integer-based power function

ipow.c

#include <stdio.h>
#include <limits.h>
#include <time.h>
#include <math.h>

ssize_t ipow(ssize_t const x, size_t n) {
	ssize_t r = 1;
	while( n > 0 ) {
		if( n & 1 ) {
			r *= x;
			n--;
		} else {
			r *= x * x;
			n -= 2;
		}
	}
	return r; 
}

ssize_t eluc_pow(ssize_t n, ssize_t p);
ssize_t fast_ipow(ssize_t x, ssize_t expo) {
	if( expo < 0 ) {
		return 0;
	} else if( expo==0 || x==1 ) {
		return 1;
	} else if( expo >= 64 ) {
		return 0x7fFFffFFFFffFFffL;
	}
	ssize_t b = 1;
	ssize_t acc = 1;
	while (b <= expo) {
		acc *= x * (expo & b);
		x *= x;
		b <<= 1;
	}
	return acc;
}

ssize_t eluc_pow(ssize_t n, ssize_t p) {
	if (p < 0) return 0; // Fractions are rounded to 0
	ssize_t b = 1;
	ssize_t acc = 1;
	while (b <= p) {
		acc *= n * (p & b);
		n *= n;
		b <<= 1;
	}
	return acc;
}


int main() {
	ssize_t x = 0*4294967296, expo = 0*2;
	scanf("%zi", &x);
	scanf("%zi", &expo);
	printf("%zi**%zi\n", x, expo);
	clock_t start   = clock();
	ssize_t res = 0;
	enum{ run_counter = 10000000 };
	for( size_t i=0 ; i < run_counter; i++ ) {
		res     = fast_ipow(x, expo);
	}
	clock_t end     = clock();
	long double elapsed = ( long double )(end - start) / ( long double )(CLOCKS_PER_SEC);
	printf("fast_ipow :: elapsed ms: %Lf\n", elapsed * 1000);
	
	start   = clock();
	for( size_t i=0 ; i < run_counter; i++ ) {
		res     = ipow(x, expo);
	}
	end     = clock();
	elapsed = ( long double )(end - start) / ( long double )(CLOCKS_PER_SEC);
	printf("ipow      :: elapsed ms: %Lf\n", elapsed * 1000);
	
	start   = clock();
	for( size_t i=0 ; i < run_counter; i++ ) {
		res     = eluc_pow(x, expo);
	}
	end     = clock();
	elapsed = ( long double )(end - start) / ( long double )(CLOCKS_PER_SEC);
	printf("eluc_pow  :: elapsed ms: %Lf\n", elapsed * 1000);
	
	start   = clock();
	for( size_t i=0 ; i < run_counter; i++ ) {
		res     = pow(x, expo);
	}
	end     = clock();
	elapsed = ( long double )(end - start) / ( long double )(CLOCKS_PER_SEC);
	printf("pow       :: elapsed ms: %Lf\n", elapsed * 1000);
	
	start   = clock();
	for( size_t i=0 ; i < run_counter; i++ ) {
		res     = powf(x, expo);
	}
	end     = clock();
	elapsed = ( long double )(end - start) / ( long double )(CLOCKS_PER_SEC);
	printf("powf      :: elapsed ms: %Lf\n", elapsed * 1000);
}