[llvm-dev] LLVM flags for Vectorization

[Renato Golin via llvm-dev]

On 20 August 2016 at 09:22, Santanu Das via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I have been analyzing the LLVM vectorizer by running some benchmarks. For
> vectorization, I have used the following flags:
>
> -O3
> -ffast-math
> -mavx2
>
> Am I missing any other flags which will improve vectorizer performance?

Hi Santanu,

These flags should be enough for Clang to vectorise your code based on
AVX2 support.

If that doesn't give you the vectorisation you want, you can try to
force the vector width or unroll factor by either command line options
or pragmas in the code:

http://llvm.org/docs/Vectorizers.html

If you don't understand why a loop is not being vectorised, you can
try the Clang diagnostics:

http://blog.llvm.org/2014/11/loop-vectorization-diagnostics-and.html

And, of course, if you spot a loop or a basic block that could have
been vectorised but wasn't, please open a bug on our bugzilla with the
results of the diagnostics and your experimentation with widths and
factors:

https://llvm.org/bugs/

Hope that helps.

cheers,
--renato