Someday support IBM XL compilers

[Paul Hargrove]

This issue is being reported as a place to keep records.
It is NOT currently a project priority to support the IBM XL compilers.

With the merge of PR#96 for PGI support on PPC64, I find we are just one minor issue (#248) away from being able to compile UPC++ with the IBM XL compilers on Summit.

I have now attempted a full CI run on Summit. Below are my notes recording the status.
The short version is that we appear to be far from seeing this as a supported compiler family.

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UPC++ at b5dfa82a
Summit compiler environment module xl/16.1.1-3

At compile time I see numerous warnings. Some are obviously harmless, such as unknown Pragmas and unused variable warnings (for ones used only in assertions). At lease one is just a straight-forward "cleanup" (issue #247).

All the tests in CI (excluding known issue234) compile except for the Open MP tests with -g. However, that appears to be a GASNet issue (bug 3998) in which an --enable-debug build is incompatible with -fopenmp.

There a numerous failures at runtime including timeouts, SEGVs, GASNet error messages regarding out-of-range ranks in AM injections, and UPC++ assertion failures. I am not making any attempt to record them here/now.
It is worth noting that there were NO failures from the GASNet tests in the same CI run.

[Dan Bonachea]

There is now a beta version of IBM XL based on LLVM:

https://community.ibm.com/community/user/power/blogs/si-yuan-zhang1/2020/11/20/ibm-cc-and-fortran-new-beta-compilers-available

Based on the fact there's been no release to their production XL compiler in over 2 years, this beta is probably where we should focus any effort in this direction.

[Paul Hargrove]

The blog post linked above has no links to join the beta program, even after I authenticate with my IBM Community ID. So, I suspect that specific beta program may have ended.

The only download link from there is for the Apr 2019 Community Edition compilers we are already using on our OpenPower-8 VM.

However, based on Bug 3741 (Mar 2018) in which we found GASNet to identify the IBM compilers in SummitDev as Clang, I am pretty sure we've been using "IBM XL based on LLVM" for roughly 3 years now.

I also cannot agree fully with "no release to their production XL compiler in over 2 years", since Summit's newest version is 16.1.1.10 released 11 Mar 2021 (3 weeks old). However, it is true that that is "Fix 10" to a 16.1.1 compiler released in Nov 2019. So, I guess it depends what you count as a release.