Fast 2D linear regression in pure C without math libraries

linear_regression.c

#include <stdio.h>
#include <stdint.h>

float pwr2(float x)
{
    return x * x;
}

float inv_sqrt(float number)
{
	const float x2 = number * 0.5f;
	const float threehalfs = 1.5f;

	union {
		float f;
		uint32_t i;
	} conv  = { .f = number };
	conv.i  = 0x5f3759df - (conv.i >> 1);
	conv.f  *= threehalfs - (x2 * conv.f * conv.f);
	return conv.f;
}

uint8_t lin_reg(int n, const float x[], const float y[], float* m, float* b, float* r2)
{
	float sumx = 0.0f; 
	float sumx2 = 0.0f;
	float sumxy = 0.0f; 
	float sumy = 0.0f;
	float sumy2 = 0.0f;

	for (int i=0; i < n; i++){ 
		sumx  += x[i];       
		sumx2 += pwr2(x[i]);  
		sumxy += x[i] * y[i];
		sumy  += y[i];      
		sumy2 += pwr2(y[i]); 
	} 

	float denom = (n * sumx2 - pwr2(sumx));
	if (denom == 0) {
		*m = 0.0f;
		*b = 0.0f;
		if (r2) {
			*r2 = 0.0f;
		}
		return 1;
	}

	*m = (n * sumxy  -  sumx * sumy) / denom;
	*b = (sumy * sumx2  -  sumx * sumxy) / denom;
	if (r2) {
		*r2 = pwr2((sumxy - sumx * sumy / n) * inv_sqrt((sumx2 - pwr2(sumx)/n) * (sumy2 - pwr2(sumy)/n)));
	}

	return 0; 
}

int main()
{
    unsigned int num;
    printf("Enter number of points: ");
    while(scanf("%u", &num) != 1) {
        printf("\r\nError when reading number of points! Try again: ");
    }
    printf("Please enter %u points in format 'x,y', one per line.\r\n", num);
    float x[num], y[num];
    for (uint16_t i = 0; i < num; i++) {
        printf("Point #%u: ", i);
        while(scanf("%f,%f", &x[i], &y[i]) != 2) {
            printf("\r\nError when reading number of points! Try again: ");
        }
    }
    printf("Performing linear regression...\r\n");
    float m, b, r2;
    if (lin_reg(num, x, y, &m, &b, &r2)) {
        printf("Singular matrix, unable to compute!\r\n");
    } else {
        printf("Fitted line: y = %.2fx + %.2f with score R2 = %.2f\r\n", m, b, r2);
    }

    return 0;
}