Documenting behaviour of ResourceRef (HWInstructionDispatchedEvent)

[rainerzufalldererste]

I've worked on a project using MCA/llvm-mca and found the behaviour to be unintuitive and not explicitly documented anywhere afaik. It's used in llvm-mca/Views/ResourcePressureView.cpp:62 and reads the value via llvm::countr_zero there. I hope the comment is appreciated and the review doesn't cause any inconveniences.

[andreadb]

LGTM Thanks!

[rainerzufalldererste]

Sorry, first diff, would you mind comitting this for me?

llvm/include/llvm/MCA/HWEventListener.h

public:
 
   // The event type. The exact meaning depends on the subtarget.
   const unsigned Type;
 
   // The instruction this event was generated for.
   const InstRef &IR;
 };
 
+// ResourceRef::first is the index of the associated Resource.
+// ResourceRef::second is a bitmask of the referenced sub-unit of the resource.
 using ResourceRef = std::pair<uint64_t, uint64_t>;
+
 using ResourceUse = std::pair<ResourceRef, ResourceCycles>;
 
 class HWInstructionIssuedEvent : public HWInstructionEvent {
 public:
   HWInstructionIssuedEvent(const InstRef &IR, ArrayRef<ResourceUse> UR)
       : HWInstructionEvent(HWInstructionEvent::Issued, IR), UsedResources(UR) {}
 
   ArrayRef<ResourceUse> UsedResources;