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

Christof Douma via llvm-dev llvm-dev at lists.llvm.org
Tue Apr 26 05:09:45 PDT 2016

Hi Phil.

You more or less answered your own question, but let me give you some more info. Maybe it is of use.

>From what I understand the SchedMachineModel  is the future, although it is not as powerful as itineraries at present. The mi-scheduler is mostly developed around out-of-orders cores, I believe (I love to hear arguments on the contrary). Some of the constraints that can be found in in-order micro architectures cannot be expressed in the per-operand scheduling model and the heuristics of the pre-RA scheduling pass is probably a bit too focussed on register pressure for in-order cores (I have no numbers, just hearsay).

There is some documentation in comments at the start of include/llvm/Target/TargetSchedule.td that you might find useful. If you are going to look at an existing scheduling model, I suggest to look at an in-order core. A good example would be AArch64/AArch64SchedA53.td. If itineraries are present, they are used by the mi-scheduler next to the SchedMachineModel to detect hazards. I think that is the only place where the mi-scheduler uses itineraries.

There are some magic numbers you need for in-order operation. Most notably MicroOpBufferSize should be set to 0 for full in-order behaviour. You also want to set CompleteModel to 0 as that prevents asserts due to instructions without scheduling information. There is a script that might help you to visualise if you have provided scheduling information in the SchedMachineModel for all instructions (utils/schedcover.py). It is very simplistic and takes as input the debug output of tablegen. There are some usage comments at the beginning.


From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of Phil Tomson via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Reply-To: Phil Tomson <phil.a.tomson at gmail.com<mailto:phil.a.tomson at gmail.com>>
Date: Wednesday, 20 April 2016 23:06
To: LLVM Developers Mailing List <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] How to get started with instruction scheduling? Advice needed.

I notice from looking at ARMScheduleA9.td that there seems to be a hybrid approach where they still have itineraries but also use SchedMachineModel:

// ===---------------------------------------------------------------------===//
// The following definitions describe the simpler per-operand machine model.
// This works with MachineScheduler and will eventually replace itineraries.

class A9WriteLMOpsListType<list<WriteSequence> writes> {
  list <WriteSequence> Writes = writes;
  SchedMachineModel SchedModel = ?;

// Cortex-A9 machine model for scheduling and other instruction cost heuristics.
def CortexA9Model : SchedMachineModel {
  let IssueWidth = 2; // 2 micro-ops are dispatched per cycle.
  let MicroOpBufferSize = 56; // Based on available renamed registers.
  let LoadLatency = 2; // Optimistic load latency assuming bypass.
                       // This is overriden by OperandCycles if the
                       // Itineraries are queried instead.
  let MispredictPenalty = 8; // Based on estimate of pipeline depth.

  let Itineraries = CortexA9Itineraries;

  // FIXME: Many vector operations were never given an itinerary. We
  // haven't mapped these to the new model either.
  let CompleteModel = 0;

I'm guessing this is probably the way forward for my case since Itineraries seem to be already mostly defined.


On Wed, Apr 20, 2016 at 1:27 PM, Phil Tomson <phil.a.tomson at gmail.com<mailto:phil.a.tomson at gmail.com>> wrote:
So if I use the SchedMachineModel method, can I just skip itineraries?


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


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<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<mailto: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

  *   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?



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