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

[Szabolcs Nagy]

The 08/10/2020 11:30, Corinna Vinschen wrote:
> Hi Szabolcs,
> 
> ok to push?
> 

this looks ok.

i would have used the arm specific macros
( __ARM_FEATURE_FMA, __ARM_FP) directly
in arm specific code.

but using HAVE_FAST_FMA{F} works too.
(note that these macros currently only
do something useful on aarch64 and arm.)


> 
> Thanks,
> Corinna
> 
> On Aug  8 15:34, Keith Packard via Newlib wrote:
> > I added some new test configurations to my CI system for picolibc and
> > discovered that when the new math code was built on 32-bit ARM
> > processors with only single-precision floating hardware, several math
> > functions were returning imprecise results. I got the expected results
> > on processors with no FPU and on processors with both 32- and 64- bit
> > FPUs.
> > 
> > I discovered that the affected functions were using the 'fma' function
> > on this hardware, even though (lacking 64-bit HW support), that
> > function was being emulated without the required precision.
> > 
> > This all boiled down to math_config.h incorrectly detecting 64-bit FMA
> > support on ARM processors.
> > 
> > This patch series contains three changes:
> > 
> >  1. fix the fast FMA process so that 32-bit ARM processors without 64-bit FMA
> >     support don't use 'fma' for the new math functions
> > 
> >  2. Add detection of fast FMAF, which 32-bit ARM processors with only
> >     32-bit FPUs *do* support.
> > 
> >  3. Add ARM versions of fma and fmaf which are used when those
> >     instructions are available.
> > 
> 
> -- 
> Corinna Vinschen
> Cygwin Maintainer
> Red Hat
> 

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