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 12:28:18 -0700

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

Sebastian Huber <[email protected]> writes:

> I think the problem is somewhere in the build system:
>
> find -name s_fma.c
> ./newlib/libm/machine/arm/s_fma.c
> ./newlib/libm/machine/aarch64/s_fma.c
> ./newlib/libm/machine/riscv/s_fma.c
> ./newlib/libm/machine/spu/s_fma.c
> ./newlib/libm/common/s_fma.c
>
> I guess the machine-specific file overrides the common file. If the 
> machine-specific file is empty due to pre-processor magic, then the 
> default implementation is still not present.

newlib shouldn't be calling fma if the underlying hardware support isn't
present -- fma is used in some math functions to improve performance
where the code can take full advantage of the additional precision of
the intermediate value.

Are you using fma directly? If your hardware supports it, the C compiler
should be directly emitting the relevant instruction sequence so you
shouldn't be seeing an undefined function appear.

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'.

I recently did some work in this area, so it's possible I broke
something in your environment that I didn't catch in mine; I don't test
newlib builds, only downstream picolibc builds.

-- 
-keith

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