The JSVALUE64 JIT does it on fast and slow paths. The 32_64 JIT does it on fast paths only. The LLInt does it on fast paths only. All implementations of this opcode ought to be profiled.
Created attachment 145507 [details] the patch
Comment on attachment 145507 [details] the patch r=me
Landed in http://trac.webkit.org/changeset/119438