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  • Author Author: mlease
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C switch statements

mlease

This week I covered several approaches to make large switch statements execute faster and save power. Switch statements are a great way to help keep your code readable but it is easy to forget they can turn into a long sequence of if-then-else statements.

 

http://cmicrotek.com/wordpress_159256135/?p=156

  • @Mike,

     

    Do you have any measured data to show the benefits of your design suggestions in terms of power reduction ?

     

    My approach would be to write code which most clearly expresses (in my case in C, VHDL or Verilog) intent and in the first instance to let the tools do the optimisation. So if I was desperate to optimise power consumption I would start with a nice clear case statement (assuming that was the obvious way to code, using an enumerated type for the switch variable for safety and clarity). Then I would want to measure any benefit from changing from the best compiler optimisation to "power crafted" code.

     

    MK

  • in reply to mlease

    Hi Mike,

     

    Also stress the fall through effect if you do not include Break statements.

    I have seen a number of applications tripped up by a slight over sight.  Leads to those gremlin type of issues.

     

    DAB


  • in reply to DAB

    DAB, thank you for your comment. I agree completely and will add that many of the errors they cause can be difficult to track down to resolve.

     

    Most of the techniques I described for making switch statements more power efficient either don't actually use switch statements or break them down into multiple smaller, more manageable switch statements. All these approaches have the benefit of making the programmer slow down, think about what they are doing and design their code instead of just throwing a bunch of cases into a switch statement.

     

    Mike

  • Hi Mike,

     

    I should temper your enthusiasm a little.

    Large Switch statement lists often lead to a world of reliablity issues.

    When you calculate the probability of error, the Switch statement leads to more software defects than any other single Software statement.

     

    So yes they are useful, but they are also dangerous if you do not build up a solid decision tree around them to reduce the possibility for errors in your code.

     

    DAB