Re: approaches to carry-flag modelling in RTL

[Hans-Peter Nilsson <hp at bitrange dot com>]

Please, when replying, also send to me, not just the list.

On Tue, 1 Nov 2011, Paulo J. Matos wrote:
> On 01/11/11 02:43, Hans-Peter Nilsson wrote:
> >
> > Not obvious or maybe I was unclear as to what I alluded?
> > In the below insn-bodies, "sub" is the insn that sets cc0 as a
> > side-effect.
> >
> > Supposed canonical form :
> >
> > (parallel
> >   [(set cc_reg) (compare ...))
> >    (set destreg) (sub ...))])
> > and:
> > (parallel
> >   [(set destreg) (sub ...))
> >    (clobber cc_reg)])

> That is very strange because if you look into RX or MN10300, they all have the
> set REG_CC as the last in the parallel.

That'd be a good reason to flip the default...except that the
i386 has it the other way round i.e. as shown above.  I think
the main reason is that it just seemed right to those port
authors.

> I wonder if it has anything to do with
> the fact that in these backends the set of the REG_CC only shows up after
> reload.

Right, it'd only matter where (also) GCC cooks up combinations
(which IIRC it doesn't if the register is only exposed
post-reload), not where only the port emits them.  N.B., it
*could* very well be that I misremember about the canonical
form, but it seems neither of us bother to search the archives,
so never mind. ;)
...oh wait, see the comments at combine.c:2824 and 3030 r180744.
I can't find anything in the docs, but that might just be my
grep-fu failing.

I'm still thinking of a generic md iterator mechanism (one that
doesn't restrict the form of the expansion in ways getting in
the way with expanding to both a clobber and a set, and in
swapped locations as above), to make the troubles go away...
But maybe expanding them by a pass through e.g. m4 would be
better than cooking up something new there.

brgds, H-P