Re: Re: [PATCH 0/3] ARM with only 32-bit floats do not have fast 64-bit FMA

Keith Packard via Newlib <[email protected]> Mon, 07 Sep 2020 10:16:24 -0700

Newsgroups	gmane.comp.lib.newlib
Message-ID	<[email protected]>

Eric Bresie via Newlib <[email protected]> writes:

> Not directly related (and as I’m not really an expert on these things, nor able to change in any way) but was looking at the code mentioned and saw line like:
>
> if (x == 0.0 || y == 0.0)
>
> return (x * y + z);
>
> If either x or y is zero would it be better to just return z and avoid
> an extra multiplication operation here?

You want to compute the correct result and get the right exceptions in
all of the delightful IEEE754 corner cases (e.g. 0 × ∞). It's easier to
just execute the two operations than to try and synthesize the right
result (which is implementation-dependent in the case of 0 × ∞ +
qNaN). The key here is that if x or y is zero, then you won't lose any
intermediate precision by performing the operation this way.

-- 
-keith

signature.asc (application/pgp-signature, 832 B)