Date: Wed, 6 Dec 2017 11:37:09 +0100 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: remquo - underlying logic * Damian McGuckin <damianm@....com.au> [2017-12-06 12:17:40 +1100]: > While my exploration with floating point numbers was less than stellar, > I did notice that when > > ex - ey < p (where p is the digits in the significant) > > you can use the > > fma > > routine to compute some appropriately rounded/truncated version of the > quotient for both remquo and fmod. And this appears to not loose any > precision for the obvious reasons. > > > From my limited testing, the speed gain for this extremely limited range > of exponent difference is huge over the standard routine in MUSL. I will do > some more testing and report in detail but it seems to be orders of > magnitude. > > Somebody might want to comment on that sort of approach. it's not clear to me how you use fma (x-(int)(x/y)*y ?), but efficient fma instruction is not available on all targets and the software implementation can be very slow. and i suspect such approach would break fenv correctness. assuming you rely on exact fma result you may need two different implementation depending on FP_FAST_FMA (which is currently missing in musl). (musl is compiled with -std=c99 so x*y+z is not contracted to fma(x,y,z) automatically when the instruction is available, you have to add -ffp-contract=fast if that's what you want, but it might break some code in musl that relies on exact arithmetics, most math code should work either way though.)
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