[llvm-dev] Is subclass of ScheduleDAGMILive a pre-RA scheduler?

[陳韋任 via llvm-dev]

Hi, Matthias.

>From the class hierarchy, ScheduleDAGMILive is also a ScheduleDAGMI. I
am wondering if there will be any problem if
we use subclass of ScheduleDAGMILive as post-RA scheduler? The best
case is ScheduleDAGMILive just waste time
on book-keeping register pressure, but I am not sure if we can still
do those book-keeping after RA.

Talk about post-RA scheduler, I see there is another class
PostRASchedulerList whose DEBUG_TYPE is post-RA-sched.
I am wondering if that means we have two post-RA schedulers in LLVM
right now? Any particular difference between those
two classes, PostMachineScheduler and PostRASchedulerList?

Thanks.

Regards,
chenwj

2017-04-25 10:46 GMT+08:00 Matthias Braun <mbraun at apple.com>:
>
>> On Apr 22, 2017, at 5:12 AM, 陳韋任 via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> Hi All,
>>
>>  The description of ScheduleDAGMILive [1] says:
>>
>>    ScheduleDAGMILive is an implementation of ScheduleDAGInstrs that
>> schedules machine instructions
>>    while updating LiveIntervals and tracking regpressure.
>>
>> Does the live interval and register pressure part of ScheduleDAGMILive
>> mean the subclass of ScheduleDAGMILive
>> is a pre-RA scheduler? I assume the post-RA scheduler no need to take
>> care of  live interval and register pressure,
>> since RA is done already.
>
> Yes ScheduleDAGMILive is used by the MachineScheduler which runs before register allocation. By default the MachineScheduler is put between register coalescing and the greedy register allocator.
>
> (There is also the PostMachineScheduler which as the name suggests runs after register allocation and that one only uses ScheduleDAGMI and does not track register pressure).
>
> - Matthias



-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj