[LLVMdev] Indexed Load and Store Intrinsics - proposal

[Hal Finkel]

----- Original Message -----
> From: "Xinmin Tian" <xinmin.tian at intel.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: dag at cray.com, "Robert Khasanov" <robert.khasanov at intel.com>, llvmdev at cs.uiuc.edu, "Ayal Zaks"
> <ayal.zaks at intel.com>
> Sent: Tuesday, December 23, 2014 8:47:12 PM
> Subject: RE: [LLVMdev] Indexed Load and Store Intrinsics - proposal
> 
> >>>I don't understand what you mean. Why do you feel the information
> >>>will be difficult to recover later from vector GEPs feeding the
> >>>scatter/gather intrinsics?
> 
> As the intrinsic only hold vector of pointers, we would have to trace
> back to GEPs, and decompose the address calculation of each
> elements, for base and offset ... etc, right?

Correct.

> For vector of pointers
> case, the process is to get base and offset, and compose them, feed
> it into intrinsic, later decompose them when needed.

Yes.

> If we keep base
> and offset, it does simplify the compose and decompose process in
> some cases, right?

It does, but it also makes the infrastructure less generic (and duplicates functionality contained within the vector GEP within the intrinsic), and complicates any code that needs to analyze the instrinsic. It is not clear that it makes it sufficiently simpler to justify that complexity.

