On 10/6/14, 3:53 AM, Cameron Zwarich wrote:
Where do most of the small strings larger than a single character that benefit
from the inline small string optimization originate, the DOM or user JS code?
That's a good question.
When the optimization was added, strings that originated in the DOM
never ended up using the inline small string optimization (because they
ended up as "external strings" instead).
Even now, strings returned via nsAString out params (which is most APIs)
end up as "external string".
The one case when DOM-provided strings get inlined on the JS side is
when we're using a DOMString out param (which a small number of DOM
methods use). See <https://bugzilla.mozilla.org/show_bug.cgi?id=934544>.
None of which really answers your question, since it depends on the call
frequencies of DOM methods. But I expect that JS-originating strings
dominate here. Could be measured.
Note that "short" in this case is 11 utf-16 code points for non-latin1
strings and 23 bytes for latin-1 strings, which actually covers a
surprising number of strings that occur in real life.
Thanks for the reference. That function takes a char16_t*, but now that they’ve
done the Latin1 optimization, it seems like it should be possible to also make
a Latin1 variant.
Absolutely. https://bugzilla.mozilla.org/show_bug.cgi?id=1038099 is
related but distinct; worth filing an explicit bug on a latin1 external
string version.
There is an interesting question of whether it will ever be a good idea for a
JS engine to make something like WTF-8 its native string type, or if the
difficulties with random access outweigh the advantages. It seems like a
daunting task to do that at this point for Gecko / SM, even if it would work in
theory.
Yeah, coevolving Gecko and SpiderMonkey here is a pain without
boil-the-ocean kind of patches. :(
That said, we've considered doing it; the hard part would be avoiding
performance problems during the (probably longish) transition period
when conversions have to happen some places.
-Boris
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