Re: pure-fns in byte-opt.el

[Mark Oteiza <mvoteiza@udel.edu>]

On 29/07/17 at 10:03pm, Andreas Schwab wrote:
>On Jul 29 2017, Mark Oteiza <mvoteiza@udel.edu> wrote:
>
>> On 29/07/17 at 08:22pm, Eli Zaretskii wrote:
>>>> From: Mark Oteiza <mvoteiza@udel.edu>
>>>> Date: Sat, 29 Jul 2017 12:43:51 -0400
>>>> Cc: emacs-devel@gnu.org
>>>>
>>>> Stefan Monnier <monnier@iro.umontreal.ca> writes:
>>>>
>>>> >> (let ((pure-fns
>>>> >> -       '(concat symbol-name regexp-opt regexp-quote string-to-syntax)))
>>>> >> +       '(concat symbol-name regexp-opt regexp-quote string-to-syntax
>>>> >> +         make-vector)))
>>>> >
>>>> > Ah, now that makes a lot more sense: make-vector is much less pure than
>>>> > string-to-char.  The above will cause the compiler to replace
>>>> >
>>>> >    (make-vector 2 ?a)
>>>> >
>>>> > with
>>>> >
>>>> >    [?a ?a]
>>>> >
>>>> > so you end with a single immediate vector being re-used over and over,
>>>> > instead of having a new vector created each time.
>>>>
>>>> But reading a literal vector still generates a new vector, no?
>>>
>>>Why do you assume it will be read?
>>
>> I guess because I don't understand how bytecode is executed.
>
>This has nothing to do with bytecode.  Try this:
>
>(let ((i 0) r)
>  (while (< i 2)
>    (let ((v (make-vector 2 0)))
>      (aset v i i)
>      (push v r))
>    (setq i (1+ i)))
>  r)

I see.  Thanks Andreas.