[llvm-dev] (RFC) Encoding code duplication factor in discriminator

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Hal Finkel via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Dehao Chen" <dehao at google.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "Xinliang David Li"
> <davidxl at google.com>
> Sent: Tuesday, November 1, 2016 4:26:17 PM
> Subject: Re: [llvm-dev] (RFC) Encoding code duplication factor in
> discriminator

> ----- Original Message -----

> > From: "Dehao Chen" <dehao at google.com>
> 
> > To: "Hal Finkel" <hfinkel at anl.gov>
> 
> > Cc: "Paul Robinson" <paul.robinson at sony.com>, "Xinliang David Li"
> > <davidxl at google.com>, "llvm-dev" <llvm-dev at lists.llvm.org>
> 
> > Sent: Tuesday, November 1, 2016 4:14:43 PM
> 
> > Subject: Re: [llvm-dev] (RFC) Encoding code duplication factor in
> > discriminator
> 

> > damn... my english is not readable at all when I try to write
> > fast...
> > trying to make some clarification below, hopefully can make it more
> > readable...
> 

> > On Tue, Nov 1, 2016 at 2:07 PM, Dehao Chen < dehao at google.com >
> > wrote:
> 

> > > Oops... pressed the wrong button and sent out early...
> > 
> 

> > > On Tue, Nov 1, 2016 at 2:01 PM, Dehao Chen < dehao at google.com >
> > > wrote:
> > 
> 

> > > > If Hal's proposal is for SamplePGO purpose, let me clarify some
> > > > design principles of SamplePGO.
> > > 
> > 
> 

> > > > The profile for sample pgo uses source location as the key to
> > > > map
> > > > the
> > > > execution count back to IR. This design is based on the
> > > > principle
> > > > that we do not want the profile to be tightly couple with
> > > > compiler
> > > > IR. Instead, profile is simple an attribute of the source code.
> > > > We
> > > > have been benefited a lot from this design that the profile can
> > > > easily be reused across different source versions and compiler
> > > > versions, or even compilers.
> > > 
> > 
> 

> > > > That being said, the design to encode more info into
> > > > discriminator
> > > > does not mean that we will change the profile. The encoded info
> > > > in
> > > > discriminator will be handled by the create_llvm_prof tool,
> > > > which
> > > > combines counts from different clones of the same source code
> > > > and
> > > > generate the combined profile data. The output profile will not
> > > > have
> > > > any cloning/dupliaction bits at all. So for the initial example
> > > > profile I provided, the output profile will be:
> > > 
> > 
> 
> > > #1: 10
> > 
> 
> > > #3: 80
> > 
> 

> > > Not:
> > 
> 

> > > #1: 10
> > 
> 
> > > #3.0x400: 70
> > 
> 

> > > #3.0x10400: 5
> > 
> 
> > > #3.0x20400: 3
> > 
> 
> > > #3.0x30400: 2
> > 
> 

Also, how does this work for vectorization? For vectorization, you're going to multiply by the duplication factor first? The issue obviously is that each vector instruction counts for VF times as many scalar instructions, and so to get equivalent scalar counts, you need to multiply by VF. I had assumed this is what you were saying when I read the initial e-mail, but if you're also using the same duplication scheme for unrolling, then I think we need some way to differentiate. 

-Hal 

> > > The goal of the proposed change, is to make profile more
> > > accurately
> > > represent the attribute of the source.
> > 
> 
> > > The non-goal of the proposed change, is to provide more context
> > > in
> > > the profile to present the behavior of program in the context of
> > > different context.
> > 
> 
> > The non-goal of the proposed change, is to provide more context in
> > the profile to present the behavior of program in different
> > contexts.'
> 
> Okay, but why? The information will be present in the profiles, why
> not encode it in the metadata and let the optimizer decide when to
> sum it up? We should even provide some utility functions that make
> this transparent for passes that don't care about the distinction.

> > > If we are pursuing more context-sensitive profile, that would be
> > > a
> > > very different design. In this design, we will need to read
> > > profile
> > > multiple times, and have the profile tightly coupled with the
> > > compiler IR/pass manager. That is doable, but I don't think that
> > > probably better suits instrumentation based PGO's domain.
> > > Comments?
> > 
> 
> > If we are pursuing more context-sensitive profile, that would be a
> > very different design in which we need to read in profiles multiple
> > times, and have the profile tightly coupled with the compiler
> > IR/pass manager. That is doable, but that probably better suits
> > instrumentation based PGO's domain. Comments?
> 
> I don't understand why you say we'd need to read the profile multiple
> times. Can you please explain? I also don't think it needs to be
> that tightly coupled; we just need each pass that generates multiple
> copies of things to have some way to generate a unique id for what
> it's doing. It's all per source location, so I don't even think we
> need any kind of complicated hashing scheme.

