Re: reducing equality tests in displaying text

[YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>]

>>>>> On Tue, 27 Jan 2009 14:26:42 +0900, Kenichi Handa <handa@m17n.org> 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
			mituharu@math.s.chiba-u.ac.jp