The SPARC instruction set specifies instruction for handling long double
values, however, no hardware implementation actually provides them. They
generate a trap, which is expected to be handled by the softfloat library.

Since traps are slow, and gcc knows better, it will never generate those 
instructions. Instead it directly calls into the C library, to functions
specified in the ABI and used to do long double math using softfloats.

The support code for this is, in our case, compiled into both the kernel and
libroot. It lives in src/system/libroot/os/arch/sparc/softfloat.c (and other
support files). This code was extracted from FreeBSD, rather than the glibc,
because that made it much easier to get it building in the kernel.