Re: reducing equality tests in displaying text

[YAMAMOTO Mitsuharu]

>>>>> On Tue, 27 Jan 2009 14:26:42 +0900, Kenichi Handa <address@hidden> said:

>> 1. Each call of assoc_no_quit (charset,_encoding_charset_alist) in
>> fontset.c can be replaced with assq_no_quit because
>> Vfont_encoding_charset_alist is a symbol-keyed alist.

> Yes, just fixed.

There's another occurrence in font_for_char.

>> 2. Also in fontset.c:

>> If `font_group' becomes Qnil in line 473, `from' and `to' are not
>> used at all.  So the most of the task of char_table_ref_and_range
>> is wasted for this case.  Because font_group == Qnil case happens
>> frequently, the char_table_ref_and_range call should be avoided for
>> this case.

> 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    
        53.5 ms emacs   x_produce_glyphs        
        49.4 ms emacs   fontset_find_font       
        36.3 ms emacs   char_table_ref  
        31.3 ms libXft.2.dylib  XftGlyphExtents 
        31.2 ms emacs   get_next_display_element        
        29.2 ms emacs   face_for_char   
        28.4 ms emacs   display_count_lines     
        28.4 ms mach_kernel     ml_set_interrupts_enabled       
        28.2 ms emacs   hash_lookup     
        24.2 ms emacs   assq_no_quit    
        24.2 ms emacs   sub_char_table_ref      
        23.3 ms emacs   validate_interval_range 
        23.2 ms emacs   sort_overlays   
        22.3 ms libXft.2.dylib  XftFontCheckGlyph       
        22.2 ms emacs   move_it_in_display_line_to      
        21.3 ms emacs   xftfont_text_extents    
        20.3 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.

                             YAMAMOTO Mitsuharu
                        address@hidden