Most optimized way to calculate modulus in C

Posted by hasanatkazmi on Stack Overflow See other posts from Stack Overflow or by hasanatkazmi
Published on 2010-04-18T09:13:42Z Indexed on 2010/04/18 9:23 UTC
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c

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assembly

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optimization

I have minimize cost of calculating modulus in C. say I have a number x and n is the number which will divide x

when n == 65536 (which happens to be 2^16):

mod = x % n (11 assembly instructions as produced by GCC) or
mod = x & 0xffff which is equal to mod = x & 65535 (4 assembly instructions)

so, GCC doesn't optimize it to this extent.

In my case n is not x^(int) but is largest prime less than 2^16 which is 65521

as I showed for n == 2^16, bit-wise operations can optimize the computation. What bit-wise operations can I preform when n == 65521 to calculate modulus.

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