How can I write an empty conversion instruction

[Richard Earnshaw]

On Sat, 2005-05-07 at 04:14, Richard Henderson wrote:
> On Fri, May 06, 2005 at 01:59:06PM -0700, Steve Ellcey wrote:
> > I was wondering if anyone could tell me how to write an (empty)
> > instruction pattern that does a truncate/extend conversion on a register
> > 'in place'.
> > 
> > All the conversions I see are like this one in ia64/ia64.md:
> > 
> > (define_insn "extendsfdf2"
> >   [(set (match_operand:DF 0 "fr_register_operand" "=f")
> >         (float_extend:DF (match_operand:SF 1 "fr_register_operand" "f")))]
> >   ""
> >   "fnorm.d %0 = %1"
> >   [(set_attr "itanium_class" "fmac")])
> > 
> > Where the source and the destination may or may not be the same
> > register.
> 
> The best way is to have a post-reload splitter that splits the insn
> into nothing at all.

Is that really valid?  I would have thought it would break the data
flow.

R.