[patch, libfortran] Speed up cshift for dim > 1

[Jerry DeLisle]

On 06/14/2017 12:41 PM, Thomas Koenig wrote:
> Hello world,
> 
> the attached patch implements a blocked algorithm for
> improving the speed of cshift for dim > 1.
> 
> It uses the fact that
> 
>   integer, dimension (n1,n2,n3) :: a, b
> 
>   b = cshift(a,shift,3)
> 
> is identical, as far as the memory locations is concerned.
> 
>   integer, dimension (n1*n2*n3) :: c, d
>   d = cshift(c, shift*n1*n2, 1)
> 
> The speedup is quite large; from being really slow for
> dim > 1, this patch makes it go even faster.
> 
> Below there are some comparisons for the attached benchmark,
> do-1.f90. gfortran-7 uses the old library version.
> 
> Interestingly, the library version is also much faster
> than an implementation of straight DO loops.
> 
> Regression-tested.  OK for trunk?
> 

OK for trunk.

Thanks,

Jerry