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hopcode
Posts:
4
Registered:
4/15/12


Exp() function reloaded
Posted:
Apr 14, 2012 10:31 PM


Hi, here my method for the exponentianal function e^x . it is a mix of 2 fundamentals:  Taylor series running at a 10 steps for decimals 1.0 < x < +1.0  a couple of basic rules of the logarithms.
Explanation. say we want to calculate e^20.3. well,
20.3 float double is 40344CCCCCCCCCCDh hexadecimal
we convert it to base2 by multiplying it by lg2(e)
20.3 x 1.4426950408889632824453128103079f result 20.3 base2 403D4965C85C0166h
it is needed to know later how much we should shift left our partial result in its integer part.
now, for the fundamental of powers we know that
n^20.3 can be rewritten as n^20 * n^0.3
also we round down 20.3 base2 to get
20.0 and 0.3 float double remainder
20.0 = 403D000000000000h 0.3 = 3FD2597217005980h
we store the remainder 0.3 now for later use.
then we convert the integral part 20.0 base2 to an integer value,
20.0 float double > integer, s after conversion s = 1Dh 29 decimal
we shift 1 by s times on the left
i = 1 << s
thus, i = 1 * 2^29 = 20000000h ( 536'870'912 decimal)
3 additional checks are needed here. to avoid overflow after 63 shifts; in the case s > 63 and checking wether s push i out of the float double capacity.
we reconvert i to float for later use 20000000h integer = 41C0000000000000h float double
we convert the float decimals we stored above in base2 to float decimals in baseE. this is because we want to use Taylor series from the e^n. i choosed 10 steps max, and having 1.0 < decimals < +1.0 works enough good. NOTE: you can extend the range of action of the Taylor series up to +8, stepping it ~20 or more times.
recall Taylor now on e^x : e^x = 1 + x + x^2 / 2! + x^3 / 3! .... + x^n / n!
now, according fundamentals of logarithms
a) if e^x = 2^q b) and generally, lgBASE(x)^n = n * lgBASE(x) then we can extend a) this way,
x * ln(e) = q * ln(2)
c) and thus x = q * ln(2) should verify the a) as true identity.
now, because ln(2) = lg10(2) / lg10(e) lg10(2) = 0.30102999566398119521373889472449f lg10(e) = 0.43429448190325179004808384911378f ln(2) = 0.69314718055994536943283387715543f
we apply c)
x = q * ln(2) where q = r our remainder
x = 0.3 base2 * 0.69314718055994536943283387715543f x = 3FC9700ADD042628 baseE
we give this x to the Taylor expansion routine. to get back
3FF3848660139D52 as result of exp() on the remainder 0.3
finally for the fundamental of powers above we multiply
41C0000000000000h * 3FF3848660139D52h n^20 base2 * n^0.3 base2 = n^20.3 base2 to get back
41C3848660139D52h that corresponds exactly
to our decimal 654904512.1532385
The resulting assmbly code can be found on my website at
http://sites.google.com/site/x64lab/home/reloadedalgorithms/myexpfunctionjexp
it is ~40 lines of code (my Taylors's exp() code + main routine) it accepts only +numbers for now, and makes no exaustive check on the floats. for those and negative values i leave it to the reader's creativity ( being e^x essentially 1 / e^x ). this is the fastest method i know, it should time ~80 cycles totally, i didnt check it yet. it's not so important.
of some relevance to me was
1) avoid the Intel Approx Math library license 2) avoid things like the cmath library 3) avoid the 2 FPU slow instructions FYL2X to compute y * log2(x) F2XM1 to compute 2^x  1 because 250/300 cycles for 2 instro it's the insanity, 100%, pure :) especially on tests i am doing from huge RNDdata outputs.
4) using Taylor series the right way, because we would need lot of steps to get the right values on plugging in large x, as in the example e^20.3
but the truetruth is that i am not yet ready for Chebyshev; simply because i need some time to understand something more of his genial calculus. if you have simplified references about him, please share it.
and not much time to write a full assembly math library. if someone is interested to contribute the library is open source, under MPL license, but assembly required, please. the library lies under the name "amrt", in the same way as my other one, "art" assemblyruntime. i will write/update it from time to time only on my needs. you find me on clax86.
Thank you all,
hopcode aka Marc Rainer Kranz
 .:x64lab:. group http://groups.google.com/group/x64lab site http://sites.google.com/site/x64lab



