From a preservation perspective, this is one of the least useful decomps ever, given how committed Capcom is to making sure RE4 is ported absolutely everywhere (kidding!)

Made using the leaked debug build and its symbols, which shows how meaningful the work the game preservation community that acquires and distributes these things is.

OTOH this is pretty off-putting:

> Where the compiler needed a particular source shape to reproduce a register choice or a schedule and no natural spelling was found, the construct is marked with a // COMPILER-DIFF: comment (644 of them: dead tests, empty asm("") launders and anchors, register T x asm("rN") pins, padding statements).

To me the value of a decomp isn't reproducing the original bytes per se (we already have the original bytes after all) - it's about reconstructing the understanding of the original game as represented by human-readable source code; getting byte-for-byte is just an indication that you've gotten it right. Needing to add a bunch of slop to force the compiler to match the original output is actually just an indication that you've gotten it wrong - and it's a demonstration of the danger of Goodhart's law, especially as it applies to AI.

You might find it interesting that I am working on AI-driven decomp of a PS1 game not by matching bytes but by having agents produce C code and test code. Agents submit a C proposal to the harness, the harness compiles their C proposal for the original function, then the test suite provided by the implementer runs against the original machine code and the compiled C code version. The line and branch coverage of both must be 100% and given identical inputs and starting RAM, the function return value and RAM state and RAM/MMIO read write sequence must be identical. This is done with a small MIPS simulator which can run all the tests extremely fast.

The reason I’m finding this is much faster than a traditional decomp is that while it’d be nice for the bytes to match, finding the perfect blend of compiler version, compiler args, permitting variables etc to try and find the perfect register assignments, etc is all very time consuming. My ultimate goal is not a byte for byte match, that’s just one way to ensure correctness. I’ve found agents are much faster and effective at reading the original assembly and understanding what’s going on then writing semantically equivalent C.

Making a function fully matching by virtue of hacks like this is mostly not harmful to the ability to understand the code, but is a useful tool in ensuring that the code as a whole really is matching and identical to the original. Otherwise, it's difficult to prove.

There is also value in decomps beyond just understanding and general interest. They can also be used to make more advanced mods, better translation patches, etc. The fidelity here matters, like having asm code accessing structures makes it hard to modify structures, but any fidelity improvement beyond pure asm is very welcome.

Of course I still prefer to try to recover the original code that caused the compiler to do what it did, but it's a really challenging problem sometimes. I've been working on decompiling code from old versions of MSVC for literally years now and you accumulate some knowledge of what things impact register allocation or the order of symbols but some of it comes from things that get fully erased from the source. Like for example, debug builds generally seem to retain symbols that aren't actually referenced anywhere, but those symbols only actually make it into an object file if they are. For functions that were only ever inlined and not actually referenced anywhere... They still wind up in the object files and thus in debug builds, despite nothing referencing them. They are also COMDAT any'd because they can appear in multiple objects legally, which means the exact object that winds up retaining it in the final linked executable is arbitrary (and the compilation flags of the object containing it, too - I bet that was fun for developers to debug.) This is incredibly useful but very challenging, needless to say. It may even be feasible to construct examples that would be legitimately infeasible to simply guess back to equivalent source, which I suspect is a major reason why until it was finally shown to be possible in larger scale projects many people wrote fully matching decomps off as a fool's errand..

Correct, this is just hacks for stuff you didn't manage to match exactly. Either that or your build environment isn't the same. Sadly, there are things which aren't really possible to reproduce in a byte-identical manner, things like exact file layout or variable declaration order, compilation order and that kinda stuff. And they might cause small but equivalent changes like different inlining/optimisation decisions, so it's really tricky to get it byte-exact.

The OOT and MM decomp had a bruteforcer tool that would reorder lines until the register allocation matched.

Certainly, it's better if you don't have these. But if I was working on something like this, I think you release when you have something that covers most of the territory, and then refine it over time.

Maybe someone else comes and helps out here and there.

> given how committed Capcom is to making sure RE4 is ported absolutely everywhere

Heh. I was just looking at the RE family on Steam. Nothing over a fiver.

I guess they really did make it more convenient than piracy.