Re: Autoparallelization

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello,

> > I have commited the following patch to parloop branch.  It implements
> > automatic parallelization for simple loops; at the moment, code
> > generation only creates static schedule into fixed number of threads
> > (specified at compile time).  This should more or less be the version
> > I would like to get merged to 4.3 (I probably won't have time to
> > implement more features in following few weeks).
> > 
> I have started going through the code.  One question regarding the
> approach: Why didn't you folks generate OMP_PARALLEL/OMP_FOR?  The
> intent was for auto-parallelization passes to target the OMP IL, not
> libgomp.

I thought about this and considered it a bit easier to start by
targetting libgomp directly, at least for the initial implementation.
Note that it does not necessarily mean much code duplication -- we still
mostly use the same functions as the omp lowering pass (with a few extra
wrappers in tree-parloop.c to ensure that the data structures used
during the optimizations, most importantly loop structures and
dominators, are preserved).

There is a functionality duplication in the loop schedule generation,
which is something I would like to fix; I believe it would be nice to
have exactly the same functions to do the code generation as the
expansion code for OMP_FOR.  If this were possible, it would not make
much sense to me to generate the OMP_PARALLEL/OMP_FOR statements just to
lower them immediatelly, considering both the compile time overheads --
an extra lower_omp pass would be necessary -- and complexity of the
changes to omp lowering code to make it work during optimizations (many
of them to the parts that would not be used by the parallelization
code).

Zdenek