[poky] About the operator "??="

[Xu, Dongxiao]

Hi Richard,

Recently when investigating the file parsing speed, I found the implementation of "??=" is another hot spot. This operator will not apply the default assignment until the end of the parse, which is different from "?=". According to current poky, the users for "??=" is mostly the scm revisions stored in poky-default-revisions.inc.

Currently the final assignment logic for "??=" is added in finalize(), which costs about 20% parsing time. 

The differences between "??=" and "?=" are straight forward, however I don't fully understand in which senario the "??=" should be used. Or in other word, why those scm revisions should use "??=". Could you help to clarify a bit? Thanks so much!

BTW, I noticed that openembedded doesn't use this operator.

Best Regards,
-- Dongxiao