Indeed it's a shitload of work. It's also possible and has been done. You don't have to only use global brute-force - you could also reverse engineer the compiler. The OOT/MM decomps achieved completeness without //COMPILERDIFF.
Correct me if I'm wrong (I'm not very well-versed in game decomp scenes) but aren't all those bytematched decomps from 90s or at the latest early 2000s games? They didn't have global optimisation (MSVC introduced it in VS .NET or 2003 I think and many games didn't use it until later)
So these are mostly problems with more advanced compilers yk
Correct, N64 and PS1 era projects can essentially always be strictly tackled one function at a time. There are very minor things (like string literal sharing) where some earlier function can affect later function, but the workarounds are comparatively simple.
On the other hand, I have heard even old MSVC is a nightmare. Various things are affected by hash table ordering (so the names of variables matter in some situations), stuff like order of #includes mattering, etc.
Yuuuup exactly. I guess this is masked because old 90s games are often more culturally significant so people haven't found these problems yet but yes, these are serious challenges.