In syncing the existing blanks to GC=Zw+Default_Ignorable I found the following exception that needs to be added in:

  <int>0x2800</int>	<!-- BRAILLE PATTERN BLANK -->

just wrote the code to generate the table from PropList.txt though, we have some code points in fonts.conf right now, which doesn't match the condition you mentioned. i.e. not (Gen_Cat=Zs plus Default_Ignorable=True).
Should we simply drop them? or was there any historical reason we added them?

Here is the missing code points:

			<int>0x00AD</int>	<!-- SOFT HYPHEN -->
			<int>0x034F</int>	<!-- COMBINING GRAPHEME JOINER -->
			<int>0x061C</int>	<!-- ARABIC LETTER MARK -->
			<int>0x115F</int>	<!-- HANGUL CHOSEONG FILLER -->
			<int>0x1160</int>	<!-- HANGUL JUNGSEONG FILLER -->
			<int>0x17B4</int>	<!-- KHMER VOWEL INHERENT AQ -->
			<int>0x17B5</int>	<!-- KHMER VOWEL INHERENT AA -->
			<int>0x180B</int>	<!-- MONGOLIAN FREE VARIATION SELECTOR ONE -->
			<int>0x180C</int>	<!-- MONGOLIAN FREE VARIATION SELECTOR TWO -->
			<int>0x180D</int>	<!-- MONGOLIAN FREE VARIATION SELECTOR THREE -->
			<int>0x180E</int>	<!-- MONGOLIAN VOWEL SEPARATOR -->
			<int>0x200B</int>	<!-- ZERO WIDTH SPACE -->
			<int>0x200C</int>	<!-- ZERO WIDTH NON-JOINER -->
			<int>0x200D</int>	<!-- ZERO WIDTH JOINER -->
			<int>0x200E</int>	<!-- LEFT-TO-RIGHT MARK -->
			<int>0x200F</int>	<!-- RIGHT-TO-LEFT MARK -->
			<int>0x202A</int>	<!-- LEFT-TO-RIGHT EMBEDDING -->
			<int>0x202B</int>	<!-- RIGHT-TO-LEFT EMBEDDING -->
			<int>0x202C</int>	<!-- POP DIRECTIONAL FORMATTING -->
			<int>0x202D</int>	<!-- LEFT-TO-RIGHT OVERRIDE -->
			<int>0x202E</int>	<!-- RIGHT-TO-LEFT OVERRIDE -->
			<int>0x2060</int>	<!-- WORD JOINER -->
			<int>0x2061</int>	<!-- FUNCTION APPLICATION -->
			<int>0x2062</int>	<!-- INVISIBLE TIMES -->
			<int>0x2063</int>	<!-- INVISIBLE SEPARATOR -->
			<int>0x2064</int>	<!-- INVISIBLE PLUS -->
			<int>0x2066</int>	<!-- LEFT-TO-RIGHT ISOLATE -->
			<int>0x2067</int>	<!-- RIGHT-TO-LEFT ISOLATE -->
			<int>0x2068</int>	<!-- FIRST STRONG ISOLATE -->
			<int>0x2069</int>	<!-- POP DIRECTIONAL ISOLATE -->
			<int>0x206A</int>	<!-- INHIBIT SYMMETRIC SWAPPING -->
			<int>0x206B</int>	<!-- ACTIVATE SYMMETRIC SWAPPING -->
			<int>0x206C</int>	<!-- INHIBIT ARABIC FORM SHAPING -->
			<int>0x206D</int>	<!-- ACTIVATE ARABIC FORM SHAPING -->
			<int>0x206E</int>	<!-- NATIONAL DIGIT SHAPES -->
			<int>0x206F</int>	<!-- NOMINAL DIGIT SHAPES -->
			<int>0x2800</int>	<!-- BRAILLE PATTERN BLANK -->
			<int>0x3164</int>	<!-- HANGUL FILLER -->
			<int>0xFEFF</int>	<!-- ZERO WIDTH NO-BREAK SPACE -->
			<int>0xFFA0</int>	<!-- HALFWIDTH HANGUL FILLER -->
			<int>0x1BCA0</int>	<!-- SHORTHAND FORMAT LETTER OVERLAP -->
			<int>0x1BCA1</int>	<!-- SHORTHAND FORMAT CONTINUING OVERLAP -->
			<int>0x1BCA2</int>	<!-- SHORTHAND FORMAT DOWN STEP -->
			<int>0x1BCA3</int>	<!-- SHORTHAND FORMAT UP STEP -->

Are you sure?  Most of what you list are space or default_ignorable.

(In reply to Behdad Esfahbod from comment #4)
> Are you sure?  Most of what you list are space or default_ignorable.

Unless I misread the format or your intention.. or I should refer another data to determine. in fact ICU seems using http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[%3ADI%3A for the reference to determine the default ignorable though, I can't find the source of the data why they are.

That is somewhat hard to parse html in C though, maybe consider using some script language to generate it at the bootstrap perhaps.

http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[%3AGC%3DZs%3A][%3ADI%3A]&abb=on&ucd=on&esc=on&g=

0020 ;SPACE
00A0 ;NO-BREAK SPACE
00AD ;SOFT HYPHEN
034F ;COMBINING GRAPHEME JOINER
061C ;ARABIC LETTER MARK
115F ;HANGUL CHOSEONG FILLER
1160 ;HANGUL JUNGSEONG FILLER
1680 ;OGHAM SPACE MARK
17B4 ;KHMER VOWEL INHERENT AQ
17B5 ;KHMER VOWEL INHERENT AA
180B..180E ;MONGOLIAN VOWEL SEPARATOR
2000..200F ;RIGHT-TO-LEFT MARK
202A..202F ;NARROW NO-BREAK SPACE
205F..206F ;NOMINAL DIGIT SHAPES
3000 ;IDEOGRAPHIC SPACE
3164 ;HANGUL FILLER
FE00..FE0F ;VARIATION SELECTOR-16
FEFF ;ZERO WIDTH NO-BREAK SPACE
FFA0 ;HALFWIDTH HANGUL FILLER
FFF0..FFF8 ;<unassigned-FFF8>
1BCA0..1BCA3 ;SHORTHAND FORMAT UP STEP
1D173..1D17A ;MUSICAL SYMBOL END PHRASE
E0000..E0FFF ;<unassigned-E0FFF>

As mentioned, the only exception we want to add to this is U+2800 BRAILLE PATTERN BLANK.

Thanks. worked out in http://cgit.freedesktop.org/~tagoh/fontconfig/log/?h=bz79956

Interesting!  I was thinking of manually coding the list...

Why do you special-case U+0020?  I see it here:
http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[%3AGC%3DZs%3A][%3ADI%3A]&abb=on&ucd=on&esc=on&g

Oh, in the regexp, it has "\ " for it.

Did you forget to add the BRAILE exception?

(In reply to Behdad Esfahbod from comment #8)
> Interesting!  I was thinking of manually coding the list...

just wanted to reduce the unnecessary effort to keep it updated and check the changes. I'm lazy person :)

> Why do you special-case U+0020?  I see it here:
> http://unicode.org/cldr/utility/list-unicodeset.
> jsp?a=[%3AGC%3DZs%3A][%3ADI%3A]&abb=on&ucd=on&esc=on&g
> 
> Oh, in the regexp, it has "\ " for it.

Ah! I missed that. have to modify the script to recognize it...

> Did you forget to add the BRAILE exception?

it's there.. I simply forgot to add a comment for that.
I'll fix them.

Updated. works now without adding 0x20.

Thanks.  lgtm.

merged into git master. thanks!