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

[anon@att.net]

Actually since the equation ( i1 + i2 .. n ) for the "for statement" is 
evaluated once, this means that alternative 2 is the faster code.  
Because in alternative 1 the system perform must reference ( K ) at 
least once and possible twice plus provide storage for K.

Now, for optimization in Ada, The RM provide a "pragma Optimize" with 
two options "Space" and "Time".  But in GNAT "pragma Optimize" is 
non-functional, instead GNAT uses GCC backend option -OX where X is
0 to 4 at the movement to perform optimization.

Now, since GNAT is a front-end to the compiler it is possible for 
GNAT to use "pragma Optimize ( Space ) ;" to remove source-level dead 
code.  While "pragma Optimize ( Time ) ;" could turn on optimization in
the backend to a preset default optimization or allow command-line 
option to override default.

Note: GCC does not have an option to remove dead code from precompile 
libraries or routines during linking.


In <0868c42e-ed44-4b36-a929-2bffb338ee34@googlegroups.com>, reinkor <reinkor@gmail.com> writes:
>I am not sure to what extent one may assume the Ada compiler 
>makes optimization.  For example, assume the following two program 
>code alternatives:
>
>
>-----------------------Alternative 1:
>   i1,i2,k : Integer;
>begin
>    (Read i1,i2 and n - do not change after)
>
>    k := i1 + i2; -- store "i1 + i2" in k to avoid recalculation (?)
>    loop
>      for i in k .. n loop
>          Do something...
>      end loop;
>    end loop;
>
>------------------------Alternative 2:
>   i1,i2 : Integer;
>begin
>    (Read i1, i2 and n - do not change after)
>
>    loop
>      for i in i1 + i2 .. n loop -- will recalculate "i1 + i2" ?
>          Do something...
>      end loop;
>    end loop;
>
>What is the "best" alternative?  Alternative 2 
>is shortest (and probably easier to read), but
>could it give slower code that for Alternative 1 ?
>
>Yes, this example is somehow over-simplified :-)
>
>