[LLVMdev] Scheduling with RAW hazards

Andrew Trick atrick at apple.com
Thu May 9 10:25:25 PDT 2013

On May 9, 2013, at 4:02 AM, Fraser Cormack <fraser at codeplay.com> wrote:

> I have an instruction that takes no operands, and produces two results, in two consecutive cycles.
> I tried both of the following to my Schedule.td file:
> InstrItinData<IIMyInstr,  [InstrStage<2,  [FuncU]>], [1, 2]>,
> InstrItinData<IIMyInstr,  [InstrStage<1,  [FuncU]>, InstrStage<1, [FuncU]>], [1, 2]>,
> From what I can see in examples, these say that the first operand is ready the cycle after issue, and the second is ready 2 cycles after issue.

Yes, they look equivalent.

> But when I issue an instruction that uses both results, it does not obey this hazard, and is issued the cycle immediately after.  Are there any target hooks I need to implement to get this scheduling correctly?

Look at -debug-only=pre-RA-sched and confirm that the DAG's edges have the correct latency.

It also prints the current cycle count each time it schedules an instruction.
DEBUG(dbgs() << "\n*** Scheduling [" << CurCycle << "]: ");

You should see a two cycle difference between MyInstr and its second dependent. The scheduler won't insert nops for you. You'd need to do that in a target-specific way.

> I noticed that my target was using the default HazardRecognizer, which is effectively disabled, so I changed it to use the ScoreboardHazardRecognizer instead. I'm also still using the SelectionDAG scheduler, but will need to change to the MI scheduler at some point, to keep up with trunk. Should either of these help?

The hazard recognizer won't help you. It only enforces pipeline hazards (other instructions that need FuncU). It's the list scheduler itself that "enforces" operand latency.

MI scheduler allows you to use a new machine model that's simpler for most people who don't need the precision of Itineraries. Maybe not important in your case.

More importantly, SDScheduler is take-it-as-is, and will go away entirely after 3.3. Whereas MI scheduler can be fixed and improved. Now would be a good time to try switching over and start filing bugs. PPC is an example of using MI scheduler out-of-box. Hexagon is an example of customizing it at a high level. You could start off like PPC with minimal customization, but eventually you may want something in between--provide a custom MachineSchedStrategy:

class MyScheduler : public MachineSchedStrategy {...}

namespace llvm {
ScheduleDAGInstrs *createMySched(MachineSchedContext *C) {
  ScheduleDAGMI *DAG = new ScheduleDAGMI(C, new MyScheduler());
  DAG->addMutation(new MyDAGMutation());
  return DAG;
} // namespace llvm

static MachineSchedRegistry
MySchedRegistry("mysched", "Custom My scheduler.", createMySched);


> Thanks,
> Fraser
> -- 
> Fraser Cormack
> Compiler Developer
> Codeplay Software Ltd
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