[llvm-dev] Questions on LLVM vectorization diagnostics

Gerolf Hoflehner via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 23 11:06:53 PDT 2016

> On Jun 23, 2016, at 8:20 AM, Dangeti Tharun kumar via llvm-dev <llvm-dev at lists.llvm.org> 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 <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?
Can you share your table with performance score per benchmark and the options you used for each compiler? Could you separate your results in integer and floating point loops? 

You might also want to compare the compilers on the code samples http://www.aartbik.com/SSE/code.html. 
> 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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