gistfile1.h

#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdarg.h>

#define NOINLINE __attribute__((noinline))

typedef unsigned char byte;

typedef struct Continuation {
	void* rip;
	void* rsp;
	void* rbp;
	byte* stack;
	struct Continuation* return_to;
	void** args;
	size_t num_args;
} Continuation;

NOINLINE void resume(Continuation* cc)
{
	__asm__ __volatile__(
		// restore registers
		"movq %0, %%rdi\n"
		"movq %1, %%rsp\n"
		"movq %2, %%rbp\n"
		// indicate to function being resumed that it was resumed
		"movq $1, %%rax\n"
		// resume!
		"movq %3, %%r8\n"
		"jmpq *%%r8\n"
	:
	: "r"(cc), "m"(cc->rsp), "m"(cc->rbp), "m"(cc->rip)
	: "%rax", "%rdi", "%rsi", "%rsp", "%r8"
	);
}

NOINLINE bool save(Continuation* cc)
{
	__asm__ __volatile__(
	// rsp is rsp, since the stack size of this function is 0
	"movq %%rsp, %0\n"
	// rbp stored on the stack
	"movq (%%rsp), %%rax\n"
	"movq %%rax, %1\n"
	 // read instruction pointer for calling function from the stack
	"movq %%rbp, %%r8\n"
	"movq 8(%%r8), %%rax\n"
	"movq %%rax, %2\n"
	: "=m"(cc->rsp), "=m"(cc->rbp), "=m"(cc->rip)
	:
	: "%rax"
	);
	return false;
}

void init_continuation(Continuation* cc, void* func, size_t num_args)
{
	cc->rip = func;
	cc->stack = (byte*)malloc(1024);
	cc->rbp = cc->rsp = cc->stack;
	cc->num_args = num_args;
	cc->args = malloc(sizeof(void*)*num_args);
}

void start_continuation(Continuation* cc, Continuation* return_to, ...) {
	cc->return_to = return_to;
	va_list ap;
	va_start(ap, return_to);
	size_t i;
	for (i = 0; i < cc->num_args; ++i) {
		cc->args[i] = va_arg(ap, void*);
	}
	va_end(ap);
	resume(cc);
}

void continuation(Continuation* cc) {
	int a = (int)cc->args[0];
	int b = (int)cc->args[1];
	int c = a + b;
	printf("hello from continuation: %d\n", c);
	if (!save(cc))
	{
		resume(cc->return_to);
	}
	else
	{
		printf("continuation was resumed... %d\n", c);
		printf("stack var c is at 0x%llx\n", &c);
	}
	resume(cc->return_to);
}

int main (int argc, char const *argv[])
{
	Continuation return_here;
	Continuation sub;
	printf("return_here is at 0x%llx\n", &return_here);
	printf("before\n");
	if (!save(&return_here)) {
		printf("returning directly\n");
		init_continuation(&sub, continuation, 2);
		start_continuation(&sub, &return_here, (void*)5, (void*)4);
		printf("after resume -- should never be reached!\n");
	} else {
		printf("returned from continuation!\n");
		if (!save(&return_here)) {
			resume(&sub);
		} else {
			printf("returned from continuation for the second time\n");
		}
	}
	printf("exiting\n");
	return 0;
}