Turn Off Optimization?

This is a discussion on Turn Off Optimization? within the C Programming forums, part of the General Programming Boards category; I'm compiling some 'C' code. I initialize some variables, then use these initialized variables in a decision block with branches ...

  1. #1
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    Turn Off Optimization?

    I'm compiling some 'C' code. I initialize some variables, then use these initialized variables in a decision block with branches that depend on the value. Since the variables have been initialized to something, they will always take the same branch.

    However these decision blocks are inside a 'while loop', and interrupts or other code can change the value, causing another branch to be taken.

    My compiler seeing the unreachable branch at compile time optimizes out the code for this branch.

    The normal way to fix this undesired optimization is to declare the variable 'volatile'. If that's done then the compiler will not consider the branch unreachable.

    I have quite a few of these decision blocks in my code, with a large number of variables that should be declared volatile. Volatile variables cause more code to be generated, since the compiler will reload the variable each time that it's used. Since I have a limited amount of code space, I don't want to declare a lot of variables volatile.

    I want to turn off compiler optimization that eliminates unreachable branches, but keeps all other optimization.

    My present optimization is -Os.

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    So leave off -O altogether.

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    Without Optimization the code exceeds the allocated space. I only want to turn off the optimization that eliminates unreachable branches.

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by danlee58 View Post
    Without Optimization the code exceeds the allocated space. I only want to turn off the optimization that eliminates unreachable branches.
    I don't see why you won't just use "volatile." Without it, various other optimizations might be made that are inappropriate. You don't want the compiler to "waste" code by reloading variables, yet that's exactly what needs to happen when more than one execution thread can change the variable.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  5. #5
    and the hat of wrongness Salem's Avatar
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    > The normal way to fix this undesired optimization is to declare the variable 'volatile'. If that's done then the compiler will not consider the branch unreachable.
    So what's wrong with this way?

    I think whatever you do, you're going to have to edit the code.

    > I have quite a few of these decision blocks in my code, with a large number of variables that should be declared volatile.
    Is each decision variable (which should be volatile) inspected only once per decision?

    Like
    Code:
    switch ( var ) {
    }
    And not like
    Code:
    switch ( var ) {
       ///
          if ( var ) 
    }

    You could try something like
    Code:
    extern int magic;
    switch ( var | magic ) {
    }
    Where magic is initialised elsewhere to 0.
    It won't change the result, but it will force the compiler to do that little bit of extra work.
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    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by brewbuck View Post
    I don't see why you won't just use "volatile." Without it, various other optimizations might be made that are inappropriate. You don't want the compiler to "waste" code by reloading variables, yet that's exactly what needs to happen when more than one execution thread can change the variable.
    Seconded.

    You're saying that you don't want the compiler to do exactly what you actually need it to do. Just declare it volatile.
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    I agree with iMalc and brewbuck - if you have a variable that changes due to interrupts or other "compiler don't follow the change" circumstances, volatile is exactly the right solution.

    Then you should be able to have full optimization and the resulting space saving.

    Is there any particular reason(s) NOT to do this?

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