🚀 go-pugleaf

In article <l7gi3fINNmsd@utkcs2.cs.utk.edu>, eijkhout@cupid.cs.utk.edu 
(Victor Eijkhout) writes...
> 
>I would like to get an idea of the difference in performance
>between inner products
> 
>       do i=1,n    x = x + a(i)*b(i)
> 
>and axpy operations
> 
>       do i=1,n   x(i) = x(i) + a*b(i)
> 
>which both have the same number of operations, but the inner product
>has an accumulation, which traditionally seems to be an
>unvectorizable idea.
> 

I tried this with VAX FORTRAN-HPO; as long as one uses the 
/ASSUME=NOACCURACY_SENSITIVE qualifier so that the dot product's reduction
transformation can be performed (the default is ACCURACY_SENSITIVE which
disables transformations that could yield different results than scalar
execution), both forms vectorize very nicely.  (The dot product form,
of course, has a final reduction step that the "axpy" form doesn't need.)
The actual vector mul-add sequences are essentially the same between the two.

Of course, one can also use the BLAS SDOT and SAXPY intrinsics, which
VAX FORTRAN-HPO will expand and vectorize (and parallelize, if you like.)

Steve Lionel                    lionel@quark.enet.dec.com
SDT Languages Group
Digital Equipment Corporation
110 Spit Brook Road
Nashua, NH 03062