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

On 02/09/2020 01:06, Keith Packard wrote:
> 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.

Our failing test is pretty basic, it just checks if fma() and fmaf() 
library functions are present as per C99. The glibc offers also a simple 
default implementation, for example:

https://sourceware.org/git/?p=glibc.git;a=blob;f=math/s_fma.c;h=4d73af4f65d511594b2395d032a135721c578484;hb=HEAD

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