[PATCH] arm: Add ARM VFPv4 VFMA instruction support in fma/fmaf (BZ 15503)

[Joseph Myers]

On Thu, 30 Oct 2025, Adhemerval Zanella Netto wrote:

> It is also on my backlog to check if we can consolidate all the generic double
> fma implementation on only one.  Current we have:
> 
>   sysdeps/ieee754/dbl-64/s_fma.c
>   sysdeps/ieee754/ldbl-96/s_fma.c 
>   sysdeps/ieee754/ldbl-128/s_fma.c
>   sysdeps/ieee754/soft-fp/s_fma.c
>   
> All but the sofp-fp uses the floating-point exceptions to change rounding and
> check for exceptions. I think the soft-fp would most likely be a better
> default even for ldbl-96/ldbl-128.

We don't have sfp-machine.h with support for hardware exceptions / 
rounding modes on all architectures that have such exceptions and rounding 
modes, which is required to use the soft-fp fma implementation for such an 
architecture.  (Likewise for the narrowing functions that also have 
sysdeps/ieee754/soft-fp/ implementations and a default implementation 
using round-to-odd that relies on changing the hardware rounding mode and 
reading hardware exception flags - in both cases I don't know how to 
performance of the two approaches compares.  Though I suspect that the 
ldbl-128 implementation of (double) fma is very unlikely to be the best 
approach in those cases where ldbl-128 itself is implemented based on 
soft-fp.)

-- 
Joseph S. Myers
josmyers@redhat.com