I would expect that for some values of length it would be deemed "too sparse" and no longer treated as a single underlying vector.
On Thursday, July 17, 2014, Katelyn Gadd <[email protected]> wrote: > What happens if you combine 'new Array(length)' + 'array[length - 1] = > <default value>'? That seems like it would produce the optimal > behavior in all cases. > > On Wed, Jul 16, 2014 at 11:40 PM, Nicholas Nethercote > <[email protected] <javascript:;>> wrote: > > On a related note: ObjectElements::VALUES_PER_HEADER is 2. Is this > > because 2 * sizeof(HeapSlot) is 16 bytes, which is equal to > > sizeof(ObjectElements)? > > > > Nick > > > > On Wed, Jul 16, 2014 at 11:34 PM, Nicholas Nethercote > > <[email protected] <javascript:;>> wrote: > >> On Wed, Jul 16, 2014 at 11:08 PM, Nicholas Nethercote > >> <[email protected] <javascript:;>> wrote: > >>> Hi, > >>> > >>> Here's some code from pdf.js: > >>> > >>> function stringToArray(str) { > >>> var length = str.length; > >>> var array = []; > >>> for (var i = 0; i < length; ++i) { > >>> array[i] = str.charCodeAt(i); > >>> } > >>> return array; > >>> } > >>> > >>> This kind of code shows up in a number of places in pdf.js. Pretty > >>> simple -- filling in an array one element at a time. > >>> > >>> Underneath the covers, this causes the array to repeatedly double in > >>> size, which is annoying. And I thought I could avoid it because we > >>> know how big the array will be in advance. So I changed the 3rd line > >>> to this: > >>> > >>> var array = new Array(length); > >>> > >>> but I still get the same sort of doubling behaviour under the covers. > >> > >> Oh wait. It depends on the value of |length|. If it's 2050 or less, > >> then the right-sized array is allocated immediately. Otherwise, it > >> allocates 10 elements and then does the doubling thing. I guess > >> there's a limit to how much we'll pre-allocate. > >> > >> So using |new Array(length)| seems like the best thing, then. > >> > >> Nick > > _______________________________________________ > > dev-tech-js-engine-internals mailing list > > [email protected] <javascript:;> > > https://lists.mozilla.org/listinfo/dev-tech-js-engine-internals > _______________________________________________ > dev-tech-js-engine-internals mailing list > [email protected] <javascript:;> > https://lists.mozilla.org/listinfo/dev-tech-js-engine-internals > _______________________________________________ dev-tech-js-engine-internals mailing list [email protected] https://lists.mozilla.org/listinfo/dev-tech-js-engine-internals
