lib: avoid quadratic shift() in startup snapshot callback

[watilde]

Micro benchmark

$ cat bench-startup-snapshot-callbacks.js
'use strict';

function runWithShift(callbacks) {
  while (callbacks.length > 0) {
    const { 0: cb, 1: data } = callbacks.shift();
    cb(data);
  }
}

function runWithIndex(callbacks) {
  for (let i = 0; i < callbacks.length; i++) {
    const { 0: cb, 1: data } = callbacks[i];
    cb(data);
  }
  callbacks.length = 0;
}

function makeCallbacks(count) {
  return Array.from({ length: count }, (_, j) => [() => {}, j]);
}

function measure(fn, count, iterations) {
  // warmup
  for (let i = 0; i < 50; i++) fn(makeCallbacks(count));

  const start = performance.now();
  for (let i = 0; i < iterations; i++) fn(makeCallbacks(count));
  return performance.now() - start;
}

const COUNTS = [10, 100, 1_000, 10_000, 100_000];

console.log('--- shift() vs for+length=0 ---\n');

for (const count of COUNTS) {
  const iters = Math.max(10, Math.floor(1_000_000 / count));
  const shiftMs = measure(runWithShift, count, iters);
  const indexMs = measure(runWithIndex, count, iters);
  const ratio = shiftMs / indexMs;

  const countStr = `count=${String(count).padStart(6)}`;
  const shiftStr = `${shiftMs.toFixed(2).padStart(10)}ms`;
  const indexStr = `${indexMs.toFixed(2).padStart(8)}ms`;
  const ratioStr = `${ratio.toFixed(ratio >= 10 ? 0 : 1).padStart(5)}x faster`;

  console.log(`${countStr}  ${shiftStr}  vs  ${indexStr}   => ${ratioStr}`);
}

Result

$ ./node bench-startup-snapshot-callbacks.js
--- shift() vs for+length=0 ---

count=    10       58.22ms  vs     49.66ms   =>   1.2x faster
count=   100       42.62ms  vs     32.54ms   =>   1.3x faster
count=  1000       53.96ms  vs     30.92ms   =>   1.7x faster
count= 10000       60.50ms  vs     28.47ms   =>   2.1x faster
count=100000    46747.20ms  vs     50.64ms   =>   923x faster

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.57%. Comparing base (d44a71a) to head (100d584).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62914      +/-   ##
==========================================
- Coverage   89.61%   89.57%   -0.04%     
==========================================
  Files         706      704       -2     
  Lines      219203   218122    -1081     
  Branches    41995    41849     -146     
==========================================
- Hits       196445   195393    -1052     
- Misses      14663    14706      +43     
+ Partials     8095     8023      -72     

Files with missing lines	Coverage Δ
lib/internal/v8/startup_snapshot.js	95.48% <100.00%> (+0.10%)	⬆️

... and 125 files with indirect coverage changes

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