[LLVMdev] Regarding BOF: Vectorization in LLVM

Hal Finkel hfinkel at anl.gov
Tue Nov 6 10:11:03 PST 2012

----- Original Message -----
> From: "Nadav Rotem" <nrotem at apple.com>
> To: "David Tweed" <david.tweed at gmail.com>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Tuesday, November 6, 2012 11:08:23 AM
> Subject: Re: [LLVMdev] Regarding BOF: Vectorization in LLVM
> Hi David!
> On Nov 6, 2012, at 3:23 AM, David Tweed <david.tweed at gmail.com>
> wrote:
> > Hi Nadav,
> > 
> > Unfortunately I'm not attending the dev meeting, but the BoF looks
> > interesting. One thing that I'd like to throw into the mix is
> > that, while dealing with autovectorisation of LLVM compiled down
> > from C-like languages (or maybe Fortran-like languages) is clearly
> > a very big area for fruitful work both algorithmically and in
> > terms of practical relevance, it'd also be interesting to see what
> > LLVM complied from languages with semantics that are more open to
> > optimization can do to indicate these things to the
> > auto-vectorizer. (I have my own personal after-hours OSS project
> > that will soon be generating LLVM IR for which vectorization will
> > be important. I don't want to implement vectorization myself
> > before generating LLVM IR, partly since hopefully the LLVM layer
> > will have a much better estimate of the costs/benefits, so having
> > some written metadata I can set to indicate various things the
> > vectorizer would otherwise deduce (non-aliasing,
> > loop-permutability, etc) would be very inter!
>  esting.)
> This is definitely something that we plan to do.  We would like to be
> able to annotate loops (using metadata or special intrinsics, etc)
> and to indicate that they are vectorizable.  This is something that
> domain specific languages can use.  This will also allow us to
> improve vectorization of C-based languages because the user will be
> able to tell the compiler that a loop is safe to vectorize. The
> Intel compiler already supports vectorizer pragmas to specify which
> loop should be vectorized and to what vectorization factor.  I don't
> plan to start working vectorization hints soon, but this is one of
> the items on the vectorizer TODO list that I am going to present at
> the BoF.

I'd like to add: Please develop a wish list of such things, and, if possible, patches.

Thanks again,

> Nadav
> > 
> > Regards,
> > Dave Tweed
> > 
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Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory

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