Calculate Pi

main.c

#include <stdlib.h>
#include <stdio.h>
#ifdef __SSE2__
	#include <emmintrin.h>
#endif

static double calc_pi(unsigned n){
	double pi, x;
#ifdef __SSE2__
	const __m128d four = _mm_set1_pd(4.0);
	__m128d partial;
#endif
	for(pi=3.0, x=2.0; n; --n, x+=4.0)
#ifdef __SSE2__
		partial = _mm_div_pd(
			four,
			_mm_mul_pd(
				_mm_set_pd(x+2.0, x),
				_mm_mul_pd(
					_mm_set_pd(x+3.0, x+1.0),
					_mm_set_pd(x+4.0, x+2.0)
				)
			)
		),
		pi += ((double*)(&partial))[0] - ((double*)(&partial))[1];
#else	
		pi += 4.0 / (x*(x+1.0)*(x+2.0)) - 4.0 / ((x+2.0)*(x+3.0)*(x+4.0));
#endif
	return pi;
}

int main(const int argc, const char* const * const argv){
	if(argc > 1)
		printf("%.17g", calc_pi(atoi(argv[1])));
	return 0;
}