Review Board 2.0.15


O3: Handle loads when the destination is the PC.

Review Request #178 - Created Aug. 13, 2010 and submitted

Information
Ali Saidi
gem5
Reviewers
Default
Min 
O3: Handle loads when the destination is the PC.
For loads that PC is the destination, check if the load
was mispredicted again when the value being loaded returns from memory

   

  


 


 

  

  









  
 


  

 
















 



 

  


 


  



   

   
Gabe Black

   
Posted  (Aug. 13, 2010, 5:14 p.m.)


  


  


  

   



  

   

   
    
    
   


  




   







    

     
     

      

       
        

         
          src/cpu/o3/lsq_unit_impl.hh
          

           (Diff revision 1)

          
         
        

       
       
        
 


        
 


       
      

     

    

    


     
Why doesn't the normal branch mispredict logic catch this?

I'm betting instructions that normally wouldn't be branches but that modify the PC aren't being marked as branches in the decoder, and that may or may not be contributing to this problem. This change may be necessary, but I'm skeptical.




  

   

   
    
    

   
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    Normally, branch misprediction is checked right after the instruction is executed because we know the result immediately (or some fixed-cycle latency but I don't believe that is being modeled). However, load instruction is a split-transaction and therefore we need to have the check at the finishing end of the split transaction.
    
    
    
   
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    I haven't actually verified what you're saying myself, but it sounds very plausible. Since most instructions in ARM and, apparently, all instructions in other ISAs won't need this code, you might want to only run it if the instruction is marked as a branch. You'd also need to make sure those instructions actually -are- marked as branches, but you'd want to do that anyway to really be correct. It would also be best, if at all possible, for this to go through the same mispredict and branch predictor updating logic as other instructions. That way we don'd duplicate the code, especially code that applies so infrequently and might not be run without really thorough testing, and we make sure the predictor gets involved and we don't mispredict on every load to the PC.
    
    
    
   
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