AMDGPU/SI: Avoid creating unnecessary copies in the SIFixSGPRCopies pass

[• tstellarAMD]

Summary

1. Don't try to copy values to and from the same register class.
2. Replace copies with of registers with immediate values with v_mov/s_mov instructions.

The main purpose of this change is to make MachineSink do a better job of
determining when it is beneficial to split a critical edge, since the pass
assumes that copies will become move instructions.

This prevents a regression in uniform-cfg.ll if we enable critical edge
splitting for AMDGPU.

[arsenm]

lib/Target/AMDGPU/SIInstrInfo.cpp

2289

DebugLocs are supposed to be passed by reference now

2291

Period

2299–2306

There are two different ways here of referring to the same, original value. It would be easier to follow if Op.getReg() was assigned to a variable at the beginning, and then consistently used instead of switching to Copy->getOperand(1).getReg(), and then you wouldn't need to worry about the order of the Op.setReg().

Could Op have a sub register index?

2763

You can use Inst.isCopy()

[• tstellarAMD]

Address review comments and rebase.

[arsenm]

LGTM

lib/Target/AMDGPU/SIRegisterInfo.cpp

1000

Extra space here

1005–1007

This should be inline in the header and be isVGPRReg to match the existing isSGPRReg (or that should have the Reg part removed)

[Eugene.Zelenko]

Looks like patch was not committed.

lib/Target/AMDGPU/SIInstrInfo.h

public:
   /// be used when it is know that the value in SrcReg is same across all
   /// threads in the wave.
   /// \returns The SGPR register that \p SrcReg was copied to.
   unsigned readlaneVGPRToSGPR(unsigned SrcReg, MachineInstr &UseMI,
                               MachineRegisterInfo &MRI) const;
 
   void legalizeOperandsSMRD(MachineRegisterInfo &MRI, MachineInstr &MI) const;
 
+  void legalizeGenericOperand(MachineBasicBlock &InsertMBB,
+                              MachineBasicBlock::iterator I,
+                              const TargetRegisterClass *DstRC,
+                              MachineOperand &Op, MachineRegisterInfo &MRI,
+                              const DebugLoc &DL) const;
+
   /// \brief Legalize all operands in this instruction.  This function may
   /// create new instruction and insert them before \p MI.
   void legalizeOperands(MachineInstr &MI) const;
 
   /// \brief Replace this instruction's opcode with the equivalent VALU
   /// opcode.  This function will also move the users of \p MI to the
   /// VALU if necessary.
   void moveToVALU(MachineInstr &MI) const;

lib/Target/AMDGPU/SIInstrInfo.cpp

void SIInstrInfo::legalizeOperandsSMRD(MachineRegisterInfo &MRI,
   // pointer value is uniform.
   MachineOperand *SBase = getNamedOperand(MI, AMDGPU::OpName::sbase);
   if (SBase && !RI.isSGPRClass(MRI.getRegClass(SBase->getReg()))) {
       unsigned SGPR = readlaneVGPRToSGPR(SBase->getReg(), MI, MRI);
       SBase->setReg(SGPR);
   }
 }
 
+void SIInstrInfo::legalizeGenericOperand(MachineBasicBlock &InsertMBB,
+                                         MachineBasicBlock::iterator I,
+                                         const TargetRegisterClass *DstRC,
+                                         MachineOperand &Op,
+                                         MachineRegisterInfo &MRI,
+                                         const DebugLoc &DL) const {
+
+  unsigned OpReg = Op.getReg();
+  unsigned OpSubReg = Op.getSubReg();
+
+  const TargetRegisterClass *OpRC = RI.getSubClassWithSubReg(
+      RI.getRegClassForReg(MRI, OpReg), OpSubReg);
+
+  // Check if operand is already the correct register class.
+  if (DstRC == OpRC)
+    return;
+
+  unsigned DstReg = MRI.createVirtualRegister(DstRC);
+  MachineInstr *Copy = BuildMI(InsertMBB, I, DL, get(AMDGPU::COPY), DstReg)
+                               .addOperand(Op);
+
+  Op.setReg(DstReg);
+  Op.setSubReg(0);
+
+  MachineInstr *Def = MRI.getVRegDef(OpReg);
+  if (!Def)
+    return;
+
+  // Try to eliminate the copy if it is copying an immediate value.
+  if (Def->isMoveImmediate())
+    FoldImmediate(*Copy, *Def, OpReg, &MRI);
+}
+
 void SIInstrInfo::legalizeOperands(MachineInstr &MI) const {
   MachineRegisterInfo &MRI = MI.getParent()->getParent()->getRegInfo();
 
