How to Indicate the Input File While Debugging a Compiler

Hi everyone,
I m a beginner in llvm, or compiler, actully. Anyway, right now I’m trying to modify some pass so I can insert an extra function call for optimization before the specific function in a test file(Let me call it test.c). However, I don’t know how to pass the test.c as the input when I use gdb to debug the source code of llvm-gcc, does anyone have any idea about it? Thanks.