CODEWITHSUNDEEP
bit-manipulationbranch
Jacko
Jacko

Reputation: 13195

Branch free saturation

I have the following calculation:

 unsigned int a;
 unsigned b = (a < 4) ? a : 4;

Is it possible to convert the second line to a branch free format?

Thanks!

Upvotes: 2

Views: 109

Answers (2)

njuffa
njuffa

Reputation: 26105

You could use a conditionally applied mask:

unsigned int a, b, t, m;
t = a - 4;
m = 0 - ((int)t < 0); // mask of all 0s or all 1s
b = (t & m) + 4; // mask all 1s: b=a-4+4;  mask all 0s: b=4

Upvotes: 1

Juliano
Juliano

Reputation: 811

Try this:

  b = (a >= 4) * 4 + (a < 4) * ((a >> 1) & 1) * 2 + (a < 4) * (a & 1);

Explanation: we are returning 4 by "zeroing" the 2 least significant bits if a >= 4. If a < 4, we use these 2 least significant bits.

Upvotes: 2

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