# floating point operations for 1/r

## floating point operations for 1/r

Hi,if i have two 3-d vectors x([x1,x2,x3]) and y,then how many flops needed to get 1/|x-y| using icc?I may interpret this functions as rsqrt((x1-y1)^2+(x2-y2)^2+(x3-y3)^2)...thanks

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I assume you're using single-precision, otherwise asking for rsqrt wouldn't make much sense...

It's not a question of the compiler you use, but of the CPU. With any recent SSE-capable CPU you will get the following if you put a 3D vector in one SSE register (which I can't generally recommend):
one subtraction (3 cycles/4 on AMD), then one multiplication (4 cycles), then two horizontal add instructions (2x 5 cycles), and then rsqrt (3 cycles).

If you had 4 x vectors and 4 y vectors this could be improved by putting the x1 values in one SSE register, the x2 values in another and so on. Then you'd calculate 3 subtractions which can be pipelined, 3 multipliations which can be pipelined, two additions and one rsqrt. Since the vertical additions are faster than the horizontal additions you'd get the result of 4 x and y vectors in basically the same time you got the one result with the vertical vectorization.

Cheers,
Matthias

Vc: SIMD Vector Classes for C++
http://code.compeng.uni-frankfurt.de/projects/vc