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

Keith Packard via Newlib <[email protected]> Tue, 01 Sep 2020 16:06:07 -0700

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

Joseph Myers <[email protected]> writes:

> But note that newlib/libm/common/s_fma.c doesn't actually do anything 
> useful; it's not a fused operation.  Implementing correct fma in software 
> is highly nontrivial, especially when you want to handle exceptions and 
> rounding modes correctly (including machine-specific differences in 
> whether tininess is detected before or after rounding).

Should we just stop providing the generic fma/fmaf implementations? That
seems like a good idea to me as it will prevent applications from
getting the wrong answer.

The fmaf one does offer increased precision by doing the operation in
double instead of float, which is 'different' from doing it in float,
but it still gets a different answer from 'a * b + c'. It also gets the
wrong exception status.

-- 
-keith

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