Re: Pointer Operations in Chapel? (for swapping variables)

[Brad C. <br...@cr...>]

Hi Max --

There is no direct equivalent to pointer swapping at the user level in 
Chapel since Chapel arrays are not pointers and in general two arrays may 
have different memory layouts that would prevent this style of 
implementation.

That said, there are cases where the implementation could implement the 
equivalent of a pointer swap as an optimization: for example, when the 
swap operator ( <=> ) is applied to two arrays declared over the same 
domain, the underlying memory buffers used to implement those arrays could 
be swapped.  Implementing this optimization would be a joint effort 
between the compiler and the domain map author (the person implementing 
the array's implementation). Unfortunately, we do not have support for 
this optimization in our compiler as of yet.

In the meantime, you can either use an array of arrays as suggested in 
Raphael's follow up mail (or equivalently, a pair of array aliases) in 
order to make the swapping logical; or you could unroll your loop manually 
two times and have each be written in terms of one of the arrays. 
Neither is quite as elegant as the proposed array swap optimization above.

Note that the C you propose isn't quite identical to the Chapel code in 
that the Chapel is only exchanging the interiors of the two arrays whereas 
the C code is exchanging the entire array contents.  This isn't a problem 
if the boundary conditions are set up identically in both arrays.  If we 
had done that in Chapel, we could have written the assignment as:

 	X = XNew;

or used the swap operator as follows:

 	X <=> XNew;

-Brad


>> // update X with next approximation
>> X[ProblemSpace] = XNew[ProblemSpace];
>
> If we were writing this in C, we would simply swap the pointers, and
> save ourselves the memory copy.
>
>> float* temp;
>> temp = X; // assume X and XNew are of type float*
>> X = XNew;
>> XNew = temp;
>
> Is there good way to do this in Chapel?  I thought of a workaround where
> we use a single large array and alternate having X be at the beginning
> of it, or half-way down (with XNew visa-versa) so that there is no
> memory transfer, but this seems a bit of a hack.
>
> thanks,
>
> Max
>
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