NEON reciprocal example

#include <arm_neon.h>
#include <math.h>
#include <stdio.h>


void run_neon_reciproc(float data_in[4], float data_out[4]) {
	float32x4_t input = vld1q_f32(data_in);
	float32x4_t out = vrecpeq_f32(input);
	//out = vmulq_f32(vrecpsq_f32(input, out), out);
	//out = vmulq_f32(vrecpsq_f32(input, out), out);
	vst1q_f32(data_out, out);
}

void run_neon_rsqrt(float data_in[4], float data_out[4]) {
	float32x4_t input = vld1q_f32(data_in);
	float32x4_t out = vrsqrteq_f32(input);
	//out = vmulq_f32(vrsqrtsq_f32(vmulq_f32(input, out), out), out);
	//out = vmulq_f32(vrsqrtsq_f32(vmulq_f32(input, out), out), out);
	vst1q_f32(data_out, out);
}

int main(void) {
	float data_in[4] = {1., 2., 3., 4.};
	//float data_in[4] = {137., 223., 331., 499.};
	float data_out[4];
	run_neon_reciproc(data_in, data_out);
	for(int i = 0; i < 4; ++i) {
		float res = 1. / data_in[i];
		printf("%d: 1./%f = %f neon out %f err %e\n", i, data_in[i], res, data_out[i], fabs(data_out[i] - res));
	}
	run_neon_rsqrt(data_in, data_out);
	for(int i = 0; i < 4; ++i) {
		float res = 1.f/sqrt(data_in[i]);
		printf("%d: 1.f/sqrt(%f) = %f neon out %f err %e\n", i, data_in[i], res, data_out[i], fabs(data_out[i] - res));
	}
	return 0;
}