The calling convention of the System V AMD64 ABI is followed on GNU/Linux. The registers RDI, RSI, RDX, RCX, R8, and R9 are used for integer and memory address arguments and XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6 and XMM7 are used for floating point arguments.
For system calls, R10 is used instead of RCX. Additional arguments are passed on the stack and the return value is stored in RAX.
Michael Matz, Jan Hubicka, Andreas Jaeger, and Mark Mitchell. System V Application Binary Interface AMD64 Architecture Processor Supplement. 0.99.6: http://x86-64.org/documentation/abi.pdf
x86 calling convention from Wikipedia: http://en.wikipedia.org/wiki/X86_calling_conventions#cite_note-AMD-9