More comments

Author: arsenm

llvm/lib/CodeGen/VirtRegMap.cpp

@@ -557,14 +557,16 @@ bool VirtRegRewriter::subRegLiveThrough(const MachineInstr &MI,
 }
 
 /// Compute a lanemask for undef lanes which need to be preserved out of the
-/// defining block for a register assignment.
+/// defining block for a register assignment for a subregister def. \p PhysReg
+/// is assigned to \p LI, which is the main range.
 LaneBitmask VirtRegRewriter::liveOutUndefPhiLanesForUndefSubregDef(
     const LiveInterval &LI, const MachineBasicBlock &MBB, unsigned SubReg,
     MCPhysReg PhysReg, const MachineInstr &MI) const {
   LaneBitmask UndefMask = ~TRI->getSubRegIndexLaneMask(SubReg);
   LaneBitmask LiveOutUndefLanes;
 
   for (const LiveInterval::SubRange &SR : LI.subranges()) {
+    // Figure out which lanes are undef live into a successor.
     LaneBitmask NeedImpDefLanes = UndefMask & SR.LaneMask;
     if (NeedImpDefLanes.any() && !LIS->isLiveOutOfMBB(SR, &MBB)) {
       for (const MachineBasicBlock *Succ : MBB.successors()) {