[llvm-dev] help/hints/suggestions/tips please: how to give _generic_ compilation for a particular ISA a non-zero LoopMicroOpBufferSize?

[Hal Finkel via llvm-dev]

On 12/16/2016 05:05 PM, Abe Skolnik via llvm-dev wrote:
> Dear all,
>
> Some benchmarking experimentation I`ve done recently -- all on AArch64 
> -- has shown that it might be beneficial for all AArch64 targets to 
> have a positive LoopMicroOpBufferSize, whereas the default that 
> applies to all ISAs seems to be zero.
Would it be the same positive value for all cores?

You could bypass the mechanism entirely and just override 
TTI::getUnrollingPreferences in the target.

   -Hal
>
> Although I`ve tried going as far down the rabbit hole as I can, I 
> haven`t found a way to set DefaultLoopMicroOpBufferSize on a per-ISA 
> basis or to change the generic AArch64 model.  I`ve searched for 
> anything that might create an ISA-specific default scheduling model or 
> that might initialize the default MCSchedModel in an ISA-specific way, 
> but I`ve come up empty.
>
> The closest I found to what I was looking for seems to be a barrier 
> for what I am trying to do; quoting 
> "lib/Target/AArch64/AArch64GenSubtargetInfo.inc" in a build dir.:
>
>     { "generic", (const void *)&NoSchedModel },
>
>   [...]
>
>   static const llvm::MCSchedModel NoSchedModel = {
>     MCSchedModel::DefaultIssueWidth,
>     MCSchedModel::DefaultMicroOpBufferSize,
>     MCSchedModel::DefaultLoopMicroOpBufferSize,
>
>
> ... where "NoSchedModel" seems to be generated from this TableGen code 
> in the source-tree file at "llvm/lib/Target/AArch64/AArch64.td":
>
>   def : ProcessorModel<"generic", NoSchedModel, [
>                      FeatureCRC,
>                      FeatureFPARMv8,
>                      FeatureNEON,
>                      FeaturePerfMon,
>                      FeaturePostRAScheduler
>                      ]>;
>
> ... wherein it`s obvious how to add something like 
> "FeatureCustomCheapAsMoveHandling" to the list of features, but I have 
> no idea how to change the LoopMicroOpBufferSize, default or otherwise.
>
> Furthermore, "NoSchedModel" seems to be a cross-ISA entity that is 
> defined by code generated from this snippet of 
> "llvm/include/llvm/Target/TargetSchedule.td":
>
>   def NoSchedModel : SchedMachineModel {
>     let NoModel = 1;
>     let CompleteModel = 0;
>   }
>
>
> ... which is made the default model in code generated from this 
> snippet of "llvm/include/llvm/Target/Target.td":
>
>   class Processor<string n, ProcessorItineraries pi, 
> list<SubtargetFeature> f> {
>
>   [...]
>
>     SchedMachineModel SchedModel = NoSchedModel;
>
>
>
> So, all in all, given the current structure of this part of LLVM, 
> there seems to be no way to make a particular ISA have defaults 
> different than the global defaults, and in AArch64 I can`t see a way 
> to make the generic model different from the default model.
>
> If editing "AArch64GenSubtargetInfo.inc" was a decent way of changing 
> the generic model, I guess I would not need any help/etc. on this 
> topic, but I`m pretty sure that editing "AArch64GenSubtargetInfo.inc" 
> cannot possibly be the basis of a good patch proposal.  [Maybe it 
> won`t even work for an experiment -- I haven`t tried it yet.]
>
>
> Regards,
>
> Abe
>
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