negative strides

["E. Robert Tisdale" <edwin at netwood dot net>]

          subroutine  dcopy(n,dx,incx,dy,incy)
    c
    c     copies a vector, x, to a vector, y.
    c     uses unrolled loops for increments equal to one.
    c     jack dongarra, linpack, 3/11/78.
    c     modified 12/3/93, array(1) declarations changed to array(*)
    c
          double precision dx(*),dy(*)
          integer i,incx,incy,ix,iy,m,mp1,n
    c
          if(n.le.0)return
    c
    c        code for unequal increments or equal increments
    c          not equal to 1
    c
          ix = 1
          iy = 1
          if(incx.lt.0)ix = (-n+1)*incx + 1
          if(incy.lt.0)iy = (-n+1)*incy + 1
          do 10 i = 1,n
            dy(iy) = dx(ix)
            ix = ix + incx
            iy = iy + incy
       10 continue
          return

The BLAS library is perfectly happy with negative strides.
Many implementations work with zero strides as well.
But the GSL vector class supports only positive strides
which means that if precludes the application program
from taking full advantage of the BLAS library.