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drstrip
Journeyman III

kronecker delta on uint4?

I want to compute the kronecker delta (i = j returns 1, otherwise 0) for each element of a stream of type uint4. If I recall correctly, conditionals operate only on the first element of a vector type, so anything based on a conditional is out. I tried abs(sign(i - j)) but abs() will not accept a uint4, presumably because it has a conditional buried in it. The best I've come up with is:

kernel uint4 kronecker_delta(uint4 i<>, uint j<>

{

   int4 k_d = sign((int4) i - (int4)j);

   return (uint4)(k_d * k_d);

}

 

My Brook+ experience is near zero, so I wonder if I'm missing some much easier/faster way to do this.

 

 

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4 Replies
riza_guntur
Journeyman III

just make it in 4 separate if

afaik that's the only way

btw, in brook+ you need to specifiy output variable explicitly

and there is no return, all basic kernel in brook+ is void

kernel void kronecker_delta(unit i<>, unit j<>, out unit4 k<>

you do the rest

 

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I should have mentioned the pseudo-code I included was a sub-kernel.

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kernel uint4 kronecker_delta(uint4 i<>, uint j<>

{

int4 k_d = sign((int4) i - (int4)j);

return (uint4)(k_d * k_d);

}

drstrip,
I'm not sure how many instructions sign extends to, but you can try this:
return (uint4)((i != j) & (int4)1);

The first conditional should give each component that is equivalent a -1 and the rest 0. By anding with 1, the -1 becomes a 1 and the zero stays the same. This should be 2 instructions.
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that's what I was looking for. I was so focused on the fact that conditionals only operate on the value of the first vector component that I ignored that the boolean operation would operate on all components and set a result for each. Your answer helped me focus on this more powerful approach. Thanks.

 

BTW, what you wrote doesnlt actually compile, you need to write

      return (uint4)(((int4) i != (int4) j) & int4(1,1,1,1));

at least that's what I came up with to get rid of compile errors.

 

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