Performance impact of let only code

[Trevor Norris]

There is one specific case where use of let has a noticeable performance impact, and where the work around, instead of just using var, is not pretty. Example:

'use strict';
const ITER = 1e7;


let t = performance.now();

// Use of "let" in for loop.
for (let i = 0; i < ITER; i++);
t = performance.now() - t;
print(t);


t = performance.now();
// Use of "var" in for loop.
for (var i = 0; i < ITER; i++);
t = performance.now() - t;
print(t);


// Output:
// 486.324
// 28.25

Those results are for d8 4.2.77.6 on a x64.release Linux build. To work around this I need to hoist the variable declaration, or to preserve scoping need to do something ridiculous like so:

{
  let i = 0;
  for (; i < ITER; i++);
}

The point is, if var is going to be completely removed then use of let in tight for loops like what's shown above need to get faster.

[Dmitry Lomov]

Thanks Trevor, I am working as we speak to make this faster.

The dirty secret is that in current V8 all 'let' and 'const' variables are actually context-allocated, not stack-allocated.

This will be fixed very soon though.

Cheers,

Dmitry

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[Trevor Norris]

On Wednesday, March 11, 2015 at 3:29:57 PM UTC-6, Dmitry Lomov wrote:

Thanks Trevor, I am working as we speak to make this faster.

The dirty secret is that in current V8 all 'let' and 'const' variables are actually context-allocated, not stack-allocated.

This will be fixed very soon though.

That's great to hear. Thanks for letting me know.

[Si Robertson]

The dirty secret is that in current V8 all 'let' and 'const' variables are actually context-allocated, not stack-allocated.

I don't suppose you know of a bug report that we can keep an eye on? I searched through the V8 issues but failed to find anything related to this.

[Trevor Norris]

On Wednesday, March 11, 2015 at 3:29:57 PM UTC-6, Dmitry Lomov wrote:

Thanks Trevor, I am working as we speak to make this faster.

The dirty secret is that in current V8 all 'let' and 'const' variables are actually context-allocated, not stack-allocated.

This will be fixed very soon though.

Hey Dmitry. Just wanted to check in on this. I just ran a test on 4.4.26 and there still seems to be a noticeable performance impact between

for (let i = 0; i < 1e7; i++) { }

and

let i = 0;

for (; i < 1e7; i++) { }

Thanks!

[Dmitry Lomov]

This is the patch: https://codereview.chromium.org/981203003/ - not yet landed.

(Debugger changes in particular turned out to be quite involved). 

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[Dmitry Lomov]

The fix has landed, and Trevor's test shows similar performance (let variation is actually a bit faster)