[CodeGen] Add target hook shouldReMaterializeTrivialRegDef

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -418,6 +418,23 @@ class LLVM_ABI TargetInstrInfo : public MCInstrInfo {
     return true;
   }
 
+  /// Returns true if CopyMI should be considered for register
+  /// definition rematerialization. Otherwise, returns false.
+  ///
+  /// Rematerialization can replace a source register with its value
+  /// from its definition. Its applied in the register coalescer,
+  /// after instruction selection and before register allocation.
+  ///
+  /// Subtargets can override this method to classify rematerialization
+  /// candidates. Note that this cannot be defined in tablegen because it
+  /// operates at a higher level.
+  virtual bool shouldReMaterializeTrivialRegDef(
+      const MachineFunction *MF, const MachineInstr &CopyMI,
+      const Register &DestReg, const Register &SrcReg,
+      const LiveIntervals *LIS) const {
+    return true;
+  }
+
   /// Re-issue the specified 'original' instruction at the
   /// specific location targeting a new destination register.
   /// The register in Orig->getOperand(0).getReg() will be substituted by

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -1029,6 +1029,14 @@ bool AArch64InstrInfo::isAsCheapAsAMove(const MachineInstr &MI) const {
   }
 }
 
+bool AArch64InstrInfo::shouldReMaterializeTrivialRegDef(
+    const MachineFunction *MF, const MachineInstr &CopyMI,
+    const Register &DestReg, const Register &SrcReg,
+    const LiveIntervals *LIS) const {
+  return !Subtarget.canLowerToZeroCycleRegMove(CopyMI, DestReg, SrcReg) &&
+         !Subtarget.canLowerToZeroCycleRegZeroing(CopyMI, DestReg, SrcReg);
+}
+
 bool AArch64InstrInfo::isFalkorShiftExtFast(const MachineInstr &MI) {
   switch (MI.getOpcode()) {
   default:
@@ -5025,6 +5033,9 @@ void AArch64InstrInfo::copyGPRRegTuple(MachineBasicBlock &MBB,
   }
 }
 
+/// NOTE: must maintain consistency with
+/// `AArch64Subtarget::canLowerToZeroCycleRegMove` and
+/// `AArch64Subtarget::canLowerToZeroCycleRegZeroing`.
 void AArch64InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
                                    MachineBasicBlock::iterator I,
                                    const DebugLoc &DL, Register DestReg,

llvm/lib/Target/AArch64/AArch64InstrInfo.h

@@ -189,6 +189,11 @@ class AArch64InstrInfo final : public AArch64GenInstrInfo {
 
   bool isAsCheapAsAMove(const MachineInstr &MI) const override;
 
+  bool shouldReMaterializeTrivialRegDef(
+      const MachineFunction *MF, const MachineInstr &CopyMI,
+      const Register &DestReg, const Register &SrcReg,
+      const LiveIntervals *LIS) const override;
+
   bool isCoalescableExtInstr(const MachineInstr &MI, Register &SrcReg,
                              Register &DstReg, unsigned &SubIdx) const override;