Using LLVM plugins with clang

The LLVM docs suggest using opt to load and run custom passes. This works, but for certain tasks, like building programs with complicated Makefiles or unusual environments, it is less than ideal. Instead, LLVM provides a hook to allow plugin modules to be run automatically when loaded by clang. This lets you add a couple of flags and use clang (and your pass) as a drop-in replacement for your normal C compiler. These files demonstrate how that works.

Using clangtool:

Two steps:

1. Build your custom LLVM pass the normal way (as a shared library with the appropriate llvm flags).

2. Run clang on your input code with the following extra options:

    clang -Xclang -load -Xclang <custom-pass>.so ...
   

That's it. Your pass will be run automatically. You can specify these with CFLAGS in a Makefile, override CC or CXX, or write a script to wrap clang and use that.

How it works:

The key is a static class called RegisterStandardPasses, which is defined in the PassManagerBuilder.h header. The constuctor for this class calls the addGlobalExtension function, which in turn adds your custom pass to the list of extensions that are loaded by default.