Re: [Emacs-diffs] master 81a1088: Tweak builtin symbol order for speed

[Pip Cet <pipcet@gmail.com>]

On Fri, Jul 12, 2019 at 6:07 AM Paul Eggert <eggert@cs.ucla.edu> wrote:
> branch: master
> commit 81a1088ee8b833cd30a3363782195d6c4d575672
> Author: Paul Eggert <eggert@cs.ucla.edu>
> Commit: Paul Eggert <eggert@cs.ucla.edu>
>
>     Tweak builtin symbol order for speed
>
>     * lib-src/make-docfile.c (compare_globals):
>     Make symbols 1 through 4 be t, unbound, error, lambda.
>     This is in addition to symbol 0 being nil.
>     This change improved ‘make compile-always’ performance by 0.6%
>     on my platform.

Hmm. I don't understand why this would improve performance this
significantly (maybe because it allows 8-bit offsets to be used rather
than 32-bit ones?). It will make debugging a tad easier, though.
XIL(0x30) for Qt is easy to remember.

Maybe it would be a good idea to make Vdead a symbol (or a tagged NULL
pointer) and treat it similarly, if this actually improves
performance.

> --->  lib-src/make-docfile.c | 21 ++++++++++++++++-----
>  1 file changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/lib-src/make-docfile.c b/lib-src/make-docfile.c
> index 4d25b0a..368150b 100644
> --- a/lib-src/make-docfile.c
> +++ b/lib-src/make-docfile.c
> @@ -641,13 +641,24 @@ compare_globals (const void *a, const void *b)
>      return ga->type - gb->type;
>
>    /* Consider "nil" to be the least, so that iQnil is zero.  That
> -     way, Qnil's internal representation is zero, which is a bit faster.  */
> +     way , Qnil's internal representation is zero, which is a bit faster.
> +     Similarly, consideer "t" to be the second-least, and so forth.  */

"consider"

>    if (ga->type == SYMBOL)
>      {
> -      bool a_nil = strcmp (ga->name, "Qnil") == 0;
> -      bool b_nil = strcmp (gb->name, "Qnil") == 0;
> -      if (a_nil | b_nil)
> -       return b_nil - a_nil;
> +      /* Common symbols in decreasing popularity order.  */
> +      static char const commonsym[][8]

I'm curious, why isn't this simply "static char *commonsym[]"?