FPU preserved registers not preserved across fibril switches
To make it possible for two fibrils to do floating point math without influencing each other, it is necessary to save/restore the preserved FPU context registers, as prescribed by the respective ABI.
The preservation of preserved FPU registers has not been consistently implemented across all architectures. E.g. mips32 and ia64 seem to be preserving some FPU state, while sparc64, ia32 and amd64 don't.
Scratch FPU registers don't need to be preserved.