Re: Why is lexical binding so slow?

[Philipp Stephani <p.stephani2@gmail.com>]

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Stefan Monnier <monnier@iro.umontreal.ca> schrieb am So., 2. Juli 2017 um
20:36 Uhr:

> > https://github.com/mooz/js2-mode/issues/426#issuecomment-312506855.
> > Why is that?
>
> Depends on specifics, so someone would have to investigate.


I that case the reason was just a dumb mistake on my part: I ran the
benchmark without byte-compiled files. With byte compilation lexical
binding becomes at least as fast as dynamic binding.


> > Shouldn't code compiled with lexical binding be at least as
> > fast as code compiled with dynamic binding?
>
> Not necessarily, no.  E.g. when you use `mapcar` and the function
> argument has free lexical variables, the lexbind code will have to build
> a closure to pass to mapcar.  The access to the lexical vars in the
> closure will usually be faster than the corresponding access to those
> vars via dynbinding, so if mapcar makes enough calls lexbind can still
> be faster, but if mapcar makes few calls, dynbind is faster.
>
>
Thanks for the explanation.

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