C++ CPU Register Usage

Question

In C++, local variables are always allocated on the stack. The stack is a part of the allowed memory that your application can occupy. That memory is kept in your RAM (if not swapped out to disk). Now, does a C++ compiler always create assembler code that stores local variables on the stack?

Take, for example, the following simple code:

int foo( int n ) {
   return ++n;
}

In MIPS assembler code, this could look like this:

foo:
addi $v0, $a0, 1
jr $ra

As you can see, I didn't need to use the stack at all for n. Would the C++ compiler recognize that, and directly use the CPU's registers?

Edit: Wow, thanks a lot for your almost immediate and extensive answers! The function body of foo should of course be return ++n;, not return n++;. :)

Answer 1

Yes. There is no rule that "variables are always allocated on the stack". The C++ standard says nothing about a stack.It doesn't assume that a stack exists, or that registers exist. It just says how the code should behave, not how it should be implemented.

The compiler only stores variables on the stack when it has to - when they have to live past a function call for example, or if you try to take the address of them.

The compiler isn't stupid. ;)