The bug in PR70289 is an assertion failure triggered by a static
variable used inside an offloaded acc region which doesn't have a data
clause associated with it. Basically, that static variable ends up in a
different lto partition, which was not streamed to the offloaded
compiler. I'm not sure if we should try to replicate the static storage
in the offloaded regions, but it probably doesn't make sense in a
parallel environment anyway.

My solution to this problem was to teach the fortran front end to create
a data clause for each reduction variable it encounters. Furthermore,
I've decided to update the semantics of the acc parallel reduction
clause such that gfortran will emit an error when a reduction variable
is private or firstprivate (note that an acc loop reduction still works
with private and firstprivate reductions, just not acc parallel
reductions). The second change is to emit a warning when an incompatible
data clause is used with a reduction, and promote that data clause to a
present_or_copy. My rationale behind the promotion is, you cannot have a
copyin reduction variable because the original variable on the host will
not be updated. Similarly, you cannot have a copyout reduction variable
because the reduction operator is supposed to combine the results of the
reduction with the original reduction variable, but in copyout the
original variable is not initialized on the accelerator. But perhaps the
copyout rule is too strict?

Tom and I are still working with the OpenACC technical committee to get
some clarification on how the reduction value should behave with respect
to data movement. In the meantime, I wanted to see if this type of
solution would be appropriate for trunk. I was trying to get this to
work in the gimplifier so that one patch could resolve the problem for
all of the front ends, but that was happening too late. Especially for
reference types.

By the way, we will also benefit from this patch too
<https://gcc.gnu.org/ml/gcc-patches/2016-01/msg00482.html>. If not for
these reduction, but for global acc variables which haven't been
properly declared.

Cesar