Re: Assuming optimization? What is best of these code alternatives?

[Stefan.Lucks@uni-weimar.de]

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On Thu, 11 Sep 2014, reinkor wrote:

>    k := i1 + i2; -- store "i1 + i2" in k to avoid recalculation (?)
>    loop
>      for i in k .. n loop
>          Do something...
>      end loop;
>    end loop;

>    loop
>      for i in i1 + i2 .. n loop -- will recalculate "i1 + i2" ?
>          Do something...
>      end loop;
>    end loop;
>
> What is the "best" alternative?

I assume you turn on the compiler's optimisation setting. I didn't try it, 
but I would expect the following:

   - If "Do something" is sufficiently simple, the Ada compiler will
     find out that i1, i2 and, for the first variant, k, don't
     change. Then the generated code should be the same.

   - If "Do something" is so complex that the Ada compiler is unable to
     find out that neither of i1, i2, and k change, you are unlikely to
     notice any difference in speed, since the time to compute i1+i2 is
     will be negligible, compared to the time for "Do something".

Nevertheless, if you want to make sure the compiler notices that neither 
i1 nor i2 change, you could declare them as constants:

    declare
      I1: constant Some_Type := Get_I1;
      I2: constant Some_Type := Get_I2;
    begin
      loop
        for I in I1+I2 .. N loop
          Do_Something
        end loop;
      end loop;
    end;

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--Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany--