By my count, it's very sparse - only 0.003% occupied. I suppose the initial size declaration could make it dense, but then it would use a considerable amount of (virtual?) memory. Unfortunately, I don't have access to Safari to test that.
Hmm… it looks like I misspoke. The "sparse mode" check that Array.prototype.sort performs in JavaScriptCore appears to be different than the its concept of sparse array storage, which is used under the conditions I described. The "sparse mode" check in the sorting code appears to be related to whether the array instance has getters/setters or properties defined via Object.defineProperty. The performance difference my code demonstrates is due to the extra overhead of iterating the sparse array storage compared to a vector, and not due to the different sorting algorithms that are employed.
