[LLVMdev] Instruction Scheduling - migration from v3.1 to v3.2

[Martin J. O'Riordan]

Thanks Andrew, that information is a great help.  I think I will rewrite our
old mechanism to use the method you describe, as we do not currently have a
Schedule.td implementation.

 

            MartinO

 

From: Andrew Trick [mailto:atrick at apple.com] 
Sent: 30 April 2013 03:21
To: Martin.ORiordan at movidius.com
Cc: llvmdev at cs.uiuc.edu; Dev
Subject: Re: [LLVMdev] Instruction Scheduling - migration from v3.1 to v3.2

 

On Apr 26, 2013, at 3:53 AM, Martin J. O'Riordan
<Martin.ORiordan at movidius.com> wrote:

I am migrating the llvm/clang derived compiler for our processor from the
v3.1 to v3.2 codebase.  This has mostly gone well except that instruction
latency scheduling is no longer happening.

The people who implemented this previously sub-classed 'ScheduleDAGInstrs'
and implemented the virtual methods 'computeLatency' and
'computeOperandLatency'.  However, these methods have been removed from
'ScheduleDAG' and 'ScheduleDAGInstrs' so are no longer invoked on our
implementation.  Instead, the correct approach seems to implement a
sub-class of 'TargetSchedModel'.

When I had a look at how other targets dealt with this transition I found
that none of them had implemented the latency scheduler the way we had, so I
couldn't just mimic a strategy used by other targets moving from v3.1 to
v3.2.  Unfortunately the people who implemented the existing solution are no
longer with the organization so I am trying to determine how to fix this
without their experience.

Is there any preferred or recommended approach for rewriting our existing
implementation to fit the 'TargetSchedModel' mechanism?

 

Hi Martin,

 

Yes, TargetSchedModel is the machine-instr scheduler's interface to the
machine model. It is not extensible. Instead, it checks the current
subtarget configuration, and either directly queries latency tables, or
calls hooks.

 

Three possible configurations are:

 

(1) No per-opcode tables

 

In this case, only one helper is used:

 

unsigned TargetInstrInfo::defaultDefLatency(const MCSchedModel *SchedModel,

                                            const MachineInstr *DefMI)
const;

 

This is currently non-virtual because it is designed to directly query the
fields in the SchedMachineModel tablegen class using another hook to select
high latency ops:

 

virtual bool isHighLatencyDef(int opc) const;

 

Instructions that are not high latency default to one cycle.

 

Example:

 

def GenericModel : SchedMachineModel {

  let LoadLatency = 4;

  let HighLatency = 10;

}

 

If it really helps, you could redeclare defaultDefLatency as a virtual hook
and override it.

 

(2) The "old" style of pipeline itineraries.

 

This is closest to the old behavior. If an itinerary is defined, you get to
override two hooks:

 

virtual int TargetInstrInfo::getOperandLatency(const InstrItineraryData
*ItinData,

                                               const MachineInstr *DefMI,
unsigned DefIdx,

                                               const MachineInstr *UseMI,

                                               unsigned UseIdx) const;

 

virtual unsigned TargetInstrInfo::getInstrLatency(const InstrItineraryData
*ItinData,

                                                  const MachineInstr *MI,

                                                  unsigned *PredCost = 0)
const;

 

Example:

 

def CortexA8Model : SchedMachineModel {

   ...

  let Itineraries = CortexA8Itineraries;

}

 

If you don't actually have any itineraries defined in you target's
Schedule.td files, you need to define some dummy itinerary with at least one
entry. Something like this may work (I haven't tried it).

 

def IIC_Default : InstrItinClass;

 

def MyItineraries : ProcessorItineraries<[

  InstrItinData<IIC_Default, [], []>

]>;

 

3) The new style of machine model.

 

Here, you don't use any hooks above the machine model. Instead you define
all possible instruction latencies and resources in your .td. For a
particular opcode, the model allows selecting the set of resources based on
hooks. But all the resources are defined in one place.

 

-Andy

 

 

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