This is a speed optimization. It's a new opcode.
Created attachment 21968 [details] patch
Created attachment 21969 [details] patch Forgot to include the SunSpider speed-up figure in the ChangeLog.
Created attachment 21981 [details] patch, updated for TOT and revised based on Geoff's feedback
Comment on attachment 21981 [details] patch, updated for TOT and revised based on Geoff's feedback r=me, but the removal of the in place constructors should be a separate patch. However, r=me on removing them as well :D
You should also probably update the op_new_array documentation.
http://trac.webkit.org/changeset/34851