AMDGPU: Make findUsedSGPR more readable

[arsenm]

Summary

Add more comments etc.

[arsenm]

Rebase

[• tstellarAMD]

LGTM.

[arsenm]

r251996

lib/Target/AMDGPU/SIInstrInfo.cpp

case AMDGPU::INSERT_SUBREG:
     if (!NewDstRC)
       return nullptr;
     return NewDstRC;
   default:
     return NewDstRC;
   }
 }
 
+// Find the one SGPR operand we are allowed to use.
 unsigned SIInstrInfo::findUsedSGPR(const MachineInstr *MI,
                                    int OpIndices[3]) const {
   const MCInstrDesc &Desc = MI->getDesc();
 
   // Find the one SGPR operand we are allowed to use.
   //
   // First we need to consider the instruction's operand requirements before
   // legalizing. Some operands are required to be SGPRs, such as implicit uses
unsigned SIInstrInfo::findUsedSGPR(const MachineInstr *MI,
   const MachineRegisterInfo &MRI = MI->getParent()->getParent()->getRegInfo();
 
   for (unsigned i = 0; i < 3; ++i) {
     int Idx = OpIndices[i];
     if (Idx == -1)
       break;
 
     const MachineOperand &MO = MI->getOperand(Idx);
-    if (RI.isSGPRClassID(Desc.OpInfo[Idx].RegClass))
-      SGPRReg = MO.getReg();
+    if (!MO.isReg())
+      continue;
 
-    if (MO.isReg() && RI.isSGPRClass(MRI.getRegClass(MO.getReg())))
-      UsedSGPRs[i] = MO.getReg();
-  }
+    // Is this operand statically required to be an SGPR based on the operand
+    // constraints?
+    const TargetRegisterClass *OpRC = RI.getRegClass(Desc.OpInfo[Idx].RegClass);
+    bool IsRequiredSGPR = RI.isSGPRClass(OpRC);
+    if (IsRequiredSGPR)
+      return MO.getReg();
 
-  if (SGPRReg != AMDGPU::NoRegister)
-    return SGPRReg;
+    // If this could be a VGPR or an SGPR, Check the dynamic register class.
+    unsigned Reg = MO.getReg();
+    const TargetRegisterClass *RegRC = MRI.getRegClass(Reg);
+    if (RI.isSGPRClass(RegRC))
+      UsedSGPRs[i] = Reg;
+  }
 
   // We don't have a required SGPR operand, so we have a bit more freedom in
   // selecting operands to move.
 
   // Try to select the most used SGPR. If an SGPR is equal to one of the
   // others, we choose that.
   //
   // e.g.
   // V_FMA_F32 v0, s0, s0, s0 -> No moves
   // V_FMA_F32 v0, s0, s1, s0 -> Move s1
 
+  // TODO: If some of the operands are 64-bit SGPRs and some 32, we should
+  // prefer those.
+
   if (UsedSGPRs[0] != AMDGPU::NoRegister) {
     if (UsedSGPRs[0] == UsedSGPRs[1] || UsedSGPRs[0] == UsedSGPRs[2])
       SGPRReg = UsedSGPRs[0];
   }
 
   if (SGPRReg == AMDGPU::NoRegister && UsedSGPRs[1] != AMDGPU::NoRegister) {
     if (UsedSGPRs[1] == UsedSGPRs[2])
       SGPRReg = UsedSGPRs[1];