Unless you are writing an optimizing compiler, you just have to understand the target language well enough to write snippets that resemble the operations of your source or intermediate language, which can be pretty little. See:

http://t3x.org/subc/cg386.c.html

Anecdotally: I have once written an AXP21164 back-end without any prior knowledge, only based on the reference manual. It was not pretty or fast, but it worked.