   // Legalize VOP2
   if (isVOP2(MI) || isVOPC(MI)) {
     legalizeOperandsVOP2(MRI, MI);
     return;
   }
if (VRC || !RI.isSGPRClass(getOpRegClass(MI, 0))) {
       RC = SRC;
     }
 
     // Update all the operands so they have the same type.
     for (unsigned I = 1, E = MI.getNumOperands(); I != E; I += 2) {
       MachineOperand &Op = MI.getOperand(I);
       if (!Op.isReg() || !TargetRegisterInfo::isVirtualRegister(Op.getReg()))
         continue;
-      unsigned DstReg = MRI.createVirtualRegister(RC);
 
       // MI is a PHI instruction.
       MachineBasicBlock *InsertBB = MI.getOperand(I + 1).getMBB();
       MachineBasicBlock::iterator Insert = InsertBB->getFirstTerminator();
 
-      BuildMI(*InsertBB, Insert, MI.getDebugLoc(), get(AMDGPU::COPY), DstReg)
-          .addOperand(Op);
-      Op.setReg(DstReg);
+      // Avoid creating no-op copies with the same src and dst reg class.  These
+      // confuse some of the machine passes.
+      legalizeGenericOperand(*InsertBB, Insert, RC, Op, MRI, MI.getDebugLoc());
     }
   }
 
   // REG_SEQUENCE doesn't really require operand legalization, but if one has a
   // VGPR dest type and SGPR sources, insert copies so all operands are
   // VGPRs. This seems to help operand folding / the register coalescer.
   if (MI.getOpcode() == AMDGPU::REG_SEQUENCE) {
     MachineBasicBlock *MBB = MI.getParent();
     const TargetRegisterClass *DstRC = getOpRegClass(MI, 0);
     if (RI.hasVGPRs(DstRC)) {
       // Update all the operands so they are VGPR register classes. These may
       // not be the same register class because REG_SEQUENCE supports mixing
       // subregister index types e.g. sub0_sub1 + sub2 + sub3
       for (unsigned I = 1, E = MI.getNumOperands(); I != E; I += 2) {
         MachineOperand &Op = MI.getOperand(I);
         if (!Op.isReg() || !TargetRegisterInfo::isVirtualRegister(Op.getReg()))
           continue;
 
         const TargetRegisterClass *OpRC = MRI.getRegClass(Op.getReg());
         const TargetRegisterClass *VRC = RI.getEquivalentVGPRClass(OpRC);
         if (VRC == OpRC)
           continue;
 
-        unsigned DstReg = MRI.createVirtualRegister(VRC);
-
-        BuildMI(*MBB, MI, MI.getDebugLoc(), get(AMDGPU::COPY), DstReg)
-            .addOperand(Op);
-
-        Op.setReg(DstReg);
+        legalizeGenericOperand(*MBB, MI, VRC, Op, MRI, MI.getDebugLoc());
         Op.setIsKill();
       }
     }
 
     return;
   }
 
   // Legalize INSERT_SUBREG
   // src0 must have the same register class as dst
   if (MI.getOpcode() == AMDGPU::INSERT_SUBREG) {
     unsigned Dst = MI.getOperand(0).getReg();
     unsigned Src0 = MI.getOperand(1).getReg();
     const TargetRegisterClass *DstRC = MRI.getRegClass(Dst);
     const TargetRegisterClass *Src0RC = MRI.getRegClass(Src0);
     if (DstRC != Src0RC) {
-      MachineBasicBlock &MBB = *MI.getParent();
-      unsigned NewSrc0 = MRI.createVirtualRegister(DstRC);
-      BuildMI(MBB, MI, MI.getDebugLoc(), get(AMDGPU::COPY), NewSrc0)
-          .addReg(Src0);
-      MI.getOperand(1).setReg(NewSrc0);
+      MachineBasicBlock *MBB = MI.getParent();
+      MachineOperand &Op = MI.getOperand(1);
+      legalizeGenericOperand(*MBB, MI, DstRC, Op, MRI, MI.getDebugLoc());
     }
     return;
   }
 
