Re: where exactly c++,c fail and Ada gets thru'

[Keith Thompson <kst-u@mib.org>]

Maciej Sobczak <no.spam@no.spam.com> writes:
> 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.

Actually, I think it does; it just doesn't define to what extent
compilers are *required* to optimize.  (In C, allowed optimizations
are determined by what's loosely called the "as-if rule"; C++ is
probably similar.)

>                                         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?

Probably the point was that range checks are an intrinsic feature of
Ada, and any decent Ada compiler will optimize them as much as
practical.  In C++, on the other hand, any range checks would have to
be done as explicit user code, and a typical C++ compiler might be
less likely to recognize an idiom that it can optimize.

A sufficiently clever C++ compiler should be able, in most cases, to
optimize as well as a sufficiently clever Ada compiler (though I
suspect Ada's aliasing rules allow for better optimizations in some
cases).  I have no idea whether current C++ compilers are
"sufficiently clever".

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
We must do something.  This is something.  Therefore, we must do this.