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

bash

# 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!)

code

[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

code

[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

code

[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

java

// ❌ 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

java

// ❌ 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

bash

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