[chrisseaton]

It uses something they call IRA and LRA on the RTL representation. But bottom line is: it's graph colouring.

https://gcc.gnu.org/onlinedocs/gccint/RTL-passes.html

There's really quite a lot of documentation and published papers when you actually look:

https://github.com/gcc-mirror/gcc/blob/751f306688508b08842d0...