Quoting Richard Henderson <rth@redhat.com>:

> See fnma for the named pattern.
>
>> Besides, there is no builtin for fmssf4 anyway.
>
> We look for all of the variants with extra negations.  Try
>
>   __builtin_fma (-a, b, c)

Thanks, that did the trick. (__builtin_fmaf to be more exact.)