But if I declare
A a(8)
as global -- compiler produce big general code. I tried to addstatic
-- it didn't help. Why?
In my experience, gcc is very reluctant if the object / function has external linkage. Since we don't have your code to compile, I made a slightly modified version of your code:
#include <cstdio>
class A {
int x;
public:
A(int x_): x(x_) {}
int f() { return x*x; }
};
A a(8);
int main() {
printf("%d", a.f());
}
I have found 2 ways to achive that the generated assembly corresponds to this:
int main() {
printf("%d", 64);
}
In words: to eliminate everything at compile time so that only the necessary minimum remains.
One way to achive this both with clang and gcc is:
#include <cstdio>
class A {
int x;
public:
constexpr A(int x_): x(x_) {}
constexpr int f() const { return x*x; }
};
constexpr A a(8);
int main() {
printf("%d", a.f());
}
gcc 4.7.2 already eliminates everything at -O1
, clang 3.5 trunk needs -O2
.
Another way to achieve this is:
#include <cstdio>
class A {
int x;
public:
A(int x_): x(x_) {}
int f() const { return x*x; }
};
static const A a(8);
int main() {
printf("%d", a.f());
}
It only works with clang at -O3
. Apparently the constant folding in gcc is not that aggressive. (As clang shows, it can be done but gcc 4.7.2 did not implement it.)