Re: Even numbered register pairs restriction on some instructions

[Matthew Malcomson <matthew dot malcomson at arm dot com>]

On 31/08/18 18:06, Jeff Law wrote:
> On 08/31/2018 09:41 AM, Matthew Malcomson wrote:
> > Hi there,
> >
> > I'm looking into whether it's possible to require even numbered
> > registers on
> > modes that need more than one hard-register to represent them. But only in
> > some cases.
> >
> > The problem is the one mentioned explicitly here
> > https://gcc.gnu.org/onlinedocs/gccint/Register-Classes.html about
> > enforcing a
> > requirement for a register pair to start with an even-numbered register.
> > I can't use TARGET_HARD_REGNO_MODE_OK as suggested in that document
> > because I'd
> > like to allow register pairs starting with odd-numbered registers in
> > general,
> > but not in some specific cases.
> >
> >  From a comment in gcc/config/sparc/constraints.md above the constraint
> > "U" it
> > looks like there isn't a way to define such a constraint when using LRA,
> > and I
> > haven't found any hook that could do so but I'm hoping there's something
> > I'm
> > missing.
> >
> > The best thing I can think of is to define a bunch of classes ZERO_ONE,
> > TWO_THREE, FOUR_FIVE, ..., define a constraint on each of those classes,
> > then
> > use all the constraints as one alternative in my pattern.
> >
> > Is there a better method?
> >
> > Is there some small change that could enable a nicer method?
> It would help to know more about when you want to allow odd registers.
For concreteness: I'm looking at TImode registers that will span 2 hard 
registers.

In most cases it's fine for these register pairs to start at either an 
odd or even hard register.

I'd like to restrict TImode registers to even registers only in one or 
two patterns.
(for concreteness: in order to use the aarch64 casp instruction)
> I'd be surprised if creating a class, but still rejecting in
> HARD_REGNO_MODE_OK worked...
Sorry: I don't know what you mean by creating a class and rejecting it 
in HARD_REGNO_MODE_OK.
As far as I can tell, HARD_REGNO_MODE_OK is only given information on 
the register number and
the mode of the value, so it knows nothing about the register class any 
constraint may be applying.

I mentioned HARD_REGNO_MODE_OK just because the documentation page I 
linked to said that was
the way to implement limiting register pairs to those beginning with an 
even numbered register and I
wanted to explain why this wasn't applicable in my case (due to not 
being able to limit the restriction
to certain patterns).
> Jeff
Matthew