They aren't inlining manually. The optimisation works by making more cases fall into an already existing fast path. edit: I might have misunderstood - I should

They aren't inlining manually. The optimisation works by making more cases fall into an already existing fast path. edit: I might have misunderstood - I should say that the patch doesn't do any inlining. The fast path is indeed the manually inlined body of the function.