Thanks for the explainations!
I did read in the Pike's paper about the syntax name+offset(FP), but I did understood that name had to be a symbol already defined, and I was looking for it in the c code. Sorry for the noise!
This led me to another question, however: I've read before that the plan9 compilers use the stack for va_list, but here the assembler is using it also for explicit parameters, right?
Is it correct to say that this means that the Plan9 compiler suite *never* follows the sysV calling convention documented at section 3.2.3 of AMD64 ABI
http://www.x86-64.org/documentation/abi.pdf and always pushes parameters to the stack?