How can I do llvm link time optimization

Question

I compile a C++ program, using for example the following.

clang++ -O4 -emit-llvm file1.cpp -c -o file1.bc 
clang++ -O4 -emit-llvm file2.cpp -c -o file2.bc 

llvm-link file1.bc file2.bc 

How can I perform link time optimization here?

Answer 1

Use opt:

clang++ -O4 -emit-llvm file1.cpp -c -o file1.bc 
clang++ -O4 -emit-llvm file2.cpp -c -o file2.bc 
llvm-link file1.bc file2.bc -o all.bc
opt -std-compile-opts -std-link-opts -O3 all.bc -o optimized.bc