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name: detecting-deoptimizations description: Traces deoptimization events where execution falls back from compiled code to interpreter. Use to detect deoptimization loops, unstable assumptions, and type instability. Zero transfers in steady-state is the goal. Many transfers indicate severe performance problems. Critical for diagnosing compilation instability.

Detecting Deoptimizations

Traces every deoptimization event where execution falls back from compiled code to interpreter. Critical for finding the most severe performance problems.

Why Deoptimizations Are Catastrophic

• Single transfer cost: Hundreds of nanoseconds (vs ~5ns for normal call)
• Deoptimization loops: 10-100x slower than pure interpreter!
• Wasted compilation: Compile → deoptimize → recompile → deoptimize = disaster

Goal: Zero transfers in steady-state execution!

When to Use This Skill

• Investigate poor performance despite successful compilation
• Profile shows methods in interpreted mode
• Debug why compiled code keeps falling back
• Verify benchmarks have properly warmed up

Quick Start

Copy# Basic deoptimization trace
<launcher> --experimental-options \
  --engine.TraceTransferToInterpreter <program>

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

# Focus on specific function
<launcher> --experimental-options \
  --engine.TraceTransferToInterpreter \
  --engine.CompileOnly=functionName \
  <program>

⚠️ REQUIRED: Fermi Verification (Every Tool Invocation)

Before running:

• Pre-calculate: Expected transfers (0-50 during warmup, 0 in steady-state)
• Smoke test: <launcher> --experimental-options --engine.TraceTransferToInterpreter -c 'print 1;' → Verify no false transfers

After running:

• Validate: Transfer count within expectation? YES / NO
• If >100 transfers: CRITICAL - Deoptimization loop, analyze repeated locations
• Save output: tool-outputs/trace-transfers-[benchmark].txt

Gate: All boxes checked? → Proceed to analysis

Key Options

Understanding Output

Transfer Event

Copy[engine] transferToInterpreter at
problemFunction(<source>:42:123-456)
callerFunction(<source>:18:78-90)
topLevelFunction(<source>:5:10-50)

Problem Patterns

Pattern 1: Deoptimization Loop (CRITICAL!)

Copy[engine] transferToInterpreter at MyNode.execute(<source>:42)
[engine] transferToInterpreter at MyNode.execute(<source>:42)
[engine] transferToInterpreter at MyNode.execute(<source>:42)
...dozens more at same location...

🚨 This is catastrophic! Often worse than never compiling.

Cause: Unstable type assumptions Fix: Implement proper type specializations, avoid mixing types

Pattern 2: Warmup Transfers (Normal)

Copy[First 5 seconds]
...many transfers...
[After 10+ seconds]
[No more transfers]  ✅

Acceptable: Decreases over time, stops after warmup

Pattern 3: Rare Path Transfers (Acceptable)

Copy[During entire execution]
[engine] transferToInterpreter at errorHandler(<source>:250)

Acceptable: Infrequent, from error handling or validation

Transfer Count Guidelines

Analysis Commands

Copy# Count total transfers
grep -c "transferToInterpreter" transfers.log

# Count unique locations
grep "transferToInterpreter at" transfers.log | sort -u | wc -l

# Find repeated locations (deoptimization loops)
grep "transferToInterpreter at" transfers.log | sort | uniq -c | sort -rn

Integration with Other Skills

Related Skills

• tracing-compilation-events - See compilation cycles
• profiling-with-cpu-sampler - Measure impact
• detecting-performance-warnings - Find barriers
• analyzing-compiler-graphs - Deep analysis

Reference

Copy<launcher> --help:engine | grep -i transfer

See PATTERNS.md for detailed problem patterns and solutions.