bug#3745: 23.0.95; emacs-23.0.95: unibyte-display-via-language-environment

[Kenichi Handa]

In article <87bpo13ks8.fsf@stupidchicken.com>, Chong Yidong 
<cyd@stupidchicken.com> writes:

> > Now `charset_unibyte' is always 0 (i.e. the same as `charset_ascii').
> Is this variable obsolete, then?

Yes, at the moment.  But, I'd like to use it for
unibyte-display-via-language-environment.

In article <87y6r560y3.fsf@stupidchicken.com>, Chong Yidong 
<cyd@stupidchicken.com> writes:

> > Now `charset_unibyte' is always 0 (i.e. the same as
> > `charset_ascii').  So, unibyte->multibyte conversion always
> > results in an eight-bit multibyte character.

> Looking through the code, I see that the variable `charset_unibyte' is
> not initialized properly.  That's the only reason it's 0.  We have to
> fix this for sure.

Yes.

> > To fix the above problem, I propose these changes for 23.1
> > and the trunk.
> >
> > (1) Fix all codes accessing charset_unibyte
> > (e.g. Funibyte_char_to_multibyte) not to refer to it.

> Can we use charset_iso_8859_1 instead of charset_unibyte, or add a line
> that says

>   charset_unibyte
>     = define_charset_internal (...);

> in syms_of_charset?

No.  Stefan's change was to make unibyte-char-to-multibyte
(and unibyte_char_to_multibyte) always returning an 8-bit
char for an 8-bit byte.  To do that, charset_unibyte must be
the same as charset_ascii, but, first of all, we don't have
to use charset_unibyte in such an operation.  We can simply
use BYTE8_TO_CHAR.

> > (2) Setup charset_unibyte correctly in Fset_charset_priority.
> >
> > (3) Fix x_produce_glyphs to do DECODE_CHAR (charset_unibyte,
> >     it->c) instead of unibyte_char_to_multibyte (it->c).

> Number 3 is not a trivial change.  IIUC, unibyte_char_to_multibyte is
> very fast.  Changing it to use DECODE_CHAR may lead to a performance
> hit.

But, using unibyte_char_to_multibyte here is a clear bug.
If the overhead by DECODE_CHAR is untolerable (I don't
believe it), we can do this:

(1) modify unibyte_char_to_multibyte to use BYTE8_TO_CHAR
    instead of the table unibyte_to_multibyte_table.
(2) Setup unibyte_to_multibyte_table for unibyte_charset.
(3) Just lookup that table in x_produce_glyphs.

---
Kenichi Handa
handa@m17n.org