Defining the rules of the calling convention
Implementing the rules of the calling convention is an important part of lowering the LLVM intermediate representation (IR) to machine code. The basic rules can be defined in the target description. Let’s have a look.
Most calling conventions follow a basic pattern: they define a subset of registers for parameter passing. If this subset is not exhausted, the next parameter is passed in the next free register. If there is no free register, then the value is passed on the stack. This can be realized by looping over the parameters and deciding how to pass each parameter to the called function while keeping track of the used registers. In LLVM, this loop is implemented inside the framework, and the state is held in a class called CCState. Furthermore, the rules are defined in the target description.
The rules are given as a sequence of conditions. If the condition holds, then an action is executed. Depending on the outcome of that...
