gcc argument register spilling on x86-64

Question

I\'m doing some experimenting with x86-64 assembly. Having compiled this dummy function:

long myfunc(long a, long b, long c, long d,
            long e, long         


        

Answer 1

A couple of reasons:

1. In the general case, an argument to a function has to be treated like a local variable because it could be stored to or have its address taken within the function. Therefore, it is simplest to just allocate a stack slot for every arguments.
2. Debug information becomes much simpler to emit with stack locations: the argument's value is always at some specific location, instead of moving around between registers and memory.

When you're looking at -O0 code in general, consider that the compiler's top priorities are reducing compile-time as much as possible and generating high-quality debugging information.