LTO

Makefile

all: run

x:
	clang++ -g -O0 -shared -fPIC x.cpp -o libx.so

y: x
	clang++ -g -O0 -shared -fPIC y.cpp -o liby.so -L . -l x

z: y
	clang++ -g -O0 z.cpp -o z -L . -lx -ly -lc++ -flto 

run: z
	LD_LIBRARY_PATH=. ./z

x.cpp

int x() { return 5; }

int a() { return 0; }
extern "C" __attribute__ ((visibility ("default"))) int b() { return 7; }

y.cpp

int x();

int y() { return x() + 3; }

int a() { return 4; }

extern "C" __attribute__ ((visibility ("default"))) int b() { return 8; }

z.cpp

#include <iostream>

int x();
int y();
int a();
//extern "C" int b();

extern "C" __attribute__ ((visibility ("default"))) int b() { return 20; }

int main() { std::cout << x() + y() + a() + b() << std::endl; }