Re: Ways to add source annotations to a loop

[David Malcolm <dmalcolm at redhat dot com>]

On Mon, 2012-12-03 at 12:19 +0100, Richard Biener wrote:
> On Mon, Dec 3, 2012 at 11:05 AM, Alexey Kravets <mr.kayrick@gmail.com> wrote:
> > Hello,
> > I am looking for a way to implement source annotation (or something
> > similar) for a for loops. Basically, I need some mechanism to mark
> > certain for loops in the source code for the GIMPLE optimization
> > passes (for C/C++ only currently).
> > For example something like this:
> >
> > int i;
> > __attribute__((marked)) // Not compilable by GCC.
> > for (i =0; i < 100; i++) {
> >    foo (i);
> > }
> >
> > or
> >
> > int i;
> >
> > #pragma marked // Can be compiled, but require additional GIMPLE tree nodes.
> > for (i =0; i < 100; i++) {
> >    foo (i);
> > }
> >
> >
> > As far as I know, for loops are lowered to jumps by C front-end and
> > natural loops are detected later, so there is no strait way of passing
> > such information.
> > OpenMP pragmas provides such functionality, but require additional
> > GIMPLE nodes to be defined and carried through all optimization
> > passes,
> > which I would prefer to avoid (if possible). So, is there any strait
> > way to implement this?
> 
> A natural way would be to attach such information to the on-the side
> loop structure tree (cfgloop.h, struct loop).  Unfortunately it is build
> quite late, so at the moment you'd have to use a combination of
> GIMPLE nodes and later transitioning it to loop struct annotations.
> There is, of course, no reason to not at some point in the future
> let frontends populate the initial loop tree and keep that up-to-date.

I don't know if this gives you what you need, but an approach that
occurred to me is to mark the iterator variable with a custom attribute:

int i __attribute__((iterator_for_one_of_the_interesting_loops));
for (i =0; i < 100; i++) {
   foo (i);
}

Perhaps you could use that to detect the loops of interest: the
initialization, the comparison, and the increment?

Hope this is helpful
Dave