Execution Guide

Tool versions

Browser resolution as in fe-01: $CHROME, newest Playwright cache build, then system Chrome.

Quick start

cd fe-03-object-shapes-and-jit/src
npm install
npm run ic            # 1. inline caches: mono/poly/megamorphic    (~15s)
npm run shapes        # 2. hidden classes, delete, dictionary mode (~20s)
npm run arrays        # 3. array element kinds                     (~25s)
npm run perspective   # 4. what any of it is worth                 (~40s)
npm run all

Run perspective even if you skip the others. It is the module.

Tuning

ITER=10000000 npm run ic
N=1000000     npm run shapes
N=20000000    npm run arrays

Why every case gets a fresh page

Micro-benchmarks contaminate each other in ways that are silent and directional:

• Inline caches are per-site and persist. A shared helper function accumulates feedback across cases, so case 1 pays warm-up and case 4 inherits a warm-but-polymorphic function. This produced a backwards result while building the module.
• JIT tier is per-function and persists. By case 3, a function may be in TurboFan; case 1 ran in Sparkplug.
• The DOM persists. A case that appends 5,000 elements makes every later layout measurement enormous.

Fresh page per case costs ~150 ms of setup and removes all three. It is not optional.

Reading the harness

lib/bench.mjs addresses three standard micro-benchmark failures:

It does not address: timer coarsening (performance.now() is quantised to ~0.1 ms in non-isolated contexts, so keep per-rep work above ~5 ms), thermal throttling on long runs, or other processes on the machine.

A warning about editing the page templates

Page scripts are embedded in JS template literals. A backtick anywhere inside them — including inside a comment — terminates the template and produces a syntax error reported at a confusing line. This happened twice while building this module. Write the (a[i] || 0) form, never `a[i] || 0`, inside a page template.