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Forum: FPGA, VHDL & Verilog VHDL optimization


VHDL optimization
von Kilian H. (jal)

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I've been wondering how much optimization the average tool performs, and 
how much I should manually try to optimize. For example, if I have two 
signals with the same assignment:
1
   sig1 <= (someSig & "000") + someOtherSig;

2
   sig2 <= (someSig & "000") + someOtherSig;

are the tools smart enough to know it's the same assignment and merge 
them? Or should I create a variable and use that?

A second example, would the below create the same logic, or is one 
preferable to the other? If so why?
1
   if sig1 = '0' then

2
     sig2 <= "0000";

3
   else

4
     sig2 <= "1111";

5
   end if;

6
7
   sig2 <= sig1 & sig1 & sig1 & sig1;


  


  




  

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    Re: VHDL optimization
  


  

    

      von
      
        -gb- (Guest)
      
      


      
    


    

      
    

    

    

  
  


  

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Write the code simple to read and understand. The tools normally 
optimize better than you could do.

  


  




  

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    Re: VHDL optimization
  


  

    

      von
      
        Christoph Z.
        (christophz)
        
      
      


      
    


    

      
    

    

    

  
  


  

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Kilian H. wrote:
> For example, if I have two
> signals with the same assignment:
> are the tools smart enough to know it's the same assignment and merge
> them?

Yes, they are smart enough to merge them even when they are declared in 
different files.

Kilian H. wrote:
> A second example, would the below create the same logic, or is one
> preferable to the other? If so why?

In the simulation it is definitely NOT equivalent (due to the fact that 
std_logic has more than '0' and '1').

I would assume that the if construct synthesizes to a multiplexer 
structure.

The second one could be written as sig2 <= (others => sig1);

  


  




  

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    Re: VHDL optimization
  


  

    

      von
      
        Kilian H.
        (jal)
        
      
      


      
    


    

      
    

    

    

  
  


  

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Thanks both, good to know the optimizers are smart enough, saves me some 
headaches :).

@ChristofZ: hadn't thought of using (others => sig1), thanks!

  


  




  

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