Re: [tree-ssa]: Loop normalization pass

[Steven Bosscher <s dot bosscher at student dot tudelft dot nl>]

Op di 14-01-2003, om 17:36 schreef Daniel Berlin:
> 
> On Tuesday, January 14, 2003, at 11:12  AM, Steven Bosscher wrote:
> 
> > Daniel Berlin wrote:
> >> Normalizes loops to start at 1 (or would we rather 0), and have
> >>  a stride of 1.
> >>
> > ---- 8< ----
> >>
> >> Since it wants loops to start at 1, rather than 0, it normalizes
> >> a *ton* of loops, and thus, is pretty well tested in that respect.
> >
> > If you are suggesting that most loops are only "normalized" because
> > their index starts at 0, then wouldn't it be better indeed to have
> > normalized loops start at 0 too?
> 
> Sure. I honestly don't care.
> All the fortran loop optimizers i have source to normalize to 1

Because the first element in a Fortran array has index 1.

> All the C loop optimizers I have source to normalize to 0.

Because the first element in a Fortran array has index 0.

> I assume this is because these are the common lower bounds for these 
> languages.
> So I was at a loss which to choose, since we have both, and I presumed 
> fortran95 is written by fortran programmers, and thus, they probably 
> start their loops at 1 as well.

But the offset of the first element from the array base is still 0, of
course.  So actually the Fortran compiler has to change all those
indices to (index - 1), e.g.,

INTEGER i, A(10), B(10)
DO, i=1:10
   A(i) = B(i)
END DO

will eventually be transformed to:

integer a[10], b[10];
for (int i = 1; i<=10; i++)
  a[i - 1] = b[i - 1];

or actually the equivalen while(1) form of this.  See what happens if
you normalize this loop to 0?

Therefore my guess is that normalizing to 0 is the best choice.

Greetz
Steven