Detecting Performance Warnings

Detects optimization barriers during Truffle compilation. Essential for finding virtual calls, type instabilities, and boundary issues that prevent peak performance.

When to Use This Skill

• Find optimization barriers preventing compilation
• Identify virtual calls that should be direct
• Detect non-constant stores and type check failures
• Locate TruffleBoundary issues in hot paths

Use EARLY in performance investigation - often reveals root causes quickly.

Quick Start

# Basic warning detection
<launcher> --experimental-options \
  --compiler.TracePerformanceWarnings=all <program>

# Combined with compilation trace (RECOMMENDED)
<launcher> --experimental-options \
  --compiler.TracePerformanceWarnings=all \
  --engine.TraceCompilation \
  <program> 2>&1 | tee warnings.log

⚠️ REQUIRED: Fermi Verification (Every Tool Invocation)

Before running:

• Pre-calculate: Expected warnings (0-10 for well-optimized code, 0 is ideal)
• Smoke test: <launcher> --experimental-options --compiler.TracePerformanceWarnings=all -c 'print 1;' → Verify no false warnings

After running:

• Validate: Warning count within expectation? YES / NO
• If many warnings (>20): Document each - these are critical optimization barriers
• Save output: tool-outputs/perf-warnings-[benchmark].txt

Gate: All boxes checked? → Proceed to analysis

Key Options

Option	Description
--compiler.TracePerformanceWarnings=all	All warning types
--compiler.TracePerformanceWarnings=call	Call-related only
--compiler.TracePerformanceWarnings=instanceof	Type checks only
--compiler.TracePerformanceWarnings=store	Store operations only

Understanding Output

Virtual Call Warning (Critical!)

[engine] Performance warning: observed virtual call at
  com.example.CallNode.call(CallNode.java:42)
  Reason: CallTarget is not constant

Fix: Add @Cached for CallTarget lookup.

Type Check Warning

[engine] Performance warning: observed type check at
  com.example.TypeNode.execute(TypeNode.java:55)
  Reason: Type is not constant

Fix: Add type-specific specializations.

Store Warning

[engine] Performance warning: observed store at
  com.example.StoreNode.execute(StoreNode.java:33)
  Reason: Store is not frame or final field

Fix: Use frame slots or final fields.

Warning Severity

Warning Type	Severity	Impact
Virtual call	Critical	Blocks inlining, 10-100x slower
Type check	High	Prevents specialization
Store	Medium	May escape optimization

Integration with Other Skills

Finding	Next Skill
Many virtual calls	analyzing-compiler-graphs for details
Type instability	detecting-deoptimizations
Still confused	fetching-truffle-documentation

Common Fixes

Virtual Call → Direct Call

// ❌ BAD
CallTarget target = lookupFunction(name);
target.call(args);

// ✅ GOOD
@Specialization(guards = "func == cachedFunc")
Object cached(@Cached("func") Function cachedFunc,
              @Cached("cachedFunc.getCallTarget()") CallTarget target) {
    return target.call(args);
}

Type Check → Specialization

// ❌ BAD
if (value instanceof Integer) { ... }

// ✅ GOOD
@Specialization
int doInt(int value) { ... }

Related Skills

• profiling-with-cpu-sampler - Identify hot functions first
• tracing-compilation-events - See compilation status
• analyzing-compiler-graphs - Deep IR analysis
• fetching-truffle-documentation - API reference

Reference

<launcher> --help:compiler:internal | grep -i trace