Re: reducing equality tests in displaying text

[YAMAMOTO Mitsuharu]

>>>>> On Wed, 28 Jan 2009 16:03:01 +0900, Kenichi Handa <address@hidden> said:

>> > When font_group is Qnil, in most cases, SUB_CHAR_TABLE_P (table) is
>> > zero.  Thus the calculation of FROM and TO must not be that slow.
>> > Did you really see the bottle-neck of the performance here?

>> Below is a part of the result of an experiment (after your change)
>> with Shark.app on Mac OS X 10.5.6. I scrolled up and down the Japanese
>> tutorial.  Emacs is compiled with Xft support and the default CFLAGS
>> (-g -O2 -Wno-pointer-sign).

>> 957.8 ms     emacs   mark_object     
>> 194.3 ms     emacs   mark_vectorlike 
>> 147.9 ms     emacs   Fgarbage_collect        
>> 145.3 ms     libfreetype.6.dylib     tt_cmap4_char_map_binary        
>> 108.9 ms     emacs   char_table_ref_and_range        
>> 98.8 ms      emacs   sub_char_table_ref_and_range    

> Are you sure that those calls mostly return nil?  Could you
> please check if this patch surely improve the performance?

Here is the result.

        750.9 ms        emacs   mark_object     
        153.4 ms        libfreetype.6.dylib     tt_cmap4_char_map_binary        
        126.7 ms        emacs   Fgarbage_collect        
        106.8 ms        emacs   mark_vectorlike 
        64.8 ms emacs   x_produce_glyphs        
        61.5 ms emacs   char_table_ref  
        47.5 ms emacs   fontset_find_font       
        33.3 ms emacs   get_next_display_element        
        30.3 ms emacs   face_for_char   
        29.3 ms libXft.2.dylib  XftGlyphExtents 
        29.3 ms emacs   assq_no_quit    
        29.3 ms emacs   find_interval   
        27.3 ms emacs   hash_lookup     
        26.3 ms emacs   make_uninit_multibyte_string    
        25.2 ms emacs   sub_char_table_ref      
        24.4 ms mach_kernel     ml_set_interrupts_enabled       
        23.2 ms emacs   next_element_from_buffer        
        22.3 ms emacs   display_count_lines     
        20.2 ms emacs   Fcons   
        19.2 ms libXft.2.dylib  XftCharIndex    

>>>> 3. Despite its argument name, `has_char' functions in font backend
>>>> drivers are mostly called for a font object instead of a font
>>>> entity.  Some font backend drivers could make use of this fact:
>>>> e.g., ftfont_has_char can avoid frequent assoc_no_quit calls if
>>>> struct ft_font has a reference to fc_charset in ft_face_cache.

>> > I've just fixed ftfont_has_char to use FT_Get_Char_Index directly.

>> But FT_Get_Char_Index is much slower than FcCharSetHasChar.
>> `tt_cmap4_char_map_binary' in the above table is called from
>> FT_Get_Char_Index.

> Hmmm.  I've just found that Xft has the function
> XftCharExists now.  I remember that it didn't exist in a
> rather old vesion.  Does your Xft library have this
> function?

It exists.  But it simply uses the `charset' member in struct _XftFont:

_X_EXPORT FcBool
XftCharExists (Display      *dpy,
               XftFont      *pub,
               FcChar32    ucs4)
{
    if (pub->charset)
        return FcCharSetHasChar (pub->charset, ucs4);
    return FcFalse;
}

Is there any reason you prefer an Xft-level routine to
fontconfig-level?  By adding some `FcCharSet *' member in struct
ftfont as I said, you don't need to "override" `has_char' function in
the xft driver, and the ftx driver can also benefit from it for free.

                                     YAMAMOTO Mitsuharu
                                address@hidden