Historically there was a notion of "clean room" reimplementation.
These days it seems people translate the actual game code into a different representation of that same code and that makes it open source? Github is full of these.
These projects start off with the original assembly code and use it actively throughout all stages. This is about as far away from clean room as you get.
When run through a disassembler, in most cases, it’s probably the first time anyone has ever looked at the assembly (assuming PlayStation or newer). There are no comments or labels or symbols. Not what most people would consider “the original assembly”.
Their comment still stands; whether it's machine code or "original assembly code" that shipped on the disk it falls under "clean room" reversing and by extension implementation.
(I like these projects but I really don't think they could defend themselves legally if you company went after them)
On the one hand, it is fairly clear that producing source code with the explicit goal of reproducing a 1:1 binary is in no way transformative, so that's out. This would be a really hard argument to even attempt.
On the other hand, these projects are mostly free, intended for owners of the game to play the original game on a different platform or in a modified format, and not likely to have a negative effect on the original work's desirability or value. And, the reproductions aren't complete and alone usually produce limited to no value to a consumer (usually, they won't start without the original game files). These are the other important factors considered in fair use determinations and generally go the way of these being OK.
So, it's hard to say. With reverse engineering and copyright in the US in general, context is crucially important; something that would be completely illegal for one purpose (ie - decompiling and recompiling a competitor's software to distribute it without a license or use it internally without purchasing it would be obviously illegal) could be OK for another one.
Then a freshly written project would be completely different with no derivative elements at all; the only commonality between the two codebases is their functional elements. You can compile a decomp into the same binary, but that's only to prove functional equivalence. The intended mode would be with a modern compiler that completely rewrites the logic (it might even have to if it writes it for a completely different computer architecture, which describes every retro console game). Then neither the source nor the compiled artifact would match.
I strongly disagree with this notion from even a conceptual (much less legal) level; the point of compilation is not to erase the algorithms the programmer implemented, just to optimize and implement them.
> You can compile a decomp into the same binary, but that's only to prove functional equivalence.
This is like saying that a translated book is only "functionally" identical to the original; there's a lot of precedent in copyright law for this not being the case, and I don't think any argument revolving around the transformativeness of the compilation process would fly at all.
Programs as written are nothing like programs as compiled, certainly not as an expressive endeavor. Books don't have an analog. There's no point where we strip all conceptual meaning from the book and leave only the procedural algorithms the semantics demand. There's no point where we replace all the individual words with autovectorized versions, or where we automatically delete impossible sentences, or remove every layer of abstraction that the programmer put in there exactly for the ability to convey ideas.
Absolutely false. There are an infinite number of programs that will compile to the same machine code. Especially when an optimizer is involved. Discovering one of those is a creative process, transformative, and protected.
Using an LLM to do it for you? I wouldn’t touch that with a 10 foot pole. Seems too close to mechanical transformation to me.
Copyright for this new code is owned by the person doing decompilation. No one knows how similar to the original it is or not, just that it compiles to the same output.
Edit to clarify: by not exist, I mean it is not publicly available.
Also, If it is an unauthorized derivative work, as I understand it then it might not even qualify for copyright protection itself.
A reimplementation or a behavior-based clone is an entirely different legal world from decompilation.
In fact, the goal of most decomp not to produce the same binary, that’s just used as a validation.
True, but as a derivative work of the original source code they don't have rights to do anything with that code without infringing copyright.