[llvm-dev] How to get started with instruction scheduling? Advice needed.

[Phil Tomson via llvm-dev]

So if I use the SchedMachineModel method, can I just skip itineraries?

Phil

On Wed, Apr 20, 2016 at 12:29 PM, Sergei Larin <slarin at codeaurora.org>
wrote:

> Target does make a difference. VLIW needs more hand-holding. For what you
> are describing it should be fairly simple.
>
>
>
> Best strategy – see what other targets do. ARM might be a good start for
> generic superscalar. Hexagon for VLIW style scheduling.
>
>
>
> Depending on what you decide, you might need different target hooks.
>
>
>
> Sergei
>
>
>
> ---
>
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
> by The Linux Foundation
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of *Phil
> Tomson via llvm-dev
> *Sent:* Wednesday, April 20, 2016 12:51 PM
> *To:* LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
> *Subject:* [llvm-dev] How to get started with instruction scheduling?
> Advice needed.
>
>
>
> I need to add instruction scheduling for a new target which is a fairly
> simple in-order execution machine.
>
> I've been watching this presentation from a 2014 LLVM dev meeting as it
> seems relevant:
>
> "SchedMachineModel: Adding and Optimizing a Subtarget"
> http://llvm.org/devmtg/2014-10/Slides/Estes-MISchedulerTutorial.pdf
>
> In this presentation the author says that there have been several ways to
> approach scheduling in LLVM over the years:
>
>    - Pre 2008: SelectionDAGISel pass creates the ScheduleDAG from the
>    SelectionDAG at the end of instruction selection
>    - ScheduleDAG works on SelectionDAG Nodes (SDNodes)
>    - Circa 2008: Post Register
>
> Allocation pass added for
>
> instruction selection ( SchedulePostRATDList
>
> works on MachineInstrs)
>
>    - Circa 2012: MIScheduler
>
> (ScheduleDAGMI) added as
>
> separate pass for pre-RA
>
> scheduling
>
>    - Circa 2014: MIScheduler
>
> adapted to optionally replace
>
> PostRA Scheduler
>
> In the presentation he goes with defining a subclass of SchedMachineModel
> in the schedule .td file. And apparently with this approach there are no
> instruction itineraries.
>
> So I'm wondering: what's the current recommended way to approach this and
> does it depend on the type or target? (in-order, superscalar, out of order,
> VLIW...)?
>
> Someone earlier started to define instruction itineraries for our target.
> Should I continue down this road or move over to the SchedMachineModel
> approach? Are there other recommended presentations/documents that I should
> be looking at?
>
>
>
> Thanks.
>
> Phil
>
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