[CodeGen] Cache Opcode in MachineInstr (llvm#96797)

This avoids the indirection through MCID when just accessing the opcode.
This uses two of the four padding bytes at the end of MachineInstr.

Author: aengelke

llvm/include/llvm/CodeGen/MachineInstr.h

@@ -304,6 +304,9 @@ class MachineInstr
   /// defined by this instruction.
   unsigned DebugInstrNum;
 
+  /// Cached opcode from MCID.
+  uint16_t Opcode;
+
   // Intrusive list support
   friend struct ilist_traits<MachineInstr>;
   friend struct ilist_callback_traits<MachineBasicBlock>;
@@ -563,7 +566,7 @@ class MachineInstr
   const MCInstrDesc &getDesc() const { return *MCID; }
 
   /// Returns the opcode of this MachineInstr.
-  unsigned getOpcode() const { return MCID->Opcode; }
+  unsigned getOpcode() const { return Opcode; }
 
   /// Retuns the total number of operands.
   unsigned getNumOperands() const { return NumOperands; }

llvm/lib/CodeGen/MachineInstr.cpp

@@ -98,7 +98,7 @@ void MachineInstr::addImplicitDefUseOperands(MachineFunction &MF) {
 MachineInstr::MachineInstr(MachineFunction &MF, const MCInstrDesc &TID,
                            DebugLoc DL, bool NoImp)
     : MCID(&TID), NumOperands(0), Flags(0), AsmPrinterFlags(0),
-      DbgLoc(std::move(DL)), DebugInstrNum(0) {
+      DbgLoc(std::move(DL)), DebugInstrNum(0), Opcode(TID.Opcode) {
   assert(DbgLoc.hasTrivialDestructor() && "Expected trivial destructor");
 
   // Reserve space for the expected number of operands.
@@ -117,7 +117,8 @@ MachineInstr::MachineInstr(MachineFunction &MF, const MCInstrDesc &TID,
 /// uniqueness.
 MachineInstr::MachineInstr(MachineFunction &MF, const MachineInstr &MI)
     : MCID(&MI.getDesc()), NumOperands(0), Flags(0), AsmPrinterFlags(0),
-      Info(MI.Info), DbgLoc(MI.getDebugLoc()), DebugInstrNum(0) {
+      Info(MI.Info), DbgLoc(MI.getDebugLoc()), DebugInstrNum(0),
+      Opcode(MI.getOpcode()) {
   assert(DbgLoc.hasTrivialDestructor() && "Expected trivial destructor");
 
   CapOperands = OperandCapacity::get(MI.getNumOperands());
@@ -143,6 +144,7 @@ void MachineInstr::setDesc(const MCInstrDesc &TID) {
   if (getParent())
     getMF()->handleChangeDesc(*this, TID);
   MCID = &TID;
+  Opcode = TID.Opcode;
 }
 
 void MachineInstr::moveBefore(MachineInstr *MovePos) {