From: walter harms <wharms@bf...>  20051108 12:44:49

mea culpa, i did some reading in the Internet and found http://www.nsc.liu.se/~boein/ifip/kyoto/enableprocs.txt realy interesting. in short: optimised means catching cases like internal overflow (x^2) if there is no danger, it falls back to sqrt(x^2+y^2). ntl: i would go for hypot() if i can not optimise it away. just my 5 cents walter Pablo d'Angelo wrote: > walter harms schrieb: >>hi pablo, >>you may like to replace sqrt(x^2+y^2) with hypot(x,y). systems have >>often a optimised version of it. > > Do know a system where hypot is acutally faster than sqrt(x^2+y^2)? > > I have just looked at the implementation of the x86_64 mathlib (as > shipped by suse libc), and I'm sure that hypot is slower there. > > sqrt(x*x+y*y): > > # math.c:616 > .loc 1 616 0 > mulsd %xmm0, %xmm2 > .LVL237: > mulsd %xmm1, %xmm1 > addsd %xmm0, %xmm1 > sqrtsd %xmm1, %xmm0 > > > hypot(x,y): > > seems to be slower since it is mainly concerned with the case where > r=x^2+y^2 becomes bigger than the largest number a double can handle. > > The fast, inexcat mode of hypot leads to > r = x*x + y*y > sqrt(r) > > Since there are a lot of if's for the exponent handling in between and > its not inlined it will probably be slower... > > ciao > Pablo > > 