[AMDGPU] Don't implement GCNHazardRecognizer::PreEmitNoops(SUnit *)

[foad]

Summary

When called from the post-RA scheduler, hazards have already been
handled by getHazardType returning NoopHazard, so PreEmitNoops always
returns zero. Remove it. NFC.

Historical note: PreEmitNoops was added to the hazard recognizer
interface as an optional feature to support dispatch group formation on
the POWER target:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20131202/197470.html
So it seems right that we shouldn't need to implement it.

We do still implement the other overload PreEmitNoops(MachineInstr *)
because that is used by the PostRAHazardRecognizer pass.

[arsenm]

Was this actually emitting nops or dead?

[foad]

In D79476#2022654, @arsenm wrote:

Was this actually emitting nops or dead?

It was called but it always returned zero.

[rampitec]

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.cpp

232

Do we still need this variable? I am not sure, it is initialized as false, but is there an alive path after this change?

[foad]

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.cpp

232

Yes I think we still need this to select different behaviour in getWaitStatesSince. For the post-RA scheduler usage, the variable just defaults to false. For standalone usage, PreEmitNoops sets it to true. I am still working on further cleanups in this area.

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.h

 public:
   GCNHazardRecognizer(const MachineFunction &MF);
   // We can only issue one instruction per cycle.
   bool atIssueLimit() const override { return true; }
   void EmitInstruction(SUnit *SU) override;
   void EmitInstruction(MachineInstr *MI) override;
   HazardType getHazardType(SUnit *SU, int Stalls) override;
   void EmitNoop() override;
-  unsigned PreEmitNoops(SUnit *SU) override;
   unsigned PreEmitNoops(MachineInstr *) override;
   unsigned PreEmitNoopsCommon(MachineInstr *);
   void AdvanceCycle() override;
   void RecedeCycle() override;
 };
 
 } // end namespace llvm
 
 #endif //LLVM_LIB_TARGET_AMDGPUHAZARDRECOGNIZERS_H

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.cpp

for (unsigned i = 0, e = std::min(WaitStates, MaxLookAhead - 1); i < e; ++i)
       EmittedInstrs.push_front(nullptr);
 
     EmittedInstrs.push_front(CurrCycleInstr);
     EmittedInstrs.resize(MaxLookAhead);
   }
   CurrCycleInstr = nullptr;
 }
 
-unsigned GCNHazardRecognizer::PreEmitNoops(SUnit *SU) {
-  IsHazardRecognizerMode = false;
-  return PreEmitNoopsCommon(SU->getInstr());
-}
-
 unsigned GCNHazardRecognizer::PreEmitNoops(MachineInstr *MI) {
   IsHazardRecognizerMode = true;
   CurrCycleInstr = MI;
   unsigned W = PreEmitNoopsCommon(MI);
   fixHazards(MI);
   CurrCycleInstr = nullptr;
   return W;
 }