How the FreeBSD developers solve problems that are not resolved by Assembly?

[françai s]

I think that the only case where you have to resort to writing binary code
manually
is when the assembler cannot output the desired code for some reason.
If I recall correctly, there is only one such case in the HelenOS
sources:

http://trac.helenos.org/browser/mainline/kernel/arch/mips32/include/debug.h?rev=mainline%2C1446.3.1

Here they manually encode the opcodes of five special debugging
instructions for the MSIM MIPS simulator (these instructions are not
part of the standard MIPS ISA, thus the assembler does not know them).

But the developers of FreeBSD not write binary code manually.

How the FreeBSD developers solve problems that are not resolved by Assembly?