Int-multiply-using-floats trickery

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

static float fnms(float a, float b, float c)
{
  return (float) -((double) a * (double) b - (double) c);
}

int main()
{
  union
  {
    float f;
    unsigned u;
  } u;

  const float a = 255.0f;
  const float aScaled = -a * pow(2.0f, 23.0f);

  for (int b=0; b < 65536; b++)
  {
    u.u = 0x3f800000 | b;
    float product = fnms(u.f, aScaled, aScaled);
    int prodAsInt = (int) product;
    if (prodAsInt != a*b)
    {
      printf("Mismatch! b=%d\n", b);
      return 1;
    }
  }

  return 0;
}