[llvm-dev] Questions on LLVM vectorization diagnostics

Ramakrishna Upadrasta via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 23 08:29:27 PDT 2016

Hi Tharun,

Thanks for this nice question! Hope this leads to good discussion and


On Thu, Jun 23, 2016 at 8:50 PM, Dangeti Tharun kumar <
cs15mtech11002 at iith.ac.in> wrote:

> Dear LLVM Community,
> I am D Tharun Kumar, masters student at Indian Institute of Technology
> Hyderabad, working in a team to improve current vectorizer in LLVM. As an
> initial study, we are studying various benchmarks to analyze and compare
> vectorizing capabilities of LLVM, GCC and ICC. We found that vectorization
> remarks given by LLVM are vague and brief, comparatively GCC and ICC are
> giving detailed diagnostics.
>    - I am interested to know why the LLVM diagnostics are brief and not
>    intuitive (making them less helpful)?
>    - In our analysis we never seen llvm trying to vectorize outer loops.
>    Is this well known? Is outer loop vectorization implemented in LLVM as in
>    GCC? (http://dl.acm.org/citation.cfm?id=1454119) If not, is someone
>    working on it?
>    - On the TSVC benchmark suite, out of a total of 151 loops, LLVM, GCC
>    and ICC vectorized 70, 82 and 112 loops respectively. Is the cause for lag
>    of LLVM the inability of LLVM’s vectorizer, or are there any (enabling)
>    optimization passes running before GCC’s vectorizer that are helping GCC
>    perform better?
>    - Loop peeling to enhance vectorization is present in GCC and ICC,
>    but, the LLVM remarks don’t say anything about alignment. Does LLVM has
>    this functionality and the vectorizer doesn’t remark about it, or it
>    doesn’t it have the functionality at all?
> Finally, we appreciate suggestions and directions for improving the
> vectorization framework of LLVM.
> I would also like to know if anyone worked or is working on improving
> vectorization remarks.
> Regards,
> Dangeti Tharun kumar
> M.TECH Computer Science
> IIT Hyderabad
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