On Wed, May 12, 2021 at 09:13:38AM +0000, Tamar Christina wrote:
> > From: Segher Boessenkool <seg...@kernel.crashing.org>
> > On Tue, May 11, 2021 at 05:37:34AM +0000, Tamar Christina via Gcc wrote:
> > > 2. Saturating abs:
> > > char sat (char a)
> > > {
> > > int tmp = abs (a);
> > > return tmp > 127 ? 127 : ((tmp < -128) ? -128 : tmp);
> > > }
> >
> > That can be done quite a bit better, branchless at least. Same for all
> > examples here probably.
>
> Do you mean in the example? Sure, I wanted to keep it simple 😊
Point taken... I think you need to look at these issues sooner rather
than later though.
> > > 2. Saturation:
> > > a) Use match.pd to rewrite the various saturation expressions into
> > min/max
> > > operations which opens up the expressions to further optimizations.
> >
> > You'll have to do the operation in a bigger mode for that. (This is also
> > true for
> > rounding, in many cases).
> >
> > This makes new internal functions more attractive / more feasible.
>
> True, but the internal function doesn't need to model the wider mode right?
> So if you're saturating an int, the internal-fn would work on int,int,int.
The ifn doesn't have to go to a wider mode, it's only if you want to
express it with more basic operations that you have to. That is the
reason I find ifns more attractive for this, yup.
> > > We could get the right instructions by using combine if we don't
> > > rewrite
> > > the instructions to an internal function, however then during
> > Vectorization
> > > we would overestimate the cost of performing the saturation. The
> > constants
> > > will the also be loaded into registers and so becomes a lot more
> > > difficult
> > > to cleanup solely in the backend.
> >
> > Combine is almost never the right answer if you want to combine more than
> > two or three RTL insns. It can be done, but combine will always write the
> > combined instruction in simplest terms, which tends to mean that if you
> > combine more insns there can be very many outcomes that you all need to
> > recognise as insns in your machine description.
>
> Yeah, ideally I would like to handle it before it gets to expand.
That is the right place yeah... but it will need some RTL work, in
simplify-rtx at least.
Segher
