[CodeGen] Return better Changed status from DetectDeadLanes

[serge-sans-paille]

LGTM

llvm/lib/CodeGen/DetectDeadLanes.cpp

private:
   LaneBitmask transferDefinedLanes(const MachineOperand &Def, unsigned OpNum,
                                    LaneBitmask DefinedLanes) const;
 
   /// Given a mask \p UsedLanes used from the output of instruction \p MI
   /// determine which lanes are used from operand \p MO of this instruction.
   LaneBitmask transferUsedLanes(const MachineInstr &MI, LaneBitmask UsedLanes,
                                 const MachineOperand &MO) const;
 
-  bool runOnce(MachineFunction &MF);
+  std::pair<bool, bool> runOnce(MachineFunction &MF);
 
   LaneBitmask determineInitialDefinedLanes(unsigned Reg);
   LaneBitmask determineInitialUsedLanes(unsigned Reg);
 
   bool isUndefRegAtInput(const MachineOperand &MO,
                          const VRegInfo &RegInfo) const;
 
   bool isUndefInput(const MachineOperand &MO, bool *CrossCopy) const;
bool DetectDeadLanes::isUndefInput(const MachineOperand &MO,
   Register MOReg = MO.getReg();
   if (Register::isVirtualRegister(MOReg)) {
     const TargetRegisterClass *DstRC = MRI->getRegClass(DefReg);
     *CrossCopy = isCrossCopy(*MRI, MI, DstRC, MO);
   }
   return true;
 }
 
-bool DetectDeadLanes::runOnce(MachineFunction &MF) {
+std::pair<bool, bool> DetectDeadLanes::runOnce(MachineFunction &MF) {
   // First pass: Populate defs/uses of vregs with initial values
   unsigned NumVirtRegs = MRI->getNumVirtRegs();
   for (unsigned RegIdx = 0; RegIdx < NumVirtRegs; ++RegIdx) {
     unsigned Reg = Register::index2VirtReg(RegIdx);
 
     // Determine used/defined lanes and add copy instructions to worklist.
     VRegInfo &Info = VRegInfos[RegIdx];
     Info.DefinedLanes = determineInitialDefinedLanes(Reg);
for (unsigned RegIdx = 0; RegIdx < NumVirtRegs; ++RegIdx) {
       const VRegInfo &Info = VRegInfos[RegIdx];
       dbgs() << printReg(Reg, nullptr)
              << " Used: " << PrintLaneMask(Info.UsedLanes)
              << " Def: " << PrintLaneMask(Info.DefinedLanes) << '\n';
     }
     dbgs() << "\n";
   });
 
+  bool Changed = false;
   bool Again = false;
   // Mark operands as dead/unused.
   for (MachineBasicBlock &MBB : MF) {
     for (MachineInstr &MI : MBB) {
       for (MachineOperand &MO : MI.operands()) {
         if (!MO.isReg())
           continue;
         Register Reg = MO.getReg();
         if (!Register::isVirtualRegister(Reg))
           continue;
         unsigned RegIdx = Register::virtReg2Index(Reg);
         const VRegInfo &RegInfo = VRegInfos[RegIdx];
         if (MO.isDef() && !MO.isDead() && RegInfo.UsedLanes.none()) {
           LLVM_DEBUG(dbgs()
                      << "Marking operand '" << MO << "' as dead in " << MI);
           MO.setIsDead();
+          Changed = true;
         }
         if (MO.readsReg()) {
           bool CrossCopy = false;
           if (isUndefRegAtInput(MO, RegInfo)) {
             LLVM_DEBUG(dbgs()
                        << "Marking operand '" << MO << "' as undef in " << MI);
             MO.setIsUndef();
+            Changed = true;
           } else if (isUndefInput(MO, &CrossCopy)) {
             LLVM_DEBUG(dbgs()
                        << "Marking operand '" << MO << "' as undef in " << MI);
             MO.setIsUndef();
+            Changed = true;
             if (CrossCopy)
               Again = true;
           }
         }
       }
     }
   }
 
-  return Again;
+  return std::make_pair(Changed, Again);
 }
 
 bool DetectDeadLanes::runOnMachineFunction(MachineFunction &MF) {
   // Don't bother if we won't track subregister liveness later.  This pass is
   // required for correctness if subregister liveness is enabled because the
   // register coalescer cannot deal with hidden dead defs. However without
   // subregister liveness enabled, the expected benefits of this pass are small
   // so we safe the compile time.
   MRI = &MF.getRegInfo();
   if (!MRI->subRegLivenessEnabled()) {
     LLVM_DEBUG(dbgs() << "Skipping Detect dead lanes pass\n");
     return false;
   }
 
   TRI = MRI->getTargetRegisterInfo();
 
   unsigned NumVirtRegs = MRI->getNumVirtRegs();
   VRegInfos = new VRegInfo[NumVirtRegs];
   WorklistMembers.resize(NumVirtRegs);
   DefinedByCopy.resize(NumVirtRegs);
 
+  bool Changed = false;
   bool Again;
   do {
-    Again = runOnce(MF);
+    bool LocalChanged;
+    std::tie(LocalChanged, Again) = runOnce(MF);
+    Changed |= LocalChanged;
   } while(Again);
 
   DefinedByCopy.clear();
   WorklistMembers.clear();
   delete[] VRegInfos;
-  return true;
+  return Changed;
 }