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

Joseph Myers <[email protected]> Tue, 1 Sep 2020 21:16:31 +0000

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

On Tue, 1 Sep 2020, Keith Packard via Newlib wrote:

> If not, then one of the two versions of fma should be getting compiled
> as they have opposite tests -- newlib/libm/machine/arm/s_fma.c checks
> for '#if HAVE_FAST_FMA' while newlib/libm/common/s_fma.c checks for
> '#if !HAVE_FAST_FMA'.

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).

-- 
Joseph S. Myers
[email protected]