Re: Constraint modifier for partially overlaping operands

[Richard Henderson <rth at redhat dot com>]

On 10/16/2009 11:04 PM, Ian Lance Taylor wrote:
> Andrew Hutchinson<andrewhutchinson@cox.net>  writes:
>
> > I can use "=" modifier to make operands use same register and early
> > clobber "&" to avoid overlaps.
> >
> > Is it possible to have or construct a contraint that permits partial
> > overlap operands. (which neither = or&  would allow)
> > The case would be  wide types taking multiple hard registers.
> >
> > eg Input r20..23 Output r22..25
>
> There is no such constraint today.  I suppose it would be possible to
> define such a constraint if it seemed useful.

I'd much prefer if the port decomposed its double word operations and 
used the lower-subreg pass to decompose the double word registers.  At 
which point the register allocator has all of the information it needs 
to do the right thing.


r~