Optimize rabinkarp_update() #191
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This adds rabinkarp.c copying rollsum.c's unrolled loop idea.
Change from a generic uint32_pow() to rabinkarp_pow() optimized using a lookup table for getting powers of RABINKARP_MULT. Use a DOMULT4() macro to unroll rabinkarp_update() in a way that means 4 multiplies could be piplined/parallelized if the hardware can do it. Each multiply is not dependent on the results of the previous multiply.
A lot of testing of many different variants of the loop unrolling seems to indicate this is the best variant of the loop-unrolling.
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This optimizes the rabinkarp_update() by correctly using unsigned constants and manually unrolling the loop for best performance. See the following for analysis that lead to this patch;
https://github.com/dbaarda/librsync-tests/blob/master/RESULTS.rst