Re: [patch,openacc] Propagate independent clause for OpenACC kernels pass

[Julian Brown <julian@codesourcery.com>]

On Thu, 20 Sep 2018 11:06:40 -0700
Cesar Philippidis <cesar@codesourcery.com> wrote:

> This is another old patch teaches the omp expansion pass how to
> propagate the acc loop independent clause to the later stages
> throughout compilation. Unfortunately, it didn't include any test
> cases. I'm not sure how effective this will be with the existing
> kernel parloops pass. But as I noted in my Cauldron talk, we would
> like to convert acc kernels regions to acc parallel regions, and this
> patch could help in that regard.
> 
> Chung-Lin, do you have anymore state on this patch?
> 
> Anyway, I bootstrapped and regtested it for x86_64 Linux with nvptx
> offloading and it didn't introduce any regressions. We do have a
> couple of other standalone kernels patches in og8, but those depend
> on other patches.

It's not surprising that there are no new tests and no regressions: the
new "marked_independent" field is not used anywhere, either within this
patch, or on the gomp4 branch where it originated, nor currently on the
og8 branch! It looks like the planned use by the parloops pass (etc.)
has not materialised so far.

Jakub asked in the following email at the time of the patch submission
for the gomp4 branch what the difference was between the new
marked_independent flag and safelen == INT_MAX:

  https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01100.html

If I understand the followup correctly,

  https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01117.html

a setting of safelen > 1 means that up to that number of loop
iterations can run together in lockstep (as if each insn in the loop
was blindly rewritten to a safelen-width SIMD equivalent) -- but
anything that happens in iteration N + 1 cannot happen before something
that happens in iteration N. Chung-Lin pointed out that OpenACC's
semantics are even less strict (allowing iterations to proceed fully
independently in an arbitrary order), so the marked_independent flag
does carry non-redundant information -- even with safelen set to
INT_MAX.

Actually I think that given the above, setting safelen to a value
greater than 32 (the warp size) may not be safe for NVPTX on OpenACC,
depending on the vagaries of warp scheduling. But that's not the
subject of this patch.

Anyway: given that the information recorded by this patch is not used
at present, and further work on the kernels pass may head in a
different direction, I'm not sure that it makes sense to commit it
at this point.

Also, it occurs to me that if the independent flag is set on loops
within kernels regions with an explicit "independent" clause, it should
also be set by default on loops in parallel regions without clauses
that disable the independent-iteration semantics.

Julian