[Mesa-dev] [RFC 0/9] Add precise/invariant semantics to TGSI

[Ilia Mirkin]

On Mon, Jun 12, 2017 at 7:57 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> FWIW surely on nv50 you could keep a single mad instruction for umad
> (sad maybe too?). (I'm actually wondering if the hw really can't do
> unfused float multiply+add as a single instruction but I know next to
> nothing about nvidia hw...)

The compiler should reassociate a mul + add into a mad where possible.
In actuality, IMAD is actually super-slow... allegedly slower than
IMUL + IADD. Not sure why. Maxwell added a XMAD operation which is
faster but we haven't figured out how to operate it yet. I'm not aware
of a muladd version of fma on fermi and newer (GL 4.0). The tesla
series does have a floating point mul+add (but no fma).