Strange deoptimzations in cycles

[vsemozhetbyt]

• Version: 7.1.0
• Platform: Windows 7 x64
• Subsystem: v8

The first iteration in cycles runs much faster than others:

[1, 2, 3, 4].forEach(() => {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
});

544
1297
1294
1301

It renews in another cycle with a different ratio:

[1, 2, 3, 4].forEach(() => {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
});

console.log('');

for (let j = 1; j <= 4; j++) {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

577
1312
1314
1298

437
1501
1501
1493

It becomes more strange with while cycle added in the end (this cycle removes the difference in the previous cycle, but it almost doubles its run time besides):

[1, 2, 3, 4].forEach(() => {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
});

console.log('');

for (let j = 1; j <= 4; j++) {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

console.log('');

let k = 4;
while (k--){
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

566
1317
1333
1289

2221
2448
2448
2507

2147
2360
2332
2427

[vsemozhetbyt]

If the while cycle comes first, the difference remains in all the cycles and a run time is not doubled in any of them:

let k = 4;
while (k--){
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

console.log('');

[1, 2, 3, 4].forEach(() => {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
});

console.log('');

for (let j = 1; j <= 4; j++) {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

492
1483
1482
1439

533
1293
1271
1300

440
1420
1423
1437

[vsemozhetbyt]

With non-empty functions that return different values, the difference remains (with less ratio, though):

let k = 4;
while (k--){
  const func = () => { return Math.random(); };
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

console.log('');

[1, 2, 3, 4].forEach(() => {
  const func = () => { return Math.random(); };
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
});

console.log('');

for (let j = 1; j <= 4; j++) {
  const func = () => { return Math.random(); };
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

1599
2844
2839
2838

1423
2688
2675
2690

1658
2832
2813
2829

[vsemozhetbyt]

However, if the function declaration is removed from cycles, the difference disappears:

const func = () => {};

let k = 4;
while (k--){
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

console.log('');

[1, 2, 3, 4].forEach(() => {
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
});

console.log('');

for (let j = 1; j <= 4; j++) {
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

483
455
436
455

452
438
448
430

465
455
436
448

At the same time, the "while cycle at the last position increases own and previous run times" issue remains and gains strength:

const func = () => {};

[1, 2, 3, 4].forEach(() => {
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
});

console.log('');

for (let j = 1; j <= 4; j++) {
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

console.log('');

let k = 4;
while (k--){
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

482
446
444
451

2268
2285
2265
2339

2422
2544
2511
2489

[vsemozhetbyt]

And some more while cycle weirdness:

1. Even empty while cycle with one iteration has the described impact:

let k = 1;
while (k--){}

for (let j = 1; j <= 4; j++) {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

505
1438
1423
1407

for (let j = 1; j <= 4; j++) {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

let k = 1;
while (k--){}

2333
2512
2495
2506

2. The leading while overcomes the trailing one:

let k = 1;
while (k--){}

for (let j = 1; j <= 4; j++) {
  const func = () => {};
  const start = Date.now();
  for (let i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

k = 1;
while (k--){}

484
1453
1394
1409

[vsemozhetbyt]

In the ES5 code the difference ratio is bigger:

var func, start, j, i;

for (j = 1; j <= 4; j++) {
  func = function () {};
  start = Date.now();
  for (i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

140
904
913
894

However, the while weirdness disappears:

var func, start, j, i, k;

for (j = 1; j <= 4; j++) {
  func = function () {};
  start = Date.now();
  for (i = 0; i < 100000000; i++) func();
  console.log(Date.now() - start);
}

k = 1;
while (k--){}

139
915
925
904

[vsemozhetbyt]

Issue distribution:

Node.js v.	v0.10.48	v0.12.17	v4.6.2	v5.12.0	v6.9.1	v8.0.0-nightly-20161121	v7.0.0-nightly-20161121 chakracore
JavaScript v.	ES5	ES5	ES6	ES6	ES6	ES6	ES6
Iteration 1	1328	109	437	437	531	453	204
Iteration 2	1326	749	1295	1295	1518	1450	205
Iteration 3	639	717	1283	1310	1500	1451	200
Iteration 4	640	734	1279	1311	1482	1466	199
while weirdness	–	–	+	+	1406 2215 1280 1279	+	–

[sam-github]

This is a pure V8 question, or possibly bug report, should be reported upstream to theml

[vsemozhetbyt]

Reported.

[bmeurer]

This is expected. We first need warm-up and only tier-up once we have a couple of cycles in the relevant function. Also note that there's only one deoptimization in forEach, and that doesn't really affect the outcome.

[bnoordhuis]

In your first example, the console.log(Date.now() - start) triggers a deopt because apparently V8 doesn't infer the return type of Date.now() correctly ("Insufficient type feedback for LHS of binary operation.")

When you remove it, it then deopts because of the const func = () => {} statement. If you move that out of the function, it stays optimized. I don't understand why that should be the case, though.

The let in for (let i = ...) slows it down quite a bit. If you replace it with var, it's about 4x faster.

Here is a revised version that has stable performance characteristics.

let start;
function before() { start = Date.now(); }
function after() { console.log(Date.now() - start); }

const func = () => {};
[1, 2, 3, 4].forEach(() => {
  before();
  for (var i = 0; i < 1e8; i++) func();
  after();
});

[vsemozhetbyt]

@bmeurer, @bnoordhuis Thank you!

However, how would you explain this remaining while weirdness:

const func = () => {};
const start = Date.now();

for (var j = 1; j <= 4; j++) {
  for (var i = 0; i < 1e8; i++) func();
}

console.log(Date.now() - start);

490

const func = () => {};
const start = Date.now();

for (var j = 1; j <= 4; j++) {
  for (var i = 0; i < 1e8; i++) func();
}

console.log(Date.now() - start);

let k = 1;
while (k--){}

5550