Advanced Profiling Tools

IONPERF — JIT code annotation for profilers

The IONPERF environment variable enables SpiderMonkey to emit JIT code metadata that profilers (samply, perf) can use to attribute samples to JIT-compiled functions and annotate disassembly with IR or source.

PERF_SPEW_DIR must be set to a directory where the jitdump files will be written. Create the directory before running:

mkdir -p artifacts/perf-spew
PERF_SPEW_DIR=artifacts/perf-spew IONPERF=ir \
    samply record --save-only -o profile.json.gz -- \
    ./obj-opt-nodebug/dist/bin/js --strict-benchmark-mode workload.js

Modes

Value	Output	Build requirement
`func`	Function-level granularity only	None
`ir`	Assembly annotated with MIR/LIR names	None
`ir-ops`	Assembly annotated with MIR/LIR including operands	`--enable-jitspew`
`ir-graph`	Structured IR graphs for visualization	`--enable-jitspew`
`src`	Assembly annotated with source lines (if available)	None

func is the lightest option — it just tells the profiler which JIT function each code region belongs to, so profiler output shows function names instead of raw addresses. Use this when you just want readable profiler output without IR detail.

ir is the most commonly useful mode for performance investigation. It annotates the generated assembly with the MIR/LIR instruction names, so you can correlate hot assembly with the compiler IR that produced it. This is invaluable when you need to understand what the JIT decided to emit for a given operation.

ir-ops adds operand details to the IR annotations (register names, constant values). Requires a jitspew-enabled build (--enable-jitspew in the mozconfig). Falls back to ir if jitspew is not available.

ir-graph emits structured IR graph data suitable for visualization tools. Also requires --enable-jitspew.

src annotates assembly with source file and line information. Useful when you want to see which JS source line produced which assembly, but depends on source info being available in the build.

When to use IONPERF

Use IONPERF=func routinely — it has negligible overhead and makes profiler output readable for JIT code.

Use IONPERF=ir when:

A hot function is JIT-compiled and you need to understand what code the JIT emitted
You’re evaluating whether the JIT is making good decisions for a specific code pattern
You want to see if a new IC stub or optimization is actually being used