[LLVMdev] Scheduling with RAW hazards

Fraser Cormack fraser at codeplay.com
Mon May 13 06:51:00 PDT 2013

On 09/05/2013 18:25, Andrew Trick wrote:
> On May 9, 2013, at 4:02 AM, Fraser Cormack <fraser at codeplay.com 
> <mailto: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.

Yes, I see the two-cycle difference between the two instructions. I 
enabled the post-RA scheduler, and noticed that it cared about the 
latencies, and started to rearrange the instructions accordingly. Is it 
necessary to use the post-RA scheduler to enforce such latencies?

>> 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.

Ah okay, thank you.

> 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);
> -Andy

I've had a quick experiment with the MI Scheduler, and have a few 
further questions. From what I can see, if I pass -enable-misched to the 
compiler, it only works above O1, though addOptimizedRegAlloc(). Is O0 
not supported without adding the pass myself in my PassConfig?

How does (or will) the MI Scheduler interact with the existing SD 
Scheduler? It seems as though they both run together at the moment.

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