Performance cost of passing by value vs. by reference or by pointer?

There is one thing nobody mentioned.

There is a certain GCC optimization called IPA SRA, that replaces “pass by reference” with “pass by value” automatically: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html (-fipa-sra)

This is most likely done for scalar types (eg. int, double, etc), that does not have non-default copy semantics and can fit into cpu registers.

This makes

void(const int &f)

probably as fast (and space optimized)

void(int f)

So with this optimization enabled, using references for small types should be as fast as passing them by value.

On the other hand passing (for example) std::string by value could not be optimized to by-reference speed, as custom copy semantics are being involved.

From what I understand, using pass by reference for everything should never be slower than manually picking what to pass by value and what to pass by reference.

This is extremely useful especially for templates:

template<class T>
void f(const T&)
{
    // Something
}

is always optimal