> Thanks again,
> Hal

> > > Thanks,
> > 
> 
> > > Dehao
> > 
> 

> > > > On Tue, Nov 1, 2016 at 1:04 PM, Hal Finkel < hfinkel at anl.gov >
> > > > wrote:
> > > 
> > 
> 

> > > > > > From: "Paul Robinson" < paul.robinson at sony.com >
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > To: "Dehao Chen" < dehao at google.com >, "Hal Finkel" <
> > > > > > hfinkel at anl.gov
> > > > > > >
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > Cc: "Xinliang David Li" < davidxl at google.com >,
> > > > > > llvm-dev at lists.llvm.org
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > Sent: Tuesday, November 1, 2016 2:15:38 PM
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > Subject: RE: [llvm-dev] (RFC) Encoding code duplication
> > > > > > factor
> > > > > > in
> > > > > > discriminator
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > As illustrated in the above example, it is not like
> > > > > > "vectorization
> > > > > > has a distinct bit". All different optimizations make
> > > > > > clones
> > > > > > of
> > > > > > code
> > > > > > which will be labeled by UIDs represented by N (e.g. 8)
> > > > > > bits.
> > > > > > In
> > > > > > this way, the space will be capped by the number of clones
> > > > > > all
> > > > > > optimizations have made, instead of # of optimizations that
> > > > > > has
> > > > > > applied. And it will be capped at 2^N-1. The cons of using
> > > > > > uid
> > > > > > is
> > > > > > that you will not know if a clone is coming from
> > > > > > vectorization
> > > > > > or
> > > > > > unroll or loop distribution.
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > Okay, but that kind of semantic mapping is important. How
> > > > > > should
> > > > > > we
> > > > > > encode/recover that information? To be clear, I'm not
> > > > > > saying
> > > > > > that
> > > > > > we
> > > > > > need to implement that up front, but there needs to be a
> > > > > > clear
> > > > > > path
> > > > > > to an implementation, because I don't want to have two
> > > > > > disjoint
> > > > > > schemes.
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > You mean that you want to know which optimization created
> > > > > > the
> > > > > > clone?
> > > > > > How would you use that info? Looks to me this will expose
> > > > > > compiler
> > > > > > implementation detail in debug info.
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > This is still doable, assume we have 15 interesting
> > > > > > optimizations
> > > > > > to
> > > > > > track, we can use 4 bits to encode the optimization type
> > > > > > that
> > > > > > created the clone. But this becomes nasty if the a clone is
> > > > > > created
> > > > > > by more than one optimizations. In that way, discriminator
> > > > > > may
> > > > > > not
> > > > > > be fit for this purpose.
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > My understanding was that the encoding scheme would allow
> > > > > > the
> > > > > > profiling analysis to correctly map execution data back to
> > > > > > the
> > > > > > original source construct, while preserving the property
> > > > > > that
> > > > > > each
> > > > > > distinct basic block would have its own discriminator
> > > > > > value.
> > > > > > That
> > > > > > is, the execution data would be attributed back to the
> > > > > > original
> > > > > > source construct, not whatever each individual optimization
> > > > > > had
> > > > > > done
> > > > > > to it, and the data for the original source construct would
> > > > > > correctly reflect the execution (e.g. profiling says you
> > > > > > got
> > > > > > 82
> > > > > > hits
> > > > > > on the original loop, rather than reporting 20 hits on the
> > > > > > unrolled-by-4 loop plus 1 each on 2 of the trailing
> > > > > > copies).
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > It sounds like Hal is thinking that the per-discriminator
> > > > > > execution
> > > > > > info would be preserved down to the point where an
> > > > > > individual
> > > > > > optimization could look at the profile for each piece, and
> > > > > > make
> > > > > > decisions on that basis.
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > I'm not clear how that would be possible, as the
> > > > > > optimization
> > > > > > would
> > > > > > have to first do the transform (or predict how it would do
> > > > > > the
> > > > > > transform) in order to see which individual-discriminator
> > > > > > counts
> > > > > > mapped to which actual blocks, and then make some kind of
> > > > > > decision
> > > > > > about whether to do the transform differently based on that
> > > > > > information. Then, if the optimization did choose to do the
> > > > > > transform differently, then that leaves the IR in a state
> > > > > > where
> > > > > > the
> > > > > > individual discriminators *cannot* map back to it. (Say you
> > > > > > unroll
> > > > > > by 2 instead of 4; then you have only 1 trailing copy, not
> > > > > > 3,
> > > > > > and
> > > > > > a
> > > > > > discriminator that maps to the second trailing copy now
> > > > > > maps
> > > > > > to
> > > > > > nothing. The individual-discriminator data becomes
> > > > > > useless.)
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > Am I expressing this well enough to show that what Hal is
> > > > > > looking
> > > > > > for
> > > > > > is not feasible?
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > Yes, it will need to predict how the transformation would
> > > > > affect
> > > > > the
> > > > > blocks produced. That does not seem problematic (at least at
> > > > > a
> > > > > coarse level). Yes, if transformations made earlier in the
> > > > > pipeline
> > > > > make different decisions, then that will invalidate later
> > > > > fine-grained data (at least potentially). I don't see how any
> > > > > of
> > > > > this makes this infeasible. We just need a way for the
> > > > > profiling
> > > > > counts, per descriminator, to remain available, and for the
> > > > > transformations themselves to know which discriminators (loop
> > > > > ids,
> > > > > or whatever) to consider.
> > > > 
> > > 
> > 
> 

> > > > > -Hal
> > > > 
> > > 
> > 
> 

> > > > > > --paulr
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > --
> > > > 
> > > 
> > 
> 

> > > > > Hal Finkel
> > > > 
> > > 
> > 
> 
> > > > > Lead, Compiler Technology and Programming Languages
> > > > 
> > > 
> > 
> 
> > > > > Leadership Computing Facility
> > > > 
> > > 
> > 
> 
> > > > > Argonne National Laboratory
> > > > 
> > > 
> > 
> 

> --

> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory

> _______________________________________________
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> llvm-dev at lists.llvm.org
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-- 

Hal Finkel 
Lead, Compiler Technology and Programming Languages 
Leadership Computing Facility 
Argonne National Laboratory 
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