Re: Optimizing of explicit temporary storage

[Richard Guenther <rguenth at tat dot physik dot uni-tuebingen dot de>]

Mike Stump wrote:
> On Sunday, October 10, 2004, at 07:48  AM, Richard Guenther wrote:
>
> > Is it legal for a conforming C++ compiler to optimize away storage 
> > allocation / deallocation?  Like in
> >
> > #include <new>
> >
> > void foo(void)
> > {
> >         int *x = new(std::nothrow) int;
> >         delete x;
> > }
> >
> > is it allowed to kill the new and delete statements?  Would it be 
> > allowed to do this for the throwing version of new, too?
>
> The as if rule is fairly powerful...  If the resulting program behaves 
> as if it had it, then the optimization is allowed.  I don't think memory 
> consumption is an externally visible effect.  Now, we might need to do 
> full program analysis to know which new is run, and to know its 
> semantics; I don't recall if the user is prohibited from adding 
> semantics to a new replacement function.

Well, I was thinking of the traditional

struct Vector {
   Vector(int size);
   Vector operator*(const Vector&);
   Vector& operator=(const Vector&);
   double *storage;
};

and doing expressions like

Vector a,b,c;
a = b*c;

where one usually considers using expression templates because of the 
temporaries.  With loop fusion one could get the speed of the expression 
template code, but are you allowed to get rid of the temporaries (and 
their allocated memory), too?

Richard.