approaches to carry-flag modelling in RTL

[Richard Henderson]

On 11/01/2011 02:59 PM, Hans-Peter Nilsson wrote:
> 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.

Yes.  The cc_reg last there is a consequence of the post-reload compare-elim pass.
Which wants to splat the CC set on top of the clobber.  Thus it must be last.

> 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.

Frankly, I'd prefer to flip the default.  It does seem to make the most sense.
We shan't do that until we tackle...

> 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.

... this, so that it can all be done automatically.

I think some sort of iterator mechanism is best, much like we
automate cond_exec and other sorts of macro-izations.

I can imagine a sort of

(define_insn_with_flags "name"
  [(set ...)
   (set (reg:cc) (...)]
  ""
  ...)

where we automatically expand to

(define_insn "name"
  [(set ...)
   (clobber (reg:cc))]
  ...)

(define_insn "name_flags"
  [(set ...)
   (set (reg:cc) (...)]
  ...)

or something like that.


r~