: indent + "# - " + toolName + "\n" (~indent+6 bytes + tool length).
// avgToolNameLen is a conservative estimate for typical Claude tool names
// (e.g., "Bash(echo)" = 10 chars, "mcp__github__issue_read" = 23 chars).
const avgToolNameLen = 16
var comment strings.Builder
comment.WriteString(indent + "# Allowed tools (sorted):\n")
comment.Grow(len(indent)*len(tools) + len("# Allowed tools (sorted):\n") + len(tools)*(len("# - \n")+avgToolNameLen))
comment.WriteString(indent)
comment.WriteString("# Allowed tools (sorted):\n")
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Copilot AI Apr 25, 2026

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The Grow(...) sizing appears to underestimate the final comment size: the header line also writes indent (line 473) but len(indent) isn’t included in the growth calculation, and avgToolNameLen=16 is described as “conservative” even though examples in the comment include longer names (e.g. 23 chars). This can still trigger reallocations in the hot path and makes the comment misleading. Consider basing the tool-length component on len(allowedToolsStr) (minus commas) or using a higher/accurate estimate, and include the header indent in the growth calculation.

See below for a potential fix:

	// Pre-size the builder to avoid reallocations using the exact output size:
	// - header line: indent + "# Allowed tools (sorted):\n"
	// - one line per tool: indent + "# - " + toolName + "\n"
	// allowedToolsStr is a comma-separated list of tool names, so subtract the commas
	// to get the total number of bytes that will be written for tool names.
	toolNameBytes := len(allowedToolsStr) - (len(tools) - 1)
	var comment strings.Builder
	comment.Grow(
		len(indent) +
			len("# Allowed tools (sorted):\n") +
			len(tools)*len(indent) +
			len(tools)*len("# - \n") +
			toolNameBytes,
	)

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Comment thread pkg/workflow/features.go
Comment on lines 19 to +22
flagLower := strings.ToLower(strings.TrimSpace(string(flag)))
featuresLog.Printf("Checking if feature is enabled: %s", flagLower)
if featuresLog.Enabled() {
featuresLog.Printf("Checking if feature is enabled: %s", flagLower)
}
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Copilot AI Apr 25, 2026

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featuresLog.Enabled() is called repeatedly within the same function (and within getFeatureValueFromFrontmatter). Since it just returns a bool, consider capturing it once in a local logEnabled := featuresLog.Enabled() and reusing it to reduce repetition and keep the logging guards easier to scan/maintain.

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@pelikhan
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@copilot review all comments

- claude_tools.go: use exact output size in Grow() based on len(allowedToolsStr)
  instead of avgToolNameLen estimate; also include header indent in calculation
- features.go: capture featuresLog.Enabled() once as logEnabled and thread it
  through getFeatureValueFromFrontmatter and isFeatureInEnvironment to avoid
  repeated calls and reduce repetition

Agent-Logs-Url: https://github.com/github/gh-aw/sessions/0df3a808-8546-4b1f-b9a4-bd245c660aa4

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
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Copilot AI commented Apr 25, 2026

@copilot review all comments

Both review suggestions addressed in ab095c5:

  • claude_tools.go: Replaced the avgToolNameLen estimate with exact sizing using len(allowedToolsStr) - (len(tools) - 1) (subtracting commas to get total tool name bytes), plus the missing header indent in the calculation.
  • features.go: Captured featuresLog.Enabled() once as logEnabled in isFeatureEnabled and threaded it through getFeatureValueFromFrontmatter and isFeatureInEnvironment as a parameter, eliminating the repeated calls.

Copilot AI requested a review from pelikhan April 25, 2026 10:44
@pelikhan pelikhan merged commit abef339 into main Apr 25, 2026
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@pelikhan pelikhan deleted the copilot/fix-compile-simple-workflow-performance branch April 25, 2026 11:00
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[performance] Regression in CompileSimpleWorkflow: +177.5% slower

4 participants