[AMDGPU][NFC] Organize code around reserving VGPR32 for AGPR copy.

This is an NFC patch in preparation to fix a bug related to always
reserving VGPR32 for AGPR copy.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D123651

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -614,7 +614,8 @@ static void indirectCopyToAGPR(const SIInstrInfo &TII,
   // Registers in the sequence are allocated contiguously so we can just
   // use register number to pick one of three round-robin temps.
   unsigned RegNo = DestReg % 3;
-  Register Tmp = AMDGPU::VGPR32;
+  Register Tmp =
+      MBB.getParent()->getInfo<SIMachineFunctionInfo>()->getVGPRForAGPRCopy();
   assert(MBB.getParent()->getRegInfo().isReserved(Tmp) &&
          "VGPR used for an intermediate copy should have been reserved.");
 

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

@@ -187,6 +187,12 @@ SIMachineFunctionInfo::SIMachineFunctionInfo(const MachineFunction &MF)
   S = F.getFnAttribute("amdgpu-gds-size").getValueAsString();
   if (!S.empty())
     S.consumeInteger(0, GDSSize);
+
+  // On GFX908, in order to guarantee copying between AGPRs, we need a scratch
+  // VGPR available at all times.
+  if (ST.hasMAIInsts() && !ST.hasGFX90AInsts()) {
+    VGPRForAGPRCopy = AMDGPU::VGPR_32RegClass.getRegister(32);
+  }
 }
 
 void SIMachineFunctionInfo::limitOccupancy(const MachineFunction &MF) {

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.h

@@ -493,6 +493,16 @@ private:
   // frame, so save it here and add it to the RegScavenger later.
   Optional<int> ScavengeFI;
 
+private:
+  Register VGPRForAGPRCopy;
+
+public:
+  Register getVGPRForAGPRCopy() const {
+    assert(VGPRForAGPRCopy &&
+           "Valid VGPR for AGPR copy must have been identified by now");
+    return VGPRForAGPRCopy;
+  }
+
 public: // FIXME
   /// If this is set, an SGPR used for save/restore of the register used for the
   /// frame pointer.

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -698,7 +698,7 @@ BitVector SIRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
   // On GFX908, in order to guarantee copying between AGPRs, we need a scratch
   // VGPR available at all times.
   if (ST.hasMAIInsts() && !ST.hasGFX90AInsts()) {
-    reserveRegisterTuples(Reserved, AMDGPU::VGPR32);
+    reserveRegisterTuples(Reserved, MFI->getVGPRForAGPRCopy());
   }
 
   for (auto Reg : MFI->WWMReservedRegs) {
@@ -1553,8 +1553,8 @@ void SIRegisterInfo::buildSpillLoadStore(
       assert(EltSize == 4);
 
       if (!TmpIntermediateVGPR) {
-        assert(MF->getRegInfo().isReserved(AMDGPU::VGPR32));
-        TmpIntermediateVGPR = AMDGPU::VGPR32;
+        TmpIntermediateVGPR = FuncInfo->getVGPRForAGPRCopy();
+        assert(MF->getRegInfo().isReserved(TmpIntermediateVGPR));
       }
       if (IsStore) {
         auto AccRead = BuildMI(MBB, MI, DL,