SetArrayElement shows up high on slice() profiles. At first glance, array_slice makes sure the result value is a dense array of sufficient size. So a lot of the stuff SetArrayElement does (e.g. ensureDenseArrayCapacity) seems unnecessary. If that's actually the case, can we fast-path it?

The current slice() fast path [1] should work quite [1] https://searchfox.org/mozilla-central/rev/30ead7d1ae5bf95b8bc0fd67b950cd46ca05e32c/js/src/jsarray.cpp#3212-3221 [2] https://searchfox.org/mozilla-central/rev/30ead7d1ae5bf95b8bc0fd67b950cd46ca05e32c/js/src/jsarray.cpp#3241-3248

Argh, hit enter while the focus was still in the duplicate-bug input box. What I meant to say: The current slice() fast path [1] should work quite nice for dense arrays. Then there's an additional hook in [2] and bug 1362753 made the non-fast path a bit faster, too. Therefore marking as dup of bug 1362753.