Patch forthcoming.
Created attachment 340254 [details] the patch
Comment on attachment 340254 [details] the patch r=me
Landed in https://trac.webkit.org/changeset/231741/webkit
<rdar://problem/40202349>
https://arewefastyet.com/#machine=29&view=single&suite=ss&subtest=unpack-code AWFY also agrees with the performance improvement :)