Bit width

[Tynan]

This is a technical detail that's invisible in the final experience. We're nowhere near any kind of performance limit where 64-bit would provide any benefit.

So no, targeting 64-bit isn't a priority. The only people who would notice would be the few left on 32-bit machines, who would no longer be able to play the game.

(You could say I should release both versions, but since it still needs to run fully optimized and playable on old 32-bit systems, we thus gain nothing since you'll all still be playing the same game with the same performance restrictions as you are now. But with a bunch of duplicate code files and extra EXEs to bloat the download and to trip up grandmas and kids who don't know what 32/64 bit means.)

[longbyte1]

The Unity Player isn't even 64-bit-ready so it's difficult to make a stable build. There's a bit of a hack to get it working but it's awfully buggy.

Over there in the KSP forums it's been debated over and over and over (along with better aerodynamics and whether the game is supposed to be realistic or arcadey), so there is no reason why we should replicate it here. Here's a summary of the 64-bit debate:
Pros - no 4GB limit, which heavily modded games often hit; larger and more precise numbers
Cons - yet another build target that developers must support; extremely unstable for many users
Verdict - 64-bit build will be thrown out for the brave and insane, but otherwise unsupported. Thanks Unity devs.

RimWorld hasn't reached the 4GB limit nor does it need bigger numbers; thus, 32-bit fulfills the game's needs in terms of bit width.

(Unity 5 might bring 64-bit or a reason to use 64-bit for cool effects and stuff. Might.)

[GlassDeviant]

That's exactly my point, Tynan. I am trying to find out whether it would be worth going completely over to 64-bit. Maintaining two code lines is a hassle, but if everyone is not living in the dark age of computing (i.e.: is using 64-bit and not 32-bit), it would be worthwhile to migrate the code and completely drop 32-bit. The limitations of 32-bit are well known, the largest of them being memory addressing, but the value of being able to work with an unsegmented, flat memory model and discard tons of legacy 32-bit library code is immense, among other things.

As for the stability, longbyte1, that's what happens when you just transfer code over and don't optimize it, don't blame 64-bit for slacking devs. 64-bit code is cleaner and by nature easier to maintain stability with if you put equal effort into it (on the part of both the Unity devs and the KSP devs).