Hi @Duske,
Good question! Yes, it is deterministic. The way arguments are mapped to registers is part of the ARM Procedure Call Standard that a compiler needs to follow for ARM. You can find all the details in Section 5.5 “Parameter Passing” but the gist is
The base standard provides for passing arguments in core registers (r0-r3) and on the stack. For subroutines that take a small number of parameters, only registers are used, greatly reducing the overhead of a call
GCC does support an assembly variant, referred to as extended-asm which lets you mix C variables with assembly. While it’s not necessary to use in this case, it can be helpful when you want to reference something that isn’t in a known register from some assembly code (such as a local variable)