[Openmp-dev] Option to control level of OpenMP support

[Hal Finkel]

----- Original Message -----
> From: "Andrey Bokhanko" <andreybokhanko at gmail.com>
> To: "cfe-dev" <cfe-dev at cs.uiuc.edu>, openmp-dev at dcs-maillist2.engr.illinois.edu
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Michael Wong" <fraggamuffin at gmail.com>, "Alexey Bataev" <a.bataev at gmx.com>
> Sent: Thursday, May 7, 2015 6:55:08 AM
> Subject: Option to control level of OpenMP support
> 
> All,
> 
> Now, with OpenMP 3.1 support completed
> (http://lists.cs.uiuc.edu/pipermail/cfe-dev/2015-May/042845.html) we
> (Intel) plan to go on with implementing 4.0 support. Others plan to
> contribute as well -- AFAIK, at least IBM has specific plans and some
> code already implemented.
> 
> However, there is no chance for us to have OpenMP 4.0 completed in
> time for next release. And in general, it is next to impossible to
> sync state of the next OpenMP standard support with be-annual
> releases
> -- invariably we will have something only partially implemented (with
> some pragmas working in full, some analyzed (with errors reported)
> but
> no actual LLVM IR generated, some not supported at all) at any given
> release time.
> 
> We can deal with this in two ways:
> 1) Enable all new developments by default, just document in release
> notes that support of OpenMP x.x version is guaranteed, but
> everything
> else is "experimental".
> 2) Enable only completed standard (3.1 at the moment) by default, and
> let users enable non-completed support with an option. Say,
> -std-openmp=n.
> 
> Personally, I believe option 2) is much more convenient for end
> users.
> 
> Opinions? (Both on what is preferable and option name.)

I think that we should, by default, enable all available OpenMP features. Specifically, even though we don't have OpenMP 4 support complete yet, having the 'simd' directives available will be really nice. However, we must only set the value of _OPENMP based on the latest standard for which we have complete support. Having an -std-openmp=<n> will also be a nice feature, allowing us to actually restrict the available functionality to the standard level provided. We can default to "whatever you have", but if the user requests a particular version, only provide features from that version (and, thus, set _OPENMP accordingly).

 -Hal

> 
> Yours,
> Andrey Bokhanko
> ==============
> Software Engineer
> Intel Compiler Team
> Intel
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory