[llvm-dev] LAA behavior on Incorrect #pragma omp simd.

[Doerfert, Johannes via llvm-dev]

Hi Rajasekhar,

thanks for reporting this.

On 06/26, Raja Sekhar Bhetala via llvm-dev wrote:
> The specification on this loop is incorrect as the parallel_accesses
> metadata indicate that there is no loop carried memory dependence, which is
> not true in this case.

First, I think the lowering is actually broken if a simdlen is given.
Since we use parallel_accesses metadata it indicates the loop is free of
dependences but #pragma simd simdlen(4) means we are allowed to assume
there are no loop carried dependences of length smaller than 4.


> Shouldn't the compiler not Vectorize if it can prove that there is a loop
> carried dependence and the Vectorizer will generate an incorrect code ?
> Or Should it blindly follow the user directive (without a warning) ?

I'm always on the fence when it comes to these questions. I think we
should blindly follow the directives but offer a flag that globally
turns on warning for such odd situations.


> It is very difficult for the user to identify the real source of the
> problem if the compiler vectorizes the loop silently. I agree its hard to
> detect incorrect specifications. But for cases, where it is easy to detect
> we should atleast dump a warning.

No default warning, that will clutter the output. On second though,
maybe if the we determine the given information is plain wrong.

Cheers,
  Johannes

-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190626/801706fe/attachment.sig>