analyze-wat

name	analyze-wat
description	Compile a source file to WAT and analyze the output for codegen inefficiencies. Use after a codegen change to verify the output is clean, when investigating performance issues, or when creating optimization issues from real compiler output.

Analyze WAT Output

Compile a source file to WAT and analyze the output for codegen inefficiencies.

When to use

• After implementing a codegen change, to verify the output is clean
• When investigating performance issues
• When creating optimization issues from real compiler output

Steps

1. Compile the source to WAT:

node --input-type=module -e "
import { compile } from './scripts/compiler-bundle.mjs';
import fs from 'fs';
const src = fs.readFileSync('SOURCE_FILE', 'utf8');
const result = compile(src, { fileName: 'test.ts', emitWat: true });
if (!result.success) { console.log('ERRORS:', result.errors.slice(0,5).map(e => e.message).join('\n')); process.exit(1); }
fs.writeFileSync('/tmp/output.wat', result.wat);
console.log('WAT:', result.wat.length, 'chars | Binary:', result.binary.length, 'bytes');
"

2. Check for known inefficiency patterns:

Pattern	How to detect	Severity
Duplicate locals	Same (local $name appearing twice with different types	Medium — bloats binary
Infinity guard on constant modulo	f64.abs + f64.const Infinity + f64.eq near f64.div/f64.trunc	High — 10 extra instructions per %
Dead drops	local.tee N followed later by drop with no intervening use	Medium — wasted stack ops
TDZ flags unused	(local $__tdz_ that are set to 1 but never checked	Low — extra locals
String concat chains	Multiple consecutive call $concat_import	Medium — N-1 intermediate strings
If-chain → br_table candidate	Same local.get + f64.const N + f64.eq + if repeated 3+ times	High — O(n) vs O(1)
f64↔i32 roundtrip	f64.convert_i32_s immediately followed by i32.trunc_sat_f64_s	High — pure waste
Redundant null guards	ref.is_null + if on provably non-null values (literals, new, struct.new)	Medium — unnecessary branches

3. For each pattern found, note:

   • Source line that produced it
   • WAT line numbers
   • How many instructions could be saved
   • Which codegen file to fix

4. Create or update issues in plan/issues/ citing specific source code and WAT output.

Example

# Analyze the default playground example
node --input-type=module -e "
import { compile } from './scripts/compiler-bundle.mjs';
import fs from 'fs';
const src = fs.readFileSync('playground/examples/dom/calendar.ts', 'utf8');
const result = compile(src, { fileName: 'calendar.ts', emitWat: true });
fs.writeFileSync('/tmp/calendar.wat', result.wat);
console.log('Binary:', result.binary.length, 'bytes');
"
# Then inspect: grep -n 'f64.const Infinity' /tmp/calendar.wat