CODEWITHSUNDEEP
c++optimization
Victor
Victor

Reputation: 688

Compiler optimization when using const bool to remove debug information

I have a class with a debug function named log(std::string s) I am working on a small embedded system, therefore I want to remove unnecessary strings from the binary.

I could use #define to achieve what I want, but I think it is a bad practice. Using -Os optimization I though I could use a const bool like so:

#include <cstdio>
#include <string>
#include <iostream>

class test
{
    const bool enable_debug = false;
    void log(std::string s)
    {
        if (enable_debug) printf("%s",s.c_str());
    }

public:
    void doStuff(int i)
    {
        log("I don't want this string to appear in the binary output");
    }
};

int main()
{
    test instance;
    char input;
    std::cin>>input;
    instance.doStuff(input);
}

Please take a look at the output here: https://godbolt.org/z/qz7csf . As you can see, the string is not removed from the assemby generated code, even though there is no way it can be used, because enable_debug is a const.

  1. So why is it still in the output?
  2. How can I create a toggle switch without using #define?

PS : in the end, I would like to have the bool variable external to the class, in order to switch all my classes at the same time.

Upvotes: 1

Views: 297

Answers (2)

n. m. could be an AI
n. m. could be an AI

Reputation: 120059

You are using std::string. The compiler is not smart enough to see through all the std::string machinery, and I doubt it is even allowed to eliminate the std::string construction. Since the string literal is used to initialise the std::string, the literal cannot be eliminated either.

If you want to save a few bytes, perhaps consider using const char* instead.

Upvotes: 2

eerorika
eerorika

Reputation: 238421

  1. So why is it still in the output?

Because the compiler didn't do deep enough analysis to find out that it is not used.

  1. How can I create a toggle switch without using #define?

Put debug functions, including their string literals into a separate translation unit, and don't link with that TU when you don't want them to show.

P.S.

The behaviour is undefined if debugging is enabled:

printf("%s",s);

Upvotes: 0

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