RFC: Enable vectorization of call instructions in the loop vectorizer

[James Molloy]

Hi all,

Attached is the first patch in a sequence to implement this behaviour in the way described in this thread.

This patch simply:
  * Changes most methods on TargetLibraryInfo to be virtual, to allow clients to override them.
  * Adds three new functions for querying the vectorizability (and scalarizability) of library functions. The default implementation returns failure for all of these queries.

Please review!

James

> -----Original Message-----
> From: Hal Finkel [mailto:hfinkel at anl.gov]
> Sent: 16 December 2013 21:10
> To: Arnold Schwaighofer
> Cc: llvm-commits; James Molloy
> Subject: Re: RFC: Enable vectorization of call instructions in the loop
> vectorizer
>
> ----- Original Message -----
> > From: "Arnold Schwaighofer" <aschwaighofer at apple.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "llvm-commits" <llvm-commits at cs.uiuc.edu>, "James Molloy"
> <James.Molloy at arm.com>
> > Sent: Monday, December 16, 2013 3:08:13 PM
> > Subject: Re: RFC: Enable vectorization of call instructions in the loop
> vectorizer
> >
> >
> > On Dec 16, 2013, at 2:59 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> >
> > > ----- Original Message -----
> > >> From: "Arnold Schwaighofer" <aschwaighofer at apple.com>
> > >> To: "James Molloy" <James.Molloy at arm.com>
> > >> Cc: "llvm-commits" <llvm-commits at cs.uiuc.edu>
> > >> Sent: Monday, December 16, 2013 12:03:02 PM
> > >> Subject: Re: RFC: Enable vectorization of call instructions in the
> > >> loop     vectorizer
> > >>
> > >>
> > >> On Dec 16, 2013, at 11:08 AM, James Molloy <James.Molloy at arm.com>
> > >> wrote:
> > >>
> > >>> Hi Renato, Nadav,
> > >>>
> > >>> Attached is a proof of concept[1] patch for adding the ability to
> > >>> vectorize calls. The intended use case for this is in domain
> > >>> specific languages such as OpenCL where tuned implementation of
> > >>> functions for differing vector widths exist and can be guaranteed
> > >>> to be semantically the same as the scalar version.
> > >>>
> > >>> I’ve considered two approaches to this. The first was to create a
> > >>> set of hooks that allow the LoopVectorizer to interrogate its
> > >>> client as to whether calls are vectorizable and if so, how.
> > >>> Renato
> > >>> argued that this was suboptimal as it required a client to invoke
> > >>> the LoopVectorizer manually and couldn’t be tested through opt. I
> > >>> agree.
> > >>
> > >> I don’t understand this argument.
> > >>
> > >> We could extend target library info with additional api calls to
> > >> query whether a function is vectorizable at a vector factor.
> > >> This can be tested by providing the target triple string (e.g
> > >> “target
> > >> triple = x86_64-gnu-linux-with_opencl_vector_lib") in the .ll file
> > >> that informs the optimizer that a set of vector library calls is
> > >> available.
> > >>
> > >> The patch seems to restrict legal vector widths dependent on
> > >> available vectorizable function calls. I don’t think this should
> > >> work like this.
> > >> I believe, there should be an api on TargetTransformInfo for
> > >> library
> > >> function calls. The vectorizer chooses the cheapest of either an
> > >> intrinsic call or a library function call.
> > >> The overall cost model determines which VF will be chosen.
> > >
> > > We don't have a good model currently for non-intrinsic function
> > > calls. Once we do, we'll want to know how expensive the vectorized
> > > versions are compared to the scalar version. Short of that, I
> > > think that a reasonable approximation is that any function calls
> > > will be the most expensive things in a loop, and the ability to
> > > vectorize them will be the most important factor in determining
> > > the vectorization factor.
> >
> > Yes and we can easily model this in the cost model by asking what is
> > the cost of a (library) function call (vectorized or not) and have
> > this return a reasonably high value.
>
> Sounds good to me.
>
>  -Hal
>
> >
> >
> > >
> > > -Hal
> > >
> > >>
> > >> Thanks,
> > >> Arnold
> > >>
> > >>
> > >> _______________________________________________
> > >> 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


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