For that matter my concern is with the expansion of PUSH and POP:
PUSH(x); y = POP();
*sp++ = x; y = *--sp;
Can gcc peephole the above, completely eliminating the assignment to the stack (which is unnecessary in our case after all)?
y = x; //desired target
Without somehow informing gcc to the contrary, gcc will assume that writing to * sp is significant, even though in our "higher-level" view the assignment to * sp is secondary to transferring the data between two locations.
Well, with full optimizations on gcc (-O3), it doesn't change anything (at least in execution time, I didn't compare generated machine codes). Wow, gcc seems really clever. Now that I know how hard it is to implement a compiler, I can only applaud :)