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

[Hal Finkel via llvm-dev]

----- 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
> 

> > 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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