[PATCH] Enable SLP-vectorization of intrinsics.

[Raul Silvera]

Well, either removing that or implementing a mechanism to fully model this
would resolve the issue.
If this is known to be widely broken the former would be the easiest to do,
of course. Who should we consult on this, llvm-dev? Should I prepare a
patch?

This impacts more than just vectorization, I can construct simple testcases
where we fail to common scalar sqrt calls.



On Wed, Jan 22, 2014 at 6:58 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Raul Silvera" <rsilvera at google.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "Nadav Rotem" <nrotem at apple.com>,
> reviews+D2535+public+071e8fd9238b048b at llvm-reviews.chandlerc.com,
> "Chandler
> > Carruth" <chandlerc at gmail.com>, "llvm-commits" <llvm-commits at cs.uiuc.edu
> >
> > Sent: Tuesday, January 21, 2014 8:45:22 PM
> > Subject: Re: [PATCH] Enable SLP-vectorization of intrinsics.
> >
> >
> >
> > I had a closer look trying to get a case to work vectorizing sqrt.
> > Unfortunately we fail to SLP vectorize even simple cases because the
> > sqrt intrinsic (as well as the other math-related intrinsics) are
> > marked as read-only, creating dependences with user stores.
> >
> >
> > I found the code in include/llvm/IR/Intrinsics.td where these
> > attributes are set, and there is a comment about them being treated
> > conservatively to model their dependence to hardware FP rounding
> > modes. This could be addressed by having a separate attribute to
> > model FP hardware settings (think of it as sqrt reading a specific
> > system memory location instead of reading all of user memory).
> >
> >
> > Has something like that been considered in the past, or is there a
> > different long-term strategy for dealing with these implicit
> > dependences?
>
> To be honest, I'm not exactly sure why we both modeling this at all.
> Clang/LLVM have a stated policy of not supporting floating-point
> environment access (#pragma STDC FENV_ACCESS ON is not supported -- Clang
> will complain if you try). Our modeling of these things is explicitly
> broken in other respects.
>
>  -Hal
>
> >
> >
> > If I manually remove the attribute we do SLP-vectorize a simple case
> > and seem to get the expected speedups from using vector sqrt
> > instructions.
> >
> >
> >
> > On Fri, Jan 17, 2014 at 12:41 PM, Hal Finkel < hfinkel at anl.gov >
> > wrote:
> >
> >
> >
> > ----- Original Message -----
> > > From: "Nadav Rotem" < nrotem at apple.com >
> >
> > > To: "Hal Finkel" < hfinkel at anl.gov >
> > > Cc:
> > > reviews+D2535+public+071e8fd9238b048b at llvm-reviews.chandlerc.com ,
> > > chandlerc at gmail.com , llvm-commits at cs.uiuc.edu ,
> > > rsilvera at google.com
> > > Sent: Friday, January 17, 2014 2:18:53 PM
> > > Subject: Re: [PATCH] Enable SLP-vectorization of intrinsics.
> > >
> >
> > > I think that we are converting some functions to llvm intrinsics.
> > > Right?
> >
> > Clang produces intrinsics for sqrt (only in fast-math mode), pow and
> > fma. I think that's it, and until you get to CodeGen, I don't think
> > we normally convert anything else at the LLVM level.
> >
> > -Hal
> >
> >
> >
> > >
> > > On Jan 17, 2014, at 12:11 PM, Hal Finkel < hfinkel at anl.gov > wrote:
> > >
> > > > ----- Original Message -----
> > > >> From: "Nadav Rotem" < nrotem at apple.com >
> > > >> To: nrotem at apple.com , rsilvera at google.com
> > > >> Cc: chandlerc at gmail.com , llvm-commits at cs.uiuc.edu
> > > >> Sent: Friday, January 17, 2014 2:06:55 PM
> > > >> Subject: Re: [PATCH] Enable SLP-vectorization of intrinsics.
> > > >>
> > > >>
> > > >> The code looks great. I was wondering if you had a chance to run
> > > >> some performance tests on it. Does it catch anything in the LLVM
> > > >> test suite?
> > > >
> > > > Why do you expect it to catch anything in the test-suite until we
> > > > get the function calls too (just as we do in the loop
> > > > vectorizer)?
> > > >
> > > > -Hal
> > > >
> > > >>
> > > >> http://llvm-reviews.chandlerc.com/D2535
> > > >> _______________________________________________
> > > >> llvm-commits mailing list
> > > >> llvm-commits at cs.uiuc.edu
> > > >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> > > >>
> > > >
> > > > --
> > > > Hal Finkel
> > > > Assistant Computational Scientist
> > > > Leadership Computing Facility
> > > > Argonne National Laboratory
> > >
> > >
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> >
> >
> >
> >
> > --
> >
> >
> > Raúl E. Silvera
> >
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>



-- 
 Raúl E. Silvera
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