Question about the C++ call convention of passing big object on Linux/x86-84

I'm trying to understand the overhead of pass object by value as a function parameter in C++/Linux/x86-64 platform.

The experimental code I used for the exploration is posted below and on godbolt.org: https://godbolt.org/z/r9Yfv4

Assume the function is unary. What I observed is: 1) If the parameter object is a 8 bytes in size, it will be put in RDI. 2) If the parameter is 16 bytes in size (contain two sub-object each 8 bytes), the two sub-objects will be put in to RDI and RSI. 3) If the parameter is bigger than 16 bytes, it will be passed via stack. I only consider integral types and pointers and the composition types of these basic types. I know passing floats/doubles is different.

The size of std::function is 32 bytes (GCC/Linux implementation, long + long + pointer + pointer = 32 bytes.). So passing std::function by value should look like pass struct Person4 defined in my code. But the output assembly shows that pass std::function is very different from pass struct Person4. It looks like std::function is passed via a pointer, am I right? Why there is such a difference?

#include <functional>    struct Person0 {    long name;  };    long GetName(Person0 p) {    return p.name;  }    struct Person1 {    long name;    long age;  };    long GetName(Person1 p) {    return p.name;  }    struct Person2 {    long name;    long age;    long height;  };    long GetName(Person2 p) {    return p.name;  }    struct Person3 {    long name;    long age;    long height;    long weight;  };    long GetName(Person3 p) {    return p.name + sizeof(p);  }    long Invoke(std::function<long(long)> f) {    return f(20) + sizeof(f);  }      int main() {    Person3 p;    p.name = 13;    p.age = 23;    p.height = 33;    p.weight = 43;    long n = GetName(p);      std::function<long(long)> ff;    Invoke(ff);    return 0;  }  

https://stackoverflow.com/questions/65838379/question-about-the-c-call-convention-of-passing-big-object-on-linux-x86-84 January 22, 2021 at 10:04AM