Tracing Inlining Decisions

Shows inlining decisions during compilation with call tree structure and reasons. Essential for understanding why calls aren't being optimized.

When to Use This Skill

• Verify critical calls are being inlined
• Understand why inlining failed
• Optimize method sizes for better inlining
• Debug recursive function compilation

Quick Start

# Basic inlining trace
<launcher> --experimental-options \
  --engine.TraceInlining <program>

# Combined with compilation trace (RECOMMENDED)
<launcher> --experimental-options \
  --engine.TraceInlining \
  --engine.TraceCompilation \
  <program> 2>&1 | tee inlining.log

# Focus on specific function
<launcher> --experimental-options \
  --engine.TraceInlining \
  --engine.CompileOnly="*hotFunction*" \
  <program>

⚠️ REQUIRED: Fermi Verification (Every Tool Invocation)

Before running:

• Pre-calculate: Expected hot helper functions to inline (list 2-5 function names)
• Smoke test: <launcher> --experimental-options --engine.TraceInlining -c 'print 1;' → Verify inlining decisions appear

After running:

• Validate: Critical functions inlined as expected? YES / NO
• If NO: Document why (budgetExhausted, TooLarge, TruffleBoundary)
• Save output: tool-outputs/trace-inlining-[benchmark].txt

Gate: All boxes checked? → Proceed to analysis

Key Options

Option	Description
--engine.TraceInlining	Show inlining decisions
--engine.TraceInliningDetails	Detailed reasoning
--engine.CompileOnly=<name>	Filter to specific function

Understanding Output

Inlined Successfully

[engine] Inlined: hasConflict
  Reason: trivialSize

Not Inlined

[engine] Not inlined: recursiveFunc
  Reason: RecursiveInlining

[engine] Not inlined: bigFunction
  Reason: budgetExhausted

Inlining Reasons

Good (Inlined)

Reason	Meaning
trivialSize	Small enough to inline
forced	@Specialization forced inline
relevant	Deemed performance-relevant

Bad (Not Inlined)

Reason	Meaning	Fix
RecursiveInlining	Recursive call	Expected for recursion
budgetExhausted	Inlining budget used	Reduce method sizes
TooLarge	Method too big	Split method
TruffleBoundary	Marked as boundary	Review boundary usage
notRelevant	Not performance-critical	May be OK

Problem Patterns

Pattern 1: Budget Exhausted for Hot Functions

Symptom: Critical function not inlined due to budget Fix: Reduce sizes of other methods, or increase budget

Pattern 2: Unexpected Boundary

Symptom: Hot call not inlined due to TruffleBoundary Fix: Remove boundary if not needed, or restructure

Pattern 3: Too Many Recursive Levels

Symptom: Recursive function shows "RecursiveInlining" Expected: Normal for recursive algorithms (Bytecode DSL limitation)

Integration with Other Skills

Finding	Next Skill
Budget issues	Refactor code, reduce sizes
Boundaries	Review TruffleBoundary usage
Still slow	profiling-with-cpu-sampler

Related Skills

• profiling-with-cpu-sampler - Identify hot functions first
• tracing-compilation-events - Overall compilation status
• detecting-performance-warnings - Find barriers
• fetching-truffle-documentation - API reference

Reference

<launcher> --help:engine | grep -i inlin