Re: Teaching GCC about fmadd variant

[Daniel Jacobowitz <drow@mvista.com>]

On Mon, Apr 22, 2002 at 05:47:10PM +0930, Daniel Glastonbury wrote:
> Hi,
>   I'm looking for some help in teaching gcc how to use the fmadd variants
> that my mips has.  Unlike the mips IV madd instruction, this mips has a
> variant that goes something like this:
> 
> mula  fs, ft    : ACC <- fs*ft
> madda fs, ft    : ACC <- ACC + fs*ft
> madd  fd, fs, ft: fd <- ACC + fs*ft
> 
> The problem is there is no general way to move to/from the ACC register.

(Over my head, but) do you still get reasonable benefit from this if
you define simple-minded patterns to move from/to the accumulator? 
i.e.:
   load fs into ACC: load 1.0 into ft; mula fs, ft
   load ACC into fd: load 0.0 into ft; madd fd, ft, ft

On the other hand, this won't help if you were actually seeing it use
the ACC unnecessarily.  I believe that would need to be worked around
with costs.  You're going to have problems with combine (discussed on
one GCC list, probably this one, a few weeks ago) getting fmadd to
work well, also.

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer