Status: Accepted
Owner: [email protected]
CC: [email protected]
Labels: Type-Bug Priority-Medium
New issue 2435 by [email protected]: Some string operations perform
sub-optimally (e.g String.fromCharCode, escape)
http://code.google.com/p/v8/issues/detail?id=2435
Firefox is faster than V8 at the following operations:
// convert a typed array to a js string
function ar2str(uint16arr) {
// break the computation into smaller arrays to avoid browser limits on
array size for .apply() call
var res = [], i = 0, len = uint16arr.length, MAX_ELEMENTS_PER_CALL =
100000;
while (i < len) {
res.push(String.fromCharCode.apply(null, uint16arr.subarray(i, i +=
MAX_ELEMENTS_PER_CALL)));
}
var r = res.join("");
return r;
}
// base64 decode
function b64_to_utf8( str ) {
return decodeURIComponent(escape(window.atob( str )));
}
A bit of digging shows that fromCharCode for arrays of codes is implemented
on the C++ side in Runtime_StringFromCharCodeArray, which is sub-optimal
because it uses general-purpose code [JSArray->Get() and String->Set()].
String escaping also uses String->Set(), even though the result is
guaranteed to be single-byte. The generic implementation carry a big
overhead: simply replacing the JSArray->Get()s with a more specialized
ElementsAccessor->Get() call improve performance on ar2str by ~20%.
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