latency-instrumentation

Latency Instrumentation & Self-Healing Compile Loop

This skill guides the agent through surgically injecting TRACE_EVENT macros into Chromium C++ files to break down uninstrumented main-thread blocking gaps ("black boxes") and verifying the changes via an automated compilation loop.

⚠️ CRITICAL DESIGN CONSTRAINT: NO OPTIMIZATIONS

This skill is strictly for data collection and instrumentation. You MUST NOT make any performance optimization or refactoring changes (such as caching, background-thread offloading, or logic restructuring). Your sole purpose is to add instrumentation to help the Trace Analyzer drill down into untraced gaps.

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1. Prerequisites & Input Context

Before executing this skill, you must have:

• target_method: The fully qualified C++ method name (e.g. LocationIconView::Update).
• category: The trace category (e.g., "omnibox", "navigation").
• instructions: Step-by-step C++ trace injection instructions (e.g., identifying loops or blocks to wrap).
• parent_session_id: The unique session ID.

Sandbox & Safety (Zero-Grant Rule)

• ALWAYS format all modified C++ files using git cl format immediately after editing.
• ALWAYS commit successful changes locally on the active E2E branch to checkpoint progress. Never create a CL or push to remote.
• If compilation fails after the self-healing loop, ALWAYS revert all uncommitted changes using git reset --hard HEAD to cleanly restore the last successful iteration's checkpoint.

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2. Workflow

Step 1: Locate target files

Use the codesearch tool to find the .cc and .h files declaring the target_method. Example:

lang:cpp "LocationIconView::Update"

If remote search yields zero results (common for unsubmitted local edits), run local ripgrep:

rg -n --type cpp "LocationIconView::Update"

Read the files using view_file.

Step 2: Identify Method Boundaries

Analyze the C++ source file to find the exact start and end lines of the target_method. Take care to match namespace scope, class qualifiers, and specific overloads.

Step 3: Surgically Inject Trace Macros

1. Parse the code within the identified method boundaries to locate internal sub-operations (e.g., heavy loops, helper function calls, or large nested blocks).
2. Wrap these blocks in scoped trace macros:

   {
     TRACE_EVENT0("category", "ParentMethodName::SubBlockName");
     // original code block
   }
   

3. Verify if #include "base/trace_event/trace_event.h" is present at the top of the C++ file. If missing, inject it programmatically inside the include section.
4. CRITICAL: Use file content modificatio tools for all modifications. NEVER use command-line tools like sed or cat << 'EOF'.
5. Run git cl format <modified_files> via run_command to format the code.

Step 4: Rebuild & Self-Healing Loop (Max 3 Attempts)

You are allowed up to 3 total compilation attempts to resolve any syntax, missing include, or structural errors:

1. Trigger Build: Run autoninja to compile:

   autoninja -C out/Default chrome
   

2. Success & Checkpoint: If it compiles successfully:
   • Run git add <modified_files> (add files manually, avoid wildcards).
   • Run git commit -m "feat(e2e-nla): Instrument [target_method]" to checkpoint your progress.
   • Report success and stop.
3. Failure & Repair Heuristics:
   • Parse the compiler stderr output.
   • Identify the compilation error (e.g., namespace collision, undeclared identifier, missing header).
   • Apply a targeted code repair using code editing tools.
   • Re-run autoninja.
4. Revert on Hard Failure: If all 3 attempts fail, run git reset --hard HEAD to revert all uncommitted modifications back to the last successful checkpoint (preserving previous successful iterations), and report failure.

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3. Output Contract

Report back with a JSON payload matching this structure:

{
  "status": "SUCCESS" | "FAILED",
  "modified_files": ["chrome/browser/...cc"],
  "compile_output": "Compiler logs or error details"
}