Re: How to use a scratch register in "jump" pattern ?

[Georg-Johann Lay <avr at gjlay dot de>]

Stelian Pop schrieb:
> On Tue, Mar 10, 2009 at 10:18:10PM +0100, Georg-Johann Lay wrote:
>
>
> > Note that no one is generating an insn that looks like "*jump". Maybe  
> > insn combine and peep2 would try to build such a pattern, but that is  
> > not what helpd you out here. Write the expander as parallel of a (set  
> > (pc) ...) and a (clobber (match_scratch ...)) so that an appropriate  
> > insn is expanded.
>
>
> Like this ?
>
> (define_expand "jump"
>   [(parallel [(set (pc) (label_ref (match_operand 0 "" "")))
>               (clobber (match_scratch:QI 1 "=&r"))])]
>   ""
>   ""
> )

Constraints are useless in expanders. You really need a QI scratch? Your 
insn looks as if HI was needed as you load hi and lo part of the scratch.

> > Also note that "*jump" is an (implicit) parallel. As  
> > constraint for the "*jump" insn use an "X" in the case you do not need  
> > the clobber reg and sth. like "=&r" in the case you really need it.
>
>
> Ok, but the decision on if I need the clobber reg or not is based
> on the 'length' attribute. So if I could write the following, but where
> I can put the calculation and the test of the 'length' attribute:
>
> (define_insn "*jump_internal"
>    [(set (pc)
>          (label_ref (match_operand 0 "" "")))
>     (clobber (match_scratch:QI 1 "X,=&r"))]
>    ""
>    "@
>    rjmp %0
>    ldih %1,hi(%l0)\n\t\n\tldil %1,lo(%l0)\n\tijmp (%1)"
> )

You cannot destinguish the two cases by means of constraints; computing 
jump distances is not yet known in global alloc. I guess reload will 
always take the first alternative: In your example #0 has no 
constraints. Same would apply if constraints #1 were same as constraints 
#1).

So you have just one alternative "=&r", and in the case of rjmp the 
scratch is unused. Note that you emit up to three instructions, so the 
-2048/2047 may crash in corner cases.

You can have a look at avr, it computes jump lengths as well but afaIr 
without needing a scratch for long jumps.

Georg-Johann