[llvm-dev] Is there a way to convert between SchedMachineModel and Itineraries?

Tue Nov 10 03:55:14 PST 2015

Although currently out of tree, the Movidius SHAVE compiler also makes extensive use of Itineraries. It has some fairly complex scheduling requirements that I have so far not figured out how to express with the SchedMachineModel approach.  It too is a VLIW processor, with predication and no instruction interlocking, so computing the correct scheduling is not just important for performance it is critical for correct code generation.

Curiously enough, a few months ago I posed a similar question, but the other way around regarding how to rewrite our scheduler to avoid using the itineraries - this thread has sort of answered that question J

Thanks,

            Martin O’Riordan - Movidius Ltd.

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Anshuman Dasgupta via llvm-dev
Sent: 09 November 2015 22:19
To: llvm-dev at lists.llvm.org; dpalermo at codeaurora.org
Subject: Re: [llvm-dev] Is there a way to convert between SchedMachineModel and Itineraries?

> Not many people work with either the machine model or itineraries.

Andrew's right; not too many people on the list work with the itineraries It was authored for the Hexagon backend and I believe the R600 uses it as well.

How complex are the bundling rules in your VLIW target? Is that documented somewhere? The complexity will determine whether I'd recommend using the DFA packetizer.

-Anshu

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On 11/9/2015 2:52 PM, Andrew Trick via llvm-dev wrote:

On Nov 9, 2015, at 10:49 AM, Rail Shafigulin <rail at esenciatech.com> wrote:

On Mon, Nov 9, 2015 at 10:31 AM, Hal Finkel <hfinkel at anl.gov> wrote:

----- Original Message -----
> From: "Rail Shafigulin via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Monday, November 9, 2015 10:09:07 AM
> Subject: Re: [llvm-dev] Is there a way to convert between SchedMachineModel   and Itineraries?
>
>
> Anybody? Does anyone at all know how to do it?

There is no direct conversion. Although they are similar models, they are different. The strength of Itineraries lies in modeling pipelines with complex hazards (especially those that are not fully pipelined, or have other similar ordering constraints). SchedMachineModel can't do that. Unless you need to represent those kinds of constraints, SchedMachineModel is preferred. We'd like to move toward using SchedMachineModel for most things.

 -Hal

Thanks for the reply Hal. I thought my thread was dead. 

I was recommended to use SchedMachineModel for my VLIW, which is what I've done (it took me a month to learn tblgen and write the scheduling model and one more month do get my head around LLVM. My experience with compilers is limited to a class I took in college a year ago). The scheduling part seems to be working (at least this is what my limited testing shows). However current DFAPacketizer is based on itineraries (DFAPacketizer.cpp, lines 66-73). I was hoping there is a way to convert between these two representations so that I wouldn't have to rewrite the packetizer. Unfortunately my experience with compilers is very limited and I still have a lot to learn. 

Right now I have two options. In both of the cases there are significant drawbacks.

1. Rewrite the scheduling model using itineraries. Unfortunately there is very little help available on this topic, I even asked on the IRC channel and nobody seems to know how it is done, since everyone is moving towards the SchedMachineModel.

Not many people work with either the machine model or itineraries.

Be careful! I think the DFAPacketizer will only be effective on a very simple itinerary (which could easily have been expressed in the new machine model instead). I’ve heard stories of it “blowing up” on large itineraries. I have no direct experience with it.

2. Write a new packetizer which will use the SchedMachineModel, however, as I said before, I have a very limited experience with compilers and this looks like some major work. I'm not afraid of it, it is just there is not much information and help available.

This looks like an opportunity for you to learn something interesting. Generating a state machine from a set of constraints is fairly straightforward. The only difficultly lies in limiting the total number of states so you don’t end up with a giant table. If you were able to express those constraints in the machine model they must not be too complicated.

If your state machine is really just modeling the number of functional units that can be used by a given VLIW bundle, then you don’t need to generate a state machine at all. All you need are counters. MachineScheduler can do this for you. It already has some support for scheduling instruction groups for a simple in-order machine (without plugging in your own scheduler at all). Currently, this isn’t fully implemented—it isn’t modeling multiple functional units per cycle. But that would be *very* easy to fix and is something I could help with. It’s just that no one has asked for it.

Andy

I would greatly appreciate any help on this

Rail

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