AOT: int.toDouble() in a hot loop is 5x slower than the same conversion written as 0.0 + x #64209

import 'dart:typed_data';

const n = 1024;
const iters = 200000;

double explicitConversion(Uint8List a) {
  var acc = 0.0;
  for (var i = 0; i < a.length; i++) {
    acc += a[i].toDouble();
  }
  return acc;
}

double implicitPromotion(Uint8List a) {
  var acc = 0.0;
  for (var i = 0; i < a.length; i++) {
    acc += a[i];
  }
  return acc;
}

double simdExplicit(Uint8List a) {
  var acc = Float32x4.zero();
  for (var i = 0; i < a.length; i += 4) {
    acc += Float32x4(
      a[i].toDouble(),
      a[i + 1].toDouble(),
      a[i + 2].toDouble(),
      a[i + 3].toDouble(),
    );
  }
  return acc.x + acc.y + acc.z + acc.w;
}

double simdPromoted(Uint8List a) {
  var acc = Float32x4.zero();
  for (var i = 0; i < a.length; i += 4) {
    acc += Float32x4(
      0.0 + a[i],
      0.0 + a[i + 1],
      0.0 + a[i + 2],
      0.0 + a[i + 3],
    );
  }
  return acc.x + acc.y + acc.z + acc.w;
}

void main() {
  final a = Uint8List(n);
  for (var i = 0; i < n; i++) {
    a[i] = (i * 37) & 0xff;
  }
  // All four compute the same sum.
  print(
    'results: ${explicitConversion(a)} ${implicitPromotion(a)} '
    '${simdExplicit(a)} ${simdPromoted(a)}',
  );

  var sink = 0.0;
  void bench(String label, double Function() f) {
    var best = 1e9;
    for (var r = 0; r < 6; r++) {
      final sw = Stopwatch()..start();
      for (var i = 0; i < iters; i++) {
        sink += f();
      }
      sw.stop();
      final dt = sw.elapsedMicroseconds / 1e6;
      if (dt < best) best = dt;
    }
    print(
      '${label.padRight(30)}${(best * 1000).toStringAsFixed(1).padLeft(7)} ms',
    );
  }

  bench('acc += x.toDouble()', () => explicitConversion(a));
  bench('acc += x', () => implicitPromotion(a));
  bench('Float32x4(x.toDouble(), ...)', () => simdExplicit(a));
  bench('Float32x4(0.0 + x, ...)', () => simdPromoted(a));
  if (sink == -1) print('never');
}