So you'd need to be more precise as to what properties you are trying
to preserve by placing a single stmt somewhere.
FWIW an earlier, abandoned attempt at solving the same problem was
discussed in the following thread, continuing through June:
https://gcc.gnu.org/ml/gcc-patches/2015-05/msg02647.html
Though the details of lowering of OpenACC constructs have changed with
Nathan's current patches, the underlying problem remains the same. PTX
requires certain operations (bar.sync) to be executed uniformly by all
threads in a CTA. IIUC this affects "JOIN" points across all
workers/vectors in a gang, in particular (though this is generic code,
other -- particularly GPU -- targets may have similar restrictions).