[cfe-dev] OpenMP in Flang and Clang

Johannes Doerfert via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 14 19:19:00 PDT 2020

On 4/14/20 9:58 AM, David Truby wrote:
 > I agree that we should align with clang's enums here in principle,
 > however my preference would still be to merge that patch so that we
 > have the checks and tests implemented as a first step, and then take a
 > more holistic look at aligning with clang on OpenMP semantic checks.

Agreed. I do not want to block your patch for this.

 > Another consideration is that clang currently tracks OpenMP 5 (and
 > only that specification) whereas we've been implementing 4.5 up to
 > now. When we try and share semantics checks with clang we will
 > probably want to do a more general version bump to OpenMP 5 at around
 > the same time, otherwise our semantics checks and our parser won't
 > match with regards to which version of the specification they
 > implement.

So this is not that simple. Clang defaults to 4.5 and has parts of 5.0
and 5.1 implemented. I suspect the situation would be similar in Flang
at some point soon. Since Clang supports to choose a version, in both
directions from the default, we already require extra logic to deal with
semantic changes. However, centralizing things and de-duplicating will
still help us. One of the latest changes in this regard was D77113. We
now explicitly specify in the macro file  what clause is available for
a directive and version (range).

I imagine C/C++/Fortran to be another degree of freedom here, as well as
the "only once" flag Flang already has. Maybe the latter should even be
extended to constraints on the set of clauses for a directive.

I hope this makes some sense.


 > Thanks
 > David Truby
 > ________________________________
 > From: flang-dev <flang-dev-bounces at lists.llvm.org> on behalf of Steve 
Scalpone via flang-dev <flang-dev at lists.llvm.org>
 > Sent: 14 April 2020 15:02
 > To: Johannes Doerfert <johannesdoerfert at gmail.com>; 
flang-dev at lists.llvm.org <flang-dev at lists.llvm.org>
 > Cc: cfe-dev at lists.llvm.org <cfe-dev at lists.llvm.org>
 > Subject: Re: [flang-dev] [cfe-dev] OpenMP in Flang and Clang
 > Flang today has overlapping enums for OpenMP directives,
 > e.g. OmpLoopDirective and OmpDirective; both of these enums
 > have names for PARALLEL DO, for example.
 > Perhaps before https://reviews.llvm.org/D77812 lands we
 > should merge the multiple sets of flang enumerations with
 > the other enumerations as Johannes describes.
 > On 4/9/20, 9:55 AM, "cfe-dev on behalf of Johannes Doerfert via 
cfe-dev" <cfe-dev-bounces at lists.llvm.org on behalf of 
cfe-dev at lists.llvm.org> wrote:
 >     External email: Use caution opening links or attachments
 >     Since we now merged F18, yay!, we should start reusing OpenMP
 >     "infrastructure" between LLVM/Clang and LLVM/Flang. This will reduce
 >     duplication (coding & maintenance), improve testing, and thereby 
 >     get us to OpenMP 5.X support faster.
 >     I started to look at `parse-tree.h` and `openmp-parsers.cpp` which
 >     contain a lot of code dealing with the OpenMP "grammar". Arguably, we
 >     don't want the duplication with
 >        `llvm/include/llvm/Frontend/OpenMP/OMPKinds.def`
 >     (and the almost empty "old" clang file `OpenMPKinds.def`).
 >     I think there are multiple steps we should take. Note that I have to
 >     list them in some order, please don't interpret this as the order I
 >     suggest we work on them.
 >        Move and merge the ENUM_CLASS definitions that can be shared into
 >        OMPKinds.def. This might require to extend the macros such that we
 >        have a flag for C/C++ or Fortran only but that should not be 
too hard.
 >        The existing Clang code (or better OpenMPIRBuilder by now) could
 >        benefit from new enum classes already in Flang, e.g. 
 >        while things like the kind of an OmpScheduleClause should only be
 >        defined once.
 >        Determine how we can merge the definitions in `parse-tree.h` 
with the
 >        ones in `OpenMPClauses.h`. I was hoping to trim down the 
latter anyway
 >        as it seems there is quite a bit of duplication in there. I 
was also
 >        hoping we could generate them from the macro file (or a table-gen
 >        file) so we don't have to write all the boilerplate when new
 >        directives and clauses are added. As an example, in the new OpenMP
 >        context selector support for Clang new selector sets, 
selectors, and
 >        traits can be added via macros in the OMPKinds.def file. 
 >        like parsing, pretty printing, error messages, will work right 
 >        Only if the existing logic to determine if the selector matches a
 >        context is not sufficient you need to modify one more place.
 >        (This is not true for implementation defined extensions that 
 >         the behavior of the entire thing but anyway.)
 >        Rethink the use of `std::tuple` in and `std::variant` in (the 
 >        parts) of parse-tree.h. The latter is used for OmpLinearClause for
 >        example. As far as I can tell, the two members of the variant 
have two
 >        common fields and one has a third, an enum. In clang this is 
 >        solved by having an "unknown" or "none" member in the enum which
 >        allows to remove the duplication and variant completely. I 
would like
 >        to replace tuples so we can give their constitutes proper names. I
 >        think a similar point was made before when we discussed 
tooling with
 >        (or extending) Flang.
 >     I hope to get some feedback and thoughts on this.
 >     Thanks,
 >        Johannes
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