[llvm-dev] Pre-RA scheduler does not generate NOPs when getHazardType() returns NoopHazard

[Hal Finkel via llvm-dev]

On 03/21/2017 08:29 AM, Alex Susu via llvm-dev wrote:
>   Hello.
>     I am not sure if this is the correct answer to the question I put 
> in the previous email, but please take a look at what I wrote in 
> another thread at 
> http://lists.llvm.org/pipermail/llvm-dev/2017-March/111293.html .
>     Basically, I describe there I was more successful to use 
> PreEmitNoops(), instead of the getHazardType() method which, when I 
> return NoopHazard, it does NOT always generate a NOP - in my case it 
> works for Store, but not for Load MachineInstructions.
>
>     Does the ScoreboardHazardRecognizer class generate (or not) a NOP 
> instruction, when instructed to in getHazardType(), depending on the 
> type of the instruction analyzed?

What do you mean? The ScoreboardHazardRecognizer does not generate NOP 
hazards. You need to override it to do that. The post-RA scheduler 
should then generate the NOPs (by calling your insertNoop).

  -Hal

>
>   Thank you,
>     Alex
>
>
> On 2/12/2017 4:10 AM, Alex Susu wrote:
>>   Hello.
>>     I am new to the schedulers implemented in the back end of LLVM.
>>     I am trying to handle data hazards in my simple processor, with 
>> instructions that
>> execute in 1 cycle.
>>
>>     I have tried the standard post-RA scheduler, implemented in
>> lib/CodeGen/PostRASchedulerList.cpp, (with a 
>> ScoreboardHazardRecognizer), but I have some
>> issues with some consecutive instructions that are separated in the 
>> final ASM listing and
>> they should not.
>>
>>     I also tried the standard pre-RA scheduler, implemented in
>> lib/CodeGen/ScheduleDAGRRList.cpp, (with a 
>> ScoreboardHazardRecognizer), but even if I
>> report that I have a data hazard in getHazardType() and return 
>> NoopHazard, the scheduler
>> will not insert a NOP instruction in the respective place where I 
>> report the hazard . For
>> me it is more convenient, in principle, to use the pre-RA scheduler, 
>> but it is not
>> generating NOPs.
>>     Could you please tell me how can I generate NOPs with a pre-RA 
>> scheduler when I am
>> already informing it that at the current place we have a NoopHazard.
>>
>>  Thank you,
>>     Alex
>>
>>   PS: Also, could you please tell me if with the standard pre-RA 
>> scheduler with
>> ScoreboardHazardRecognizer I could also employ other useful program 
>> instructions when
>> dealing with hazards, instead of using NOPs.
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory