Re: [PATCH, 4/16] Implement -foffload-alias

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Nov 13, 2015 at 12:29:51PM +0100, Richard Biener wrote:
> > thanks for the explanation. Filed as PR68331 - '[meta-bug] fipa-pta issues'.
> > 
> > Any feedback on the '#pragma GCC offload-alias=<none|pointer|all>' bit above?
> > Is that sort of what you had in mind?
> 
> Yes.  Whether that makes sense is another question of course.  You can
> annotate memory references with MR_DEPENDENCE_BASE/CLIQUE yourself
> as well if you know dependences without the users intervention.

I really don't like even the GCC offload-alias, I just don't see anything
special on the offload code.  Not to mention that the same issue is already
with other outlined functions, like OpenMP tasks or parallel regions, those
aren't offloaded, yet they can suffer from worse alias/points-to analysis
too.

We simply have some compiler internal interface between the caller and
callee of the outlined regions, each interface in between those has
its own structure type used to communicate the info;
we can attach attributes on the fields, or some flags to indicate some
properties interesting from aliasing POV.  We don't really need to perform
full IPA-PTA, perhaps it would be enough to a) record somewhere in cgraph
the relationship in between such callers and callees (for offloading regions
we already have "omp target entrypoint" attribute on the callee and a
singler caller), tell LTO if possible not to split those into different
partitions if easily possible, and then just for these pairs perform
aliasing/points-to analysis in the caller and the result record using
cliques/special attributes/whatever to the callee side, so that the callee
(outlined OpenMP/OpenACC/Cilk+ region) can then improve its alias analysis.

	Jakub