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

bash

# 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

code

[engine] Inlined: hasConflict
  Reason: trivialSize

Not Inlined

code

[engine] Not inlined: recursiveFunc
  Reason: RecursiveInlining

code

[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

bash

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