   // Legalize MIMG
   if (isMIMG(MI)) {
     MachineOperand *SRsrc = getNamedOperand(MI, AMDGPU::OpName::srsrc);
     if (SRsrc && !RI.isSGPRClass(MRI.getRegClass(SRsrc->getReg()))) {
while (!Worklist.empty()) {
     unsigned NewDstReg = AMDGPU::NoRegister;
     if (HasDst) {
       // Update the destination register class.
       const TargetRegisterClass *NewDstRC = getDestEquivalentVGPRClass(Inst);
       if (!NewDstRC)
         continue;
 
       unsigned DstReg = Inst.getOperand(0).getReg();
+      if (Inst.isCopy() &&
+          TargetRegisterInfo::isVirtualRegister(Inst.getOperand(1).getReg()) &&
+          NewDstRC == RI.getRegClassForReg(MRI, Inst.getOperand(1).getReg())) {
+        // Instead of creating a copy where src and dst are the same register
+        // class, we just replace all uses of dst with src.  These kinds of
+        // copies interfere with the heuristics MachineSink uses to decide
+        // whether or not to split a critical edge.  Since the pass assumes
+        // that copies will end up as machine instructions and not be
+        // eliminated.
+        addUsersToMoveToVALUWorklist(DstReg, MRI, Worklist);
+        MRI.replaceRegWith(DstReg, Inst.getOperand(1).getReg());
+        Inst.getOperand(0).setReg(DstReg);
+        continue;
+      }
+
       NewDstReg = MRI.createVirtualRegister(NewDstRC);
       MRI.replaceRegWith(DstReg, NewDstReg);
     }
 
     // Legalize the operands
     legalizeOperands(Inst);
 
     if (HasDst)

lib/Target/AMDGPU/SIRegisterInfo.h

public:
 
   unsigned findUnusedRegister(const MachineRegisterInfo &MRI,
                               const TargetRegisterClass *RC,
                               const MachineFunction &MF) const;
 
   unsigned getSGPRPressureSet() const { return SGPRSetID; };
   unsigned getVGPRPressureSet() const { return VGPRSetID; };
 
+  const TargetRegisterClass *getRegClassForReg(const MachineRegisterInfo &MRI,
+                                               unsigned Reg) const;
   bool isVGPR(const MachineRegisterInfo &MRI, unsigned Reg) const;
 
   bool isSGPRPressureSet(unsigned SetID) const {
     return SGPRPressureSets.test(SetID) && !VGPRPressureSets.test(SetID);
   }
   bool isVGPRPressureSet(unsigned SetID) const {
     return VGPRPressureSets.test(SetID) && !SGPRPressureSets.test(SetID);
   }

lib/Target/AMDGPU/SIRegisterInfo.cpp

switch(WaveCount) {
       case 7:  return 72;
       case 6:  return 80;
       case 5:  return 96;
       default: return 103;
     }
   }
 }
 
-bool SIRegisterInfo::isVGPR(const MachineRegisterInfo &MRI,
+const TargetRegisterClass*
+SIRegisterInfo::getRegClassForReg(const MachineRegisterInfo &MRI,
                                   unsigned Reg) const {
-  const TargetRegisterClass *RC;
   if (TargetRegisterInfo::isVirtualRegister(Reg))
-    RC = MRI.getRegClass(Reg);
-  else
-    RC = getPhysRegClass(Reg);
+    return  MRI.getRegClass(Reg);
+
+  return getPhysRegClass(Reg);
+}
 
-  return hasVGPRs(RC);
+bool SIRegisterInfo::isVGPR(const MachineRegisterInfo &MRI,
+                            unsigned Reg) const {
+  return hasVGPRs(getRegClassForReg(MRI, Reg));
 }