Re: [PATCH] Fix optimization regression in constant folder

[Ian Lance Taylor <iant at google dot com>]

kenner@vlsi1.ultra.nyu.edu (Richard Kenner) writes:

> > TYPE_OVERFLOW_WRAPS seems clearly correct for sizetypes (if not, why
> > not, and give us an example).
> 
> For the reason you just gave: it suppresses some optimizations.

It doesn't, though.  Having TYPE_OVERFLOW_WRAPS be true for a type
does not cause gcc to avoid any optimizations.

(Not having TYPE_OVERFLOW_UNDEFINED be true does avoid optimizations;
having TYPE_OVERFLOW_WRAPS be true does not.)


> > I really want to move toward a semantics for sizetypes which does not
> > involve mysterious checks for TYPE_IS_SIZETYPE.  If we can't do that,
> > we are going to keep breaking the way you want it to work.
> 
> I see it exactly the opposite way around: sizetype means "handle this
> computations in the most efficient way possible" (like my EXACT_DIV_EXPR).
> So any optimization conditionalized on an attribute of the type can simply
> can "|| TYPE_IS_SIZETYPE".  But if we go the approach of defining flags in
> a special way to deal with case and then get rid of TYPE_IS_SIZETYPE, we 
> have to go through exactly the same problem as now if some other 
> attributes are added to types that suppress only some optimizations.

I think that approach winds up being confusing.  We have to think
about types in terms of what operations are permitted on them.

Ian