Do you have any specialized tools (code navigation tools, for example) that you use when first encountering these large piles of code? I'd love to hear some recommendations; I have to deal with large (only sometimes bad, but always large) piles of vendor code. I'm currently staring a pile of 900kloc of pretty nice code but it's a /lot/ of code.
Not OP, but I imagine this is fairly language specific. This is where Java shines. Keep in mind I mostly work with services not applications.
My process looks like this:
Step one: Identify sources of reflection, this is the triskyest. Hopefully the only dependencies are open source, so you generally know what they do and grep can usually find the rest.
Step Two: go code spelunking. Find your entry points. Find your main() or framework equivalent. Find callsites for rest endpoints, rpc, jmx, etc.
Step three: find other "external request processing" endpoints. Do you have timer threads? Reading a Kafka stream and acting per record? Etc.
Once you understand those, you can interpret where most any stacktrace is from. Good old Intellij or Eclipse can give you all the callsites for a functions as you root around. You should slowly get a feel for which part of the code things get called from.
Now start asking questions like: what data is shared between these entry points? What's mutable? Is it all done safely?
Hopefully this wasn't too narrow an example. I'd imagine it'll apply to any services.
I'm working in Linux wifi drivers, all in C. Giant complicated protocol with giant complicated code. I've been tinkering with Microsoft VSCode, CLion, and Sourcetrail. Vim + Ctags seems to work well in the beginning but only gives pinpoint answers (can find trees, but little view of the forest). Still experimenting.
It depends upon the platform, the language and the toolsets. For instance, for C, a trick I've used is to (if I can) use different C compilers and crank the warnings/errors to 11 and fix every complaint (or try to---it can be daunting to attempt this all at once).
Other tricks---run the code through linters or other stylistic nit-pickers and fix those too. I haven't yet used a code reformatter, but that's a quick way to get code into a consistent style.
I'm all in C, too: kernel drivers from wifi chipset vendors. There's one C compiler we can use (gcc) and we're even restricted to working with a specific version (because cross compiling). We have to be very careful about changing any of the code because we have to integrate any changes into the next drop of the vendors' code. Static analysis is our best bet. Is a really interesting problem.
