Use common register class

Remove target hook. Instead, use common register class to
check if two phis are allowed to reuse the same temp.

Author: jgu222

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -2158,7 +2158,7 @@ class LLVM_ABI TargetInstrInfo : public MCInstrInfo {
     return TargetOpcode::COPY;
   }
 
-  /// During PHI elimination lets target to make necessary checks and
+  /// During PHI eleimination lets target to make necessary checks and
   /// insert the copy to the PHI destination register in a target specific
   /// manner.
   virtual MachineInstr *createPHIDestinationCopy(
@@ -2168,7 +2168,7 @@ class LLVM_ABI TargetInstrInfo : public MCInstrInfo {
         .addReg(Src);
   }
 
-  /// During PHI elimination lets target to make necessary checks and
+  /// During PHI eleimination lets target to make necessary checks and
   /// insert the copy to the PHI destination register in a target specific
   /// manner.
   virtual MachineInstr *createPHISourceCopy(MachineBasicBlock &MBB,
@@ -2180,17 +2180,6 @@ class LLVM_ABI TargetInstrInfo : public MCInstrInfo {
         .addReg(Src, 0, SrcSubReg);
   }
 
-  /// During PHI elimination lets target to decide if two phis can use the
-  /// same register \p Reg when they have the same rhs. Register \p Reg has
-  /// been used for the first phi and \p PHIReg is the DestReg of the second
-  /// Phi. This function is to check if the second phi can reuse \p Reg as
-  /// its temporary register.
-  /// The default is to allow reuse.
-  virtual bool allowPHIReuse(Register Reg, Register PHIReg,
-                             const MachineFunction &MF) const {
-    return true;
-  }
-
   /// Returns a \p outliner::OutlinedFunction struct containing target-specific
   /// information for a set of outlining candidates. Returns std::nullopt if the
   /// candidates are not suitable for outlining. \p MinRepeats is the minimum

llvm/lib/CodeGen/PHIElimination.cpp

@@ -377,10 +377,21 @@ void PHIEliminationImpl::LowerPHINode(MachineBasicBlock &MBB,
     // typically those created by tail duplication. Typically, an identical PHI
     // node can't occur, so avoid hashing/storing such PHIs, which is somewhat
     // expensive.
+
+    // canReuse() checks if two phis of the same rhs have the common sub class
+    // for their lhs's and allow reuse if so. (useful for some GPU targets)
+    auto canReuse = [](Register Reg0, Register Reg1, MachineFunction &MF) {
+      auto &MRI = MF.getRegInfo();
+      auto *RC0 = MRI.getRegClass(Reg0);
+      auto *RC1 = MRI.getRegClass(Reg1);
+      return MF.getSubtarget().getRegisterInfo()->getCommonSubClass(RC0, RC1) !=
+             nullptr;
+    };
+
     Register *Entry = nullptr;
     if (AllEdgesCritical)
       Entry = &LoweredPHIs[MPhi];
-    if (Entry && *Entry && TII->allowPHIReuse(*Entry, DestReg, MF)) {
+    if (Entry && *Entry && canReuse(*Entry, DestReg, MF)) {
       // An identical PHI node was already lowered. Reuse the incoming register.
       IncomingReg = *Entry;
       reusedIncoming = true;