Re: wide-int branch now up for public comment and review

[Kenneth Zadeck <zadeck at naturalbridge dot com>]

On 08/28/2013 05:08 AM, Richard Biener wrote:
> On Sun, 25 Aug 2013, Mike Stump wrote:
>
> > On Aug 25, 2013, at 12:26 AM, Richard Sandiford <rdsandiford@googlemail.com> wrote:
> > > (2) Adding a new namespace, wi, for the operators.  So far this
> > >     just contains the previously-static comparison functions
> > >     and whatever else was needed to avoid cross-dependencies
> > >     between wi and wide_int_ro (except for the debug routines).
> > It seems reasonable; I don't see anything I object to.  Seems like most of the time, the code is shorter (though, you use wi, which is fairly short).  It doesn't seem any more complex, though, knowing how to spell the operation wide_int:: v wi:: is confusing on the client side.  I'm torn between this and the nice things that come with the patch.
> >
> > > (3) Removing the comparison member functions and using the static
> > >     ones everywhere.
> > I've love to have richi weigh in (or someone else that wants to play the
> > role of C++ coding expert)?  I'd defer to them?
> Yeah - wi::lt (a, b) is much better than a.lt (b) IMHO.  It mimics how
> the standard library works.
>
> > > The idea behind using a namespace rather than static functions
> > > is that it makes it easier to separate the core, tree and rtx bits.
> > Being able to separate core, tree and rtx bits gets a +1 in my book.  I
> > do understand the beauty of this.
> Now, if you look back in discussions I wanted a storage
> abstraction anyway.  Basically the interface is
>
> class wide_int_storage
> {
>    int precision ();
>    int len ();
>    element_t get (unsigned);
>    void set (unsigned, element_t);
> };
>
> and wide_int is then templated like
>
> template <class storage>
> class wide_int : public storage
> {
> };
>
> where RTX / tree storage classes provide read-only access to their
> storage and a rvalue integer rep to its value.
>
> You can look at my example draft implementation I posted some
> months ago.  But I'll gladly wiggle on the branch to make it
> more like above (easy step one: don't access the wide-int members
> directly but via accessor functions)
you are of course welcome to do this.   But there were two questions 
that i never got an answer to.

1) how does doing this help in any way.   Are their clients that will 
use this?

2) isn't this just going to slow wide-int down?  It seems clear that 
there is a real performance cost in that now there is going to be an 
extra step of indirection on each access to the underlying data structures.

I will point out we have cut down on the copying by sharing the 
underlying value from trees or rtl when the value is short lived. But 
given that most constants only take a single hwi to represent, getting 
rid of the copying seems like a very small payback for the increase in 
access costs.
> > > IMO wide-int.h shouldn't know about trees and rtxes, and all routines
> > > related to them should be in tree.h and rtl.h instead.  But using
> > > static functions means that you have to declare everything in one place.
> > > Also, it feels odd for wide_int to be both an object and a home
> > > of static functions that don't always operate on wide_ints, e.g. when
> > > comparing a CONST_INT against 16.
> Indeed - in my sample the wide-int-rtx-storage and wide-int-tree-storage
> storage models were declared in rtl.h and tree.h and wide-int.h did
> know nothing about them.
this could be done for us too, it has nothing to do with the storage model.
> > Yes, though, does wi feel odd being a home for comparing a CONST_INT and
> > 16?  :-)
> >
> > > I realise I'm probably not being helpful here.
> > Iterating on how we want to code to look like is reasonable.  Prettying
> > it up where it needs it, is good.
> >
> > Indeed, if the code is as you like, and as richi likes, we'll then our
> > mission is just about complete.  :-)  For this patch, I'd love to defer
> > to richi (or someone that has a stronger opinion than I do) to say,
> > better, worse?
> The comparisons?  Better.
richard s is making these static.
> Thanks,
> Richard.