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bug#56347: Optimize/simplify STRING_SET_MULTIBYTE
From: |
Eli Zaretskii |
Subject: |
bug#56347: Optimize/simplify STRING_SET_MULTIBYTE |
Date: |
Sat, 02 Jul 2022 19:24:02 +0300 |
> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: 56347@debbugs.gnu.org
> Date: Sat, 02 Jul 2022 12:12:06 -0400
>
> STRING_SET_MULTIBYTE is fundamentally evil because it changes the nature
> of an object. Its current definition (like that of STRING_SET_UNIBYTE)
> is rather scary (it sometimes changes the nature of the arg passed to
> it, and sometimes replaces the arg with something else).
But do we have any alternatives?
> >> - /* STRING is a pure-ASCII string, so we can convert it (or,
> >> - rather, its copy) to multibyte and use that thereafter. */
> >> - Lisp_Object string_copy = Fconcat (1, &string);
> >> - STRING_SET_MULTIBYTE (string_copy);
> >> - string = string_copy;
> >> + /* STRING is a pure-ASCII string, so we can treat it as multibyte. */
> >
> > Did you actually try your change in the situations where this problem
> > pops up?
>
> I don't even know how to go about doing that, no.
Make a character-composition rule that composes, say, two '-'
characters, and then display a buffer where you have adjacent dashes.
> > AFAIR, the code makes a copy of the string for good reasons:
> > the rest of handling of the string down the line barfs if we keep a
> > multibyte string here.
>
> [ I assume you meant "barfs if we keep a *uni*byte string here". ]
Yes.
> Where?
I don't remember, sorry.
> >> -#define STRING_SET_MULTIBYTE(STR) \
> >> - do { \
> >> - if (XSTRING (STR)->u.s.size == 0) \
> >> - (STR) = empty_multibyte_string; \
> >> - else \
> >> - XSTRING (STR)->u.s.size_byte = XSTRING (STR)->u.s.size; \
> >> +#define STRING_SET_MULTIBYTE(STR) \
> >> + do { \
> >> + eassert (XSTRING (STR)->u.s.size > 0); \
> >> + XSTRING (STR)->u.s.size_byte = XSTRING (STR)->u.s.size; \
> >> } while (false)
> >>
> >> /* Convenience functions for dealing with Lisp strings. */
> >
> > You want to disallow uses of empty_multibyte_string? why?
>
> No, I want to reduce the scope of semantics of the macro, e.g. so it can
> be implemented as a function rather than a macro and so it doesn't
> magically substitute empty_multibyte_string into a variable that held
> something else.
But the effect is that you disallow calling STRING_SET_MULTIBYTE on an
empty string, isn't it?
bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Stefan Monnier, 2022/07/02