http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56843
--- Comment #2 from Bill Schmidt <wschmidt at gcc dot gnu.org> 2013-04-04 16:12:31 UTC --- Regarding the last point, I found this in the user manual: "The double-precision square root estimate instructions are not generated by default on low-precision machines, since they do not provide an estimate that converges after three steps." That seems to indicate someone decided the libcall is better than a four-step iteration. That doesn't necessarily seem obvious to me.
