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bug#56347: Optimize/simplify STRING_SET_MULTIBYTE
From: |
Stefan Monnier |
Subject: |
bug#56347: Optimize/simplify STRING_SET_MULTIBYTE |
Date: |
Sat, 02 Jul 2022 12:12:06 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux) |
>> The patch below simplifies code around STRING_SET_MULTIBYTE.
>> Any objection?
> Rationale?
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).
>> --- a/src/composite.c
>> +++ b/src/composite.c
>> @@ -1879,11 +1879,7 @@ Otherwise (for terminal display), FONT-OBJECT must be
>> a terminal ID, a
>> for (i = SBYTES (string) - 1; i >= 0; i--)
>> if (!ASCII_CHAR_P (SREF (string, i)))
>> error ("Attempt to shape unibyte text");
>> - /* 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.
> 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". ]
Where? AFAICT `string` is only used in the subsequent code by passing
it to `fill_gstring_header` and that function only passes that arg to
`fetch_string_char_advance_no_check` and that function only looks at the
string's SDATA, so as long as the sequence of bytes is consistent with
a multibyte string (which we just checked with the ASCII_CHAR_P loop),
I don't see any problem.
>> --- a/src/lisp.h
>> +++ b/src/lisp.h
>> @@ -1637,12 +1637,10 @@ #define STRING_SET_UNIBYTE(STR)
>> \
>>
>> /* Mark STR as a multibyte string. Assure that STR contains only
>> ASCII characters in advance. */
>> -#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.
Stefan
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Stefan Monnier, 2022/07/01
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Eli Zaretskii, 2022/07/02
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE,
Stefan Monnier <=
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Eli Zaretskii, 2022/07/02
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Stefan Monnier, 2022/07/02
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Eli Zaretskii, 2022/07/02
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Stefan Monnier, 2022/07/02
- bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Eli Zaretskii, 2022/07/02
bug#56347: Optimize/simplify STRING_SET_MULTIBYTE, Stefan Monnier, 2022/07/02