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From: "Martin Krischik" <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: where exactly c++,c fail and Ada gets thru'
Date: 28 Apr 2006 00:00:23 -0700
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Date: 2006-04-28T00:00:23-07:00
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Maciej Sobczak wrote:

> Martin Krischik wrote:
>
> > Now say we have a suitable C++ range template:
> >
> > typedef range<int, 1, 10> Y_Type;
> > typedef range<Y_Type, 1, 5> X_Type;
> >
> > Y_Type F (X_Type X)
> >   {
> >   return X + 3;
> >   }
> >
> > Y_Type G (X_Type X)
> >   {
> >   return X + 7;
> >   }
> >
> > Now anyone up to the challenge to define:
> >
> > template <typename Base_Type, Base_Type First, Base_Type Last>
> > range::operator = (Base_Type right)
> >
> > template <typename Base_Type, Base_Type First, Base_Type Last>
> > range::operator + (Base_Type right)
> >
> > template <typename Base_Type, Base_Type First, Base_Type Last>
> > range::range (Base_Type right)
> >
> > in such a way that it works like Ada - inclusive the *realistic* change that
> > the compiler will optimize away the range check in F ().
>
> The C++ standard does not define to what extent the compilers are
> allowed to optimize the resulting code. This is the quality of
> implementation issue and an area of competition between compiler
> vendors. Without any further checks I can claim that it is possible for
> the compiler to optimize it the way you want.
>
> And without any further checks I take the risk to claim that the Ada
> standard does not require any conforming compiler to optimize range
> checks away as you described (or please throw some references) -
> similarly, this is the QoI issue.
>
> So - what was your point?

The point is - of corse -  it that the semantic tree of the Ada
compiler contains the information that X will be in the range of (1 ..
5) and then the optimizer can easily calculate that (1 .. 5) + 3 < 10
and then optimize away the check.

The sematinc tree of the C++ will contain the information that X is in
the range of (-2*32 .. +2*32-1) and that is a pretty useless
information to the optimizer.

Martin