Macros for tail recursion in C.
Background
Using recursion for iteration is a pattern used in many functional languages. For longer iterations this might lead to stack overflow due to the amount of stack frames needed for the recursive function calls. A common way to solve this is by using tail recursion. This is done by reusing the old stack frame when the last statement of a function is a recursive call to the same function.
This is of course not needed in C due to its language constructs for iteration, but the best way to learn something is by hacking it together with a couple of filthy macros. So take this for what it is.
Usage
The macro tail_call_args(argc, args...) takes six or less arguments and
updates the parameter registers according to System V AMD64 ABI specification.
A function pointer to the current pointer the needs to be passed to the
macro tail_call(func) for the actual jump.
No expressions may be passed as arguments to tail_call_args the input to the
assembly instructions needs to be valid memory at compile time.
Using compiler optimizations while using the tail recursion macros will probably make everything blow up due to it being undefined actions.
Example
#include "tail_recursion.h"
...
void rec(long n)
{
if (n == 0) {
/* Done */
return;
} else {
n -= 1;
tail_call_args(1, n); /* Set arguments */
tail_call(rec); /* Call rec using tail recursion */
}
}