On Sat, 11 Mar 2006 13:44:52 +0100, Pianoman wrote: >Hi, I need to perform very fast squareroot (sqrt) operations on double type >nukmbers. >I tried this >function mysqrt(a:double):double; >var y,yn:double; >begin >yn:=a; >repeat >y:=yn; >yn:=(y*y+a)/(2*y); >until abs(yn-y) < 10e-16; >mysqrt:=yn; >end; >but it is quite slow (FPC uses Heron iterations) which is 10 times faster >than this code but I need even faster sqrt routin. >Any ideas for optimizing the function written above would be appreciated. >Pianoman Once upon a time, I did this in single precision:
1) I made a crude 1st guess by manufacturing, in Assembler, a linear fit mantissa - 2 different fits depending if exponent was odd or even - and the appropriate exponent. The worst case error of this is easily calculated. Process takes 2 floating ops and some integer stuff taking out and putting back the exponent. 2) I then explicitly coded the iterations required to converge in this worst case: just 3 floating ops per (or is it 2 plus integer exponent manipulation?) and it didn't take very many. No test. Saving the test saved more time than was wasted doing unnecessary iterations, at least on the machine I did it on. This may not be so in double precision ... Best of luck. Andrew Bennett, Avondale Vineyard, Nova Scotia. _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
