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V8 has optimized new JavaScript language features (2018)
11–20 of 53 posts
Re: V8 has optimized new JavaScript language features (2018)
#12> const performs a lot better in optimized code than var or let This puzzles me; if only ever one value is assigned, I would have expected at least let to perform identically to const in optimised code, because I expect the optimiser to look at the let and say “never reassigned, turn it into a const ”. By the sound of it, I’m wrong, and I’d be interested to know why I’m wrong.
JavaScript is very dynamic. Here I define a variable with `let` and then change it: let foo = 10; eval("fo"+"o = 10"); I could "obfuscate" that eval assignment as much as I like. So you can't completely statically analyse `let` variables. That said, it's likely you'd end up with perf very close to `const` in a very "hot" part of your code, since a good JIT compiler like V8 will eventually make "assumptions" about you…
An interesting point, however direct "eval" is a special case.
It's already known that functions containing direct "eval" are not subject to the same level of performance optimisations as other functions. There is no way to obscure the call to direct "eval" itself; the compiler knows clearly whether it occurs.
Without "eval" appearing syntactically inside a function's scope, there is no dynamic access to "let" variables, and there's no need for the JIT code to check the assumption at run time.
Despite no other assignments, the "let" variable does change value: It has the assigned value after the "let", and the "temporal dead zone" value before it in the same scope. However "const" also has this property so it's not obvious why there would be a speed difference.
Re: V8 has optimized new JavaScript language features (2018)
#13Buried in there is that we finally get named capture groups on regular expressions. What a day!
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Re: V8 has optimized new JavaScript language features (2018)
#14Buried in there is that we finally get named capture groups on regular expressions. What a day!
2018, so it's been a couple years.
Re: V8 has optimized new JavaScript language features (2018)
#15Earlier quoted context omitted.
JavaScript is very dynamic. Here I define a variable with `let` and then change it: let foo = 10; eval("fo"+"o = 10"); I could "obfuscate" that eval assignment as much as I like. So you can't completely statically analyse `let` variables. That said, it's likely you'd end up with perf very close to `const` in a very "hot" part of your code, since a good JIT compiler like V8 will eventually make "assumptions" about you…
An interesting point, however direct "eval" is a special case. It's already known that functions containing direct "eval" are not subject to the same level of performance optimisations as other functions. There is no way to obscure the call to direct "eval" itself; the compiler knows clearly whether it occurs. Without "eval" appearing syntactically inside a function's scope, there is no dynamic access to "let" variab…
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Re: V8 has optimized new JavaScript language features (2018)
#16This should be (2018).
Added. Thanks!
Re: V8 has optimized new JavaScript language features (2018)
#17> const performs a lot better in optimized code than var or let This puzzles me; if only ever one value is assigned, I would have expected at least let to perform identically to const in optimised code, because I expect the optimiser to look at the let and say “never reassigned, turn it into a const ”. By the sound of it, I’m wrong, and I’d be interested to know why I’m wrong.
The const keyword also guarantees that once a value is assigned to its slot it won't change in the future. As a result TurboFan skips loading and checking const slot values slots each time they are accessed (Function Context Specialization) https://github.com/thlorenz/v8-perf/blob/master/language-fea...
I think you may have misread my comment?
Re: V8 has optimized new JavaScript language features (2018)
#18> const performs a lot better in optimized code than var or let This puzzles me; if only ever one value is assigned, I would have expected at least let to perform identically to const in optimised code, because I expect the optimiser to look at the let and say “never reassigned, turn it into a const ”. By the sound of it, I’m wrong, and I’d be interested to know why I’m wrong.
The const keyword also guarantees that once a value is assigned to its slot it won't change in the future. As a result TurboFan skips loading and checking const slot values slots each time they are accessed (Function Context Specialization) https://github.com/thlorenz/v8-perf/blob/master/language-fea...
Right... but the question was 'why can't the same thing be done with let'.
The answer is probably a combination of 'they haven't gotten around to it yet', 'they don't see the need', 'they don't want the complexity', and 'they don't want to spend compile time doing that.'
Re: V8 has optimized new JavaScript language features (2018)
#19In stark contrast: "Using const/let instead of var can make JavaScript code run 10× slower in Webkit" ;P.
Re: V8 has optimized new JavaScript language features (2018)
#20In stark contrast: "Using const/let instead of var can make JavaScript code run 10× slower in Webkit" ;P. https://news.ycombinator.com/item?id=24844353
I know you jest, but I don't think that's a good example of "contrast" between the engines. That was just a bug/oversight and fixed in a couple of weeks after it was reported [1].