[RegisterScavenging] Fix assert in scavengeRegisterBackwards

[craigblackmore]

Summary

According to the documentation, if a spill is required to make a
register available and AllowSpill is false, then NoRegister should be
returned, however, this scenario was actually triggering an assertion
failure.

This patch moves the assertion after the handling of AllowSpill.

Authored by: Lewis Revill

[arsenm]

llvm/lib/CodeGen/RegisterScavenging.cpp

578–579

This debug message is going to be misleading since the scavenging failed

[craigblackmore]

I have updated the patch so that Scavenged free register: will not be printed if no register is found.

[craigblackmore]

llvm/lib/CodeGen/RegisterScavenging.cpp

578–579

Thanks, I've pushed an update to fix this.

llvm/lib/CodeGen/RegisterScavenging.cpp

Register RegScavenger::scavengeRegisterBackwards(const TargetRegisterClass &RC,
   // Find the register whose use is furthest away.
   MachineBasicBlock::iterator UseMI;
   ArrayRef<MCPhysReg> AllocationOrder = RC.getRawAllocationOrder(MF);
   std::pair<MCPhysReg, MachineBasicBlock::iterator> P =
       findSurvivorBackwards(*MRI, MBBI, To, LiveUnits, AllocationOrder,
                             RestoreAfter);
   MCPhysReg Reg = P.first;
   MachineBasicBlock::iterator SpillBefore = P.second;
-  assert(Reg != 0 && "No register left to scavenge!");
   // Found an available register?
-  if (SpillBefore == MBB.end()) {
+  if (Reg != 0 && SpillBefore == MBB.end()) {
     LLVM_DEBUG(dbgs() << "Scavenged free register: " << printReg(Reg, TRI)
                << '\n');
     return Reg;
   }
 
   if (!AllowSpill)
     return 0;
 
+  assert(Reg != 0 && "No register left to scavenge!");
+
   MachineBasicBlock::iterator ReloadAfter =
     RestoreAfter ? std::next(MBBI) : MBBI;
   MachineBasicBlock::iterator ReloadBefore = std::next(ReloadAfter);
   if (ReloadBefore != MBB.end())
     LLVM_DEBUG(dbgs() << "Reload before: " << *ReloadBefore << '\n');
   ScavengedInfo &Scavenged = spill(Reg, RC, SPAdj, SpillBefore, ReloadBefore);
   Scavenged.Restore = &*std::prev(SpillBefore);
   LiveUnits.removeReg(Reg);