Construct a global variable via non-default construction function, why it CAN be allocated into .bss section?

Question

i write two pieces of cpp source code, the only difference is one file specify a non-default construction function, while the other one is not:

// code for `test1.cpp`
class Derived {
    private:
        int         non_const_int_val=1;

    public:
        Derived(int a) : non_const_int_val(a) {
        }

} dev(0x2f2f2f2f);

int main() {
    Derived derivedObj(0x55555555), derivedObj_copy(dev);

    return 0;
}

// code for `test2.cpp`
class Derived {
    private:
        int         non_const_int_val=1;

    public:

} dev;

int main() {
    Derived derivedObj, derivedObj_copy(dev);

    return 0;
}

in test2.cpp, the global variable dev will be allocated into .data section, since it is constructed and initialized, and we can verify it as check the elf:

>> objdump -x test2.out | c++filt | grep -E -i "dev"                    
0000000000404028 g     O .data  0000000000000004              dev

While to my surprise, it doesn't hold for test1.cpp and i fail to figure it out (i think it should be initialized as well.): it locates in .bss section.

>> objdump -x test1.out | c++filt | grep -E -i "dev"
000000000040115a l     F .text  0000000000000015              _GLOBAL__sub_I_dev
000000000040402c g     O .bss   0000000000000004              dev

i use g++ with gcc 10.2.0 version.

So how can we account for such wired behaviour?
Or is there some profound difference between construction and initialization (plz forgive me if i get some core concepts wrong.)?
useful references are also preferred.

Thanks.

I dump the elf and pin-point the behavior which confuse me.

Artyer · Accepted Answer · 2024-04-25 13:59:28Z

1

In your second snippet, dev is constant initialized. It is initialized by calling a constructor. The default constructor is constexpr, making it a constant expression.

In your first snippet, since your constructor isn't constexpr, it is dynamically initialized. It's zero-initialized and then the actual initializer is run when the program starts (inside ._GLOBAL__sub_I_dev).

If you compile with some optimizations enabled, the compiler should be able to see that dev(0x2f2f2f2f) is essentially a constant and it is allowed to optimize it to be a constant too.

answered Apr 25, 2024 at 13:59

Artyer

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