🚀 go-pugleaf

In article <764@ubbpc.UUCP> wgh@ubbpc.UUCP (William G. Hutchison) writes:
  [ vectorizing in Fortran versus C ]
> In C, on the other hand, my impression is that the programmer must do his/her
> vectorizing "by hand"; that compilers cannot do as much of the work
> automatically.

The Convex compiler easily recognizes all the standard loop idioms. With
-va it treats all formal array parameters as unaliased; I've never had
to use any directives other than this.

>  Would you please post a page or so of a representative sample of your
> vectorized C so we can look at the coding techniques?  Thanks!

There's nothing really special... Here's a slightly interesting example.
The compiler knows that x and y are unaliased. neg() is a vectorizable
operation.

#ifdef VECTOR
   for (i =     0; i <  k; i++) y[i] = neg(x[i + r - k]);
   for (i =     k; i <  r; i++) y[i] = x[i - k];
   for (i =     0; i <  r; i++) x[i] = y[i];
#else
   for (i =     0; i <  k; i++) y[i] = x[i + r - k];
   for (i = r - 1; i >= k; i--) x[i] = x[i - k];
   for (i =     0; i <  k; i++) x[i] = neg(y[i]);
#endif

In this ANSIfied age I should set up a third version, like the second
but based on memmove(); but I doubt that'll be faster than the first on
most vector machines.

---Dan