Byte-compiling and lexical scoping for performance

[Clément Pit--Claudel]

Hi emacs-devel,

The article at http://nullprogram.com/blog/2016/12/11/ is an interesting read:

> Faster Elfeed Search Through JIT Byte-code Compilation
> 
> Today I pushed an update for Elfeed that doubles the speed of the 
> search filter in the worse case. […]
> 
> There’s one last thing to watch out for. Everything so far has been 
> compiled with lexical scope. […]
> 
> Lexical scope has better performance. Here are all the benchmarks
> with the default dynamic scope: […]
> 
> It halves the performance […] Under dynamic scope, local variables
> use the varref opcode — a global variable lookup — instead of the
> stack-ref opcode — a simple array index.

(sending this here and not to -discuss because IIRC we have few examples of 
lexical binding being used for performance — json.el is the only one I remember)

Cheers,
Clément.

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