I've been doing the same, porting (getting to compile and seemingly run) an old c and c++ codebase from 32bit solaris to 64bit linux. For a while I was spending my days alternating between "how did this ever compile" and "how did this ever run" when debugging. It took a lot of modifications to get it to compile on a now 6 year old version of GCC, then I got to debug a lot of issues with type sizes, endianess and posix differences. The team responsible for it is still in favor of keeping it 32bit unfortunately, so we can't even do a desperately needed valgrind run. I wish I could look at the history but I don't have access to the clear case repository I think they're still using.

We've got another codebase of similar vintage that is preventing us from deploying spectre patches...