[llvm-dev] InstrItin and SchedWriteRes

[Andrew Trick via llvm-dev]

> On Mar 26, 2018, at 5:18 AM, Pedro Lopes via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> From what I can understand from analyzing several *.td files, there are two ways of specifying scheduling information for a specific target, either using SchedWriteRes and InstrItinClass/Data.
> 
> Specifically looking at ARMScheduleA9.td, I can find both representations and a comment (in the beggining of the file):
> 
> // This section contains legacy support for itineraries. This is
> // required until SD and PostRA schedulers are replaced by MachineScheduler.
> 
> This poses several question as to:
> 1) which representation gives the better cost estimation?
> 2) AFAIK, MachineScheduler can also take advantage from InstrItineraries with the ScoreboardHazardRecognizer for better cost estimation and scheduling.

As the comment indicates, this was done for legacy reasons, to allow targets that already defined itineraries to migrate away from SelectionDAG scheduling, without defining a new machine model. What makes you think it was done "for better cost estimation and scheduling”?

> What is the reasoning for having scheduling information distributed in two distinct representations? Is it the lack of micro-architectural details or was it a design decision?

Itineraries classes already existed for ARM, but provided no way to express microarchitectural details, like some vector operations and dependencies on immediate values. (This was compensated for by totally out-of-band logic in the backend to completely override scheduling decisions. That logic is both difficult to maintain, and incompatible with the goal of defining a self-contained machine model.) It also made simple concepts like register bypass very difficult to express and poorly communicated the machine model. The implementation of itineraries (ScoreboardHazardChecker) also needlessly wastes compile time for all but a rare class of VLIW cpus.

> Another question is if one could opt, which representation should me used to represent scheduling information for new targets?

I can’t imagine why someone want to use itineraries for a new target. Maybe you have a reason?

-Andy

> Best Regards,
> Tiago
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