how to make sure g++ will not inline this?

This is a discussion on how to make sure g++ will not inline this? within the C++ Programming forums, part of the General Programming Boards category; I don't worry too much! Just a little curious though. I will use -Os. Done! Decided!...

  1. #31
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    I don't worry too much! Just a little curious though.
    I will use -Os. Done! Decided!

  2. #32
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    Quote Originally Posted by matsp View Post
    However, if you put functions in a header file, they should be marked static, or part of a class.

    If the inline keyword gets ignored and definitions for non-static functions inside header files will cause linker problems, a non-static function can never be inlined. Is this correct?

  3. #33
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    Global functions CAN be inlined, but you always need to also have a global copy too (because some source that doesn't include the source may be calling the function). If the function is marked static, the compiler can avoid generating the code for the function if it's not called within the current unit.

    By the way, the keyword "inline" does not allow the function to be defined twice. It needs to be "static inline" to allow it to be declared twice.

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  4. #34
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    inline forces the compiler to allow multiple definitions across compilation units, whether the function is static or not. (And in this sense, the compiler isn't allowed to ignore inline.) However, if it's not static, all definitions must be exactly the same (no diagnostic required), or undefined behaviour will result. Effectively, this means you can have one definition in a header file, included by whoever wants the functionality.
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  5. #35
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    Quote Originally Posted by CornedBee View Post
    inline forces the compiler to allow multiple definitions across compilation units, whether the function is static or not. (And in this sense, the compiler isn't allowed to ignore inline.) However, if it's not static, all definitions must be exactly the same (no diagnostic required), or undefined behaviour will result. Effectively, this means you can have one definition in a header file, included by whoever wants the functionality.
    Actually, I tried "inline func()" in a header file, and then included that header file twice, and gcc complained about it.

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  6. #36
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    Thanks, that is good to know!
    It would be good if matsp issue could be solved though. Could you post your full example please?

  7. #37
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    I don't understand the original problem. The question is, how to prevent the compiler from inlining a function. The answer is, define the function in a .cpp file, not a .h file. Simple as that.

    If the function is in a .cpp file, then other modules don't have access to its source implementation, and the ONLY choice is to call the function.

    Writing the code in the .h file, either as an in-class-definition inline or outside the class using the "inline" keyword, still doesn't guarantee that the compiler will inline it.

    In sum, it is easy to PREVENT inlining, although there is no (portable) way to FORCE it. I don't see what the source of the original question was.

  8. #38
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    The question is, how to prevent the compiler from inlining a function. The answer is, define the function in a .cpp file, not a .h file. Simple as that.
    And indeed, that solution was suggested in post #5, as I noted earlier.
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  9. #39
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    Quote Originally Posted by matsp View Post
    Actually, I tried "inline func()" in a header file, and then included that header file twice, and gcc complained about it.
    "allow multiple definitions across compilation units"

    Not very clear, that. No, you can't include the header twice, but there's absolutely no reason to do that anyway. That's what header guards are for.

    The issue I'm talking about is when you have two source files that include the header containing the definition.
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  10. #40
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    Quote Originally Posted by brewbuck View Post
    I don't understand the original problem. The question is, how to prevent the compiler from inlining a function. The answer is, define the function in a .cpp file, not a .h file. Simple as that.

    If the function is in a .cpp file, then other modules don't have access to its source implementation, and the ONLY choice is to call the function.
    I'm pretty sure that the "Link-time code generation" setting in newer versions of VS specifically enables things like inlining functions from other compilation units, assuming you have it turned on.

    Anyway, I see what the misunderstanding manav had was. Inlining is not directly related to code size. Inlining can make the executable bigger, and in other cases it can also make it smaller. I think he is aware of this now.
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  11. #41
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    Quote Originally Posted by iMalc View Post
    I'm pretty sure that the "Link-time code generation" setting in newer versions of VS specifically enables things like inlining functions from other compilation units, assuming you have it turned on.

    Anyway, I see what the misunderstanding manav had was. Inlining is not directly related to code size. Inlining can make the executable bigger, and in other cases it can also make it smaller. I think he is aware of this now.
    Yes, I think both gcc and Visual Studio allows "whole program optimization" [that's the gcc term], which means that the compiler will take "all" source code into account when performing optimization. Although you have to request that [at least in the versioins of gcc that I've been using]

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  12. #42
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    Whole Program Optimization is MS's term. GCC doesn't support it yet, except via the -combine switch, which, frankly, is a hack and doesn't play well with the structure of traditional makefiles.
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