 -Hal

> 
> Xinmin
> 
> -----Original Message-----
> From: Hal Finkel [mailto:hfinkel at anl.gov]
> Sent: Tuesday, December 23, 2014 5:49 PM
> To: Tian, Xinmin
> Cc: dag at cray.com; Khasanov, Robert; llvmdev at cs.uiuc.edu; Zaks, Ayal
> Subject: Re: [LLVMdev] Indexed Load and Store Intrinsics - proposal
> 
> ----- Original Message -----
> > From: "Xinmin Tian" <xinmin.tian at intel.com>
> > To: "Hal Finkel" <hfinkel at anl.gov>, "Ayal Zaks"
> > <ayal.zaks at intel.com>
> > Cc: dag at cray.com, "Robert Khasanov" <robert.khasanov at intel.com>,
> > llvmdev at cs.uiuc.edu
> > Sent: Tuesday, December 23, 2014 7:36:44 PM
> > Subject: RE: [LLVMdev] Indexed Load and Store Intrinsics - proposal
> > 
> > For non-zero case,  most of time (e.g. a[i]), the "base" is
> > uniform,
> 
> Agreed.
> 
> > the property helps to identify and perform neighboring
> > gather/scatter
> > optimizations in some applications. If the information is
> > preserved,
> > later, it may not easy to recover this "uniform" information.
> 
> I don't understand what you mean. Why do you feel the information
> will be difficult to recover later from vector GEPs feeding the
> scatter/gather intrinsics?
> 
>  -Hal
> 
> >  Non-zero case is not designed only for x86, basically, it helps to
> > preserve certain program information.
> > 
> > Thanks,
> > Xinmin
> > 
> > -----Original Message-----
> > From: llvmdev-bounces at cs.uiuc.edu
> > [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Hal Finkel
> > Sent: Tuesday, December 23, 2014 5:17 PM
> > To: Zaks, Ayal
> > Cc: dag at cray.com; Khasanov, Robert; llvmdev at cs.uiuc.edu
> > Subject: Re: [LLVMdev] Indexed Load and Store Intrinsics - proposal
> > 
> > ----- Original Message -----
> > > From: "Ayal Zaks" <ayal.zaks at intel.com>
> > > To: "Philip Reames" <listmail at philipreames.com>, dag at cray.com,
> > > "Elena Demikhovsky" <elena.demikhovsky at intel.com>
> > > Cc: "Robert Khasanov" <robert.khasanov at intel.com>,
> > > llvmdev at cs.uiuc.edu
> > > Sent: Monday, December 22, 2014 8:05:43 AM
> > > Subject: Re: [LLVMdev] Indexed Load and Store Intrinsics -
> > > proposal
> > > 
> > > > Why shouldn't the IR representation simply be a load from a
> > > > vector
> > > > of arbitrary pointers?
> > > 
> > > Such a load could indeed serve as a general form of a gather or
> > > scatter. As Elena responded, we can propose two distinct
> > > intrinsics:
> > > one with a vector of pointers, and another with (non-zero) base,
> > > a
> > > vector of indices, and a scale implicitly inferred from the
> > > element
> > > type.
> > > 
> > > The motivation for the latter stems from vectorizing a load or
> > > store
> > > to "b[i]", where b is invariant. Broadcasting b and using a
> > > vector
> > > gep to feed a vector of pointers, to be pattern matched and
> > > folded
> > > later, may work.
> > 
> > I would like you to explore this direction, where we use a vector
> > GEP
> > and the intrinsic simply takes a vector of pointers. The backend
> > should pattern-match this as appropriate. I see no reason why we
> > can't
> > make this work, especially because we don't have any real uses of
> > vector GEPs now, so we can *define* the canonical optimized form of
> > them to be conducive to the kind of pattern matching we'd like to
> > perform in the backends.
> > 
> > This, I imagine, will require some additional infrastructure work.
> > Currently, GEPs, including vector GEPs, are expanded very early
> > during
> > SDAG building, and the form produced may not be appropriate for
> > reliable pattern matching during later lowering phases. The way
> > this
> > is done is not set in stone, however, and we can certainly change
> > it
> > (including via the introduction of new SDAG nodes) to keep the
> > necessary information together in compact form.
> > 
> > Thanks again,
> > Hal
> > 
> > > The alternative intrinsic proposed keeps b scalar and uses a
> > > vector
> > > of indices for i. In any case, it's important to recognize such
> > > common patterns, at-least for x86, so could deserve an x86
> > > intrinsic. But it's a general pattern that could potentially
> > > serve
> > > other implementations; any other gathers to consider atm?
> > > 
> > > Documentation indeed needs to be provided.
> > > 
> > > Ayal.
> > > 
> > > 
> > > -----Original Message-----
> > > From: llvmdev-bounces at cs.uiuc.edu
> > > [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Philip Reames
> > > Sent: Sunday, December 21, 2014 20:25
> > > To: dag at cray.com; Demikhovsky, Elena
> > > Cc: Khasanov, Robert; llvmdev at cs.uiuc.edu
> > > Subject: Re: [LLVMdev] Indexed Load and Store Intrinsics -
> > > proposal
> > > 
> > > 
> > > On 12/18/2014 11:56 AM, dag at cray.com wrote:
> > > > "Demikhovsky, Elena" <elena.demikhovsky at intel.com> writes:
> > > >
> > > >> Semantics:
> > > >> For i=0,1,…,N-1: if (Mask[i]) {*(BaseAddr +
> > > >> VectorOfIndices[i]*Scale)
> > > >> = VectorValue[i];}
> > > >> VectorValue: any float or integer vector type.
> > > >> BaseAddr: a pointer; may be zero if full address is placed in
> > > >> the
> > > >> index.
> > > >> VectorOfIndices: a vector of i32 or i64 signed or unsigned
> > > >> integer values.
> > > > What about the case of a gather/scatter where the BaseAddr is
> > > > zero
> > > > and the indices are pointers?  Must we do a ptrtoint?  llvm.org
> > > > is
> > > > down at the moment but I don't think we currently have a vector
> > > > ptrtoint.
> > > I would be opposed to any representation which required the
> > > introduction of ptrtoint casts by the vectorizer.  If it were the
> > > only option available, I could be argued around, but I think we
> > > should try to avoid this.
> > > 
> > > More generally, I'm somewhat hesitant of representing a scatter
> > > with
> > > explicit base and offsets at all.  Why shouldn't the IR
> > > representation simply be a load from a vector of arbitrary
> > > pointers?
> > >  The backend can pattern match the actual gather instructions it
> > > supports and scalarize the rest.  The proposal being made seems
> > > very
> > > specific to the current generation of x86 hardware.
> > > 
> > > p.s. Where is the documentation for the existing mask load
> > > intrinsics?
> > > I can't find it with a quick search through the LangRef.
> > > 
> > > Philip
> > > 
> > > 
> > > 
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> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > 
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory