Re: reducing equality tests in displaying text

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

>>>>> On Wed, 28 Jan 2009 16:03:01 +0900, Kenichi Handa <handa@m17n.org> 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
				mituharu@math.s.chiba-u.ac.jp