[llvm-dev] convergent operations in OpenMP

[Mehdi AMINI via llvm-dev]

On Wed, Jul 28, 2021 at 7:03 AM Johannes Doerfert via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On 7/28/21 4:24 AM, Sameer Sahasrabuddhe wrote:
> > Nicolai Hähnle writes:
> >
> >> On Wed, Jul 28, 2021 at 8:15 AM Sameer Sahasrabuddhe <
> sameer at sbuddhe.net>
> >>> My first attempt was to modify the Sink optimization, which currently
> >>> does not sink an operation if it is convergent. The change additionally
> >>> checks to see if divergent control flow is present to prevent sinking:
> >>>
> >>> https://reviews.llvm.org/D106859
> >>>
> >>> But this broke OpenMP lit tests like this one:
> >>>
> >>> openmp/tools/archer/tests/barrier/barrier.c
> >>>
> >>> The problem is that OpenMP builtins currently rely on the "convergent"
> >>> attribute to convey barrier semantics, even on CPU targets. I am
> >>> guessing that the actual implementation on a CPU will use other native
> >>> primitives like atomics. But it seems reasonable to say "convergent"
> and
> >>> expect it to mean exactly what it says, without relying on the
> >>> underlying implementation.
> >>>
> >> Out of curiosity, since I'm not familiar with the whole OpenMP setup:
> what
> >> does the IR currently look like at the point where the test gets broken?
> > Good question! I am equally unfamiliar with the OpenMP flow. The IR that
> > I get from "clang -emit-llvm ..." contains calls to runtime functions
> > and I don't know how to link those into the IR. I am currently working
> > on the simple fact that my review request is showing failures with a
> > bunch of "libarcher" tests.
> >
> > But now it seems I am getting failures locally with or without my
> > change. So, false alarm, maybe?
>
> Libarcher tests fail for all phab reviews for a while. That has nothing
> to do with your patch.
>

It was said to have been fixed a couple of days ago, I'd try to rebase and
retry!
Also check if you can reproduce locally.

Cheers,

-- 
Mehdi



>
> As far as I can tell there is no use of `convergent` on the CPU,
> or at least it would not mean much.
>
> ~ Johannes
>
>
> > Sameer.
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