>> a) A bug in bignumcompare for 64bit Windows. Patch is here:
>> http://lists.gnu.org/archive/html/emacs-devel/2018-08/msg00487.html
>> ...
>> b) fmod_float has a bug:
>> http://lists.gnu.org/archive/html/emacs-devel/2018-08/msg00442.html
>>
>> c) Extend Fexpt to support bignums. Patch is here:
>> http://lists.gnu.org/archive/html/emacs-devel/2018-08/msg00503.html
>>
>> d) Extend Fceiling, Ffloor, Fround and Ftruncate to support bignums by
>>    updating rounding_driver.

I worked on these and installed patches to master that should do (a), (b), and 
(c). For (d) I wrote the attached patch, and plan to test it a bit more before 
installing, as it's the hairiest.