Search code examples
androidcarmintrinsicsneon

Divide by floating-point number using NEON intrinsics

I'm processing an image by four pixels at the time, this on a armv7 for an Android application.

I want to divide a float32x4_t vector by another vector but the numbers in it are varying from circa 0.7 to 3.85, and it seems to me that the only way to divide is using right shift but that is for a number which is 2^n.

Also, I'm new in this, so any constructive help or comment is welcomed.

Example:

How can I perform these operations with NEON intrinsics?

float32x4_t a = {25.3,34.1,11.0,25.1};
float32x4_t b = {1.2,3.5,2.5,2.0};
//    somthing like this
float32x4 resultado = a/b; // {21.08,9.74,4.4,12.55}
Solution

  • The NEON instruction set does not have a floating-point divide.

    If you know a priori that your values are not poorly scaled, and you do not require correct rounding (this is almost certainly the case if you're doing image processing), then you can use a reciprocal estimate, refinement step, and multiply instead of a divide:

    // get an initial estimate of 1/b.
float32x4_t reciprocal = vrecpeq_f32(b);

// use a couple Newton-Raphson steps to refine the estimate.  Depending on your
// application's accuracy requirements, you may be able to get away with only
// one refinement (instead of the two used here).  Be sure to test!
reciprocal = vmulq_f32(vrecpsq_f32(b, reciprocal), reciprocal);
reciprocal = vmulq_f32(vrecpsq_f32(b, reciprocal), reciprocal);

// and finally, compute a/b = a*(1/b)
float32x4_t result = vmulq_f32(a,reciprocal);