Remove unnecessary bitvector clear

[trentxintong]

Summary

Remove unnecessary bitvector clear.

[davide]

Can you add an assertion that the bitmap is empty/has no bits set at the beginning of the function?

[trentxintong]

My understanding is only 1 class DeadMachineInstructionElim object will be constructed and used for all the functions in the module by the pass manager. However the LivePhysRegs is re-assigned at the beginning of the runOnMachineFunction for every basic block. So the LivePhysRegs.clear() is really unnecessary.

[trentxintong]

Ping. thanks

[davide]

@qcolombet Quentin, can you please take a look at this one?

[qcolombet]

Although this is correct, I don't think that is useful. We could add functionalities to this pass that would make use of that information (e.g., a dump method that we could call from the debugger anywhere in the program) and having the state being clean, seems like a good thing to me.

Unless I am mistaken, this shouldn't have any performance implication, so the clean state seems worth it.
Am I missing something?

[trentxintong]

In D31834#793133, @qcolombet wrote:

Although this is correct, I don't think that is useful. We could add functionalities to this pass that would make use of that information (e.g., a dump method that we could call from the debugger anywhere in the program) and having the state being clean, seems like a good thing to me.

Unless I am mistaken, this shouldn't have any performance implication, so the clean state seems worth it.
Am I missing something?

I agree that the performance implication is small. I am not sure where you want to check for whether the state is clean or not.

[qcolombet]

Sorry, that one slipped through!

I agree that the performance implication is small. I am not sure where you want to check for whether the state is clean or not.

In the debugger for instance. Nevertheless, having a clean state at the end of a pass does not seem a bad thing to me.

lib/CodeGen/DeadMachineInstructionElim.cpp

for (MachineBasicBlock::reverse_iterator MII = MBB.rbegin(),
             for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
               LivePhysRegs.set(*AI);
           }
         }
       }
     }
   }
 
-  LivePhysRegs.clear();
   return AnyChanges;
 }