question on LLVM

hi, nice to meet you

I’m a newbie on LLVM. I’m not sure if I’m right with emailing here;;
If not, please pardon me.

Studying code generator(IR-to-native), I’ve stuck in the part of pass initialization.

I want to know how to execute those initialize(xxxx)Pass(PassRegistry&) functions, in the file of include\llvm\InitializePasses.h.

There are only declarations of them, not definitions.
Is it related with some initialization macros somewhere else?