I have a question about speculative execution and pipelining in CPUs, and I'm wondering: should I ask it on Stack Overflow? Or is it a software-only site?
I would like for Stack Overflow to accept such a question, and it has fielded processor-specific question in the past.
However you would have to very clearly write the question so that it appears to be a problem a regular programmer might run into, and that it's something they can solve via changing their code.
If you can't change the behavior of the problem, or solve it with a code change, then it doesn't belong on Stack Overflow.
So if you are asking theoretical questions about computer architecture then it probably doesn't apply, and the best bet is probably the Computer Science Stack Exchange, although that still might be a poor fit.
If you are asking people to enumerate all the ways that the current line of processors deal with such issues, then you probably should go to Super User.
But if you have a piece of assembly code that demonstrates the problem, and are asking for a solution, or if you notice a performance problem and you have a suspicion, and code that demonstrates the problem, then you should be safe on Stack Overflow.
However: there are very few people on Stack Overflow who would be able to answer your question with regards to all of today's most common processors.
I was just informed there is a site at http://electronics.stackexchange.com/
I would suggest starting there.
This question can be answered by someone who knows processor architectures very well. Ideally a processor designer, but that's a very rare breed. It could also be a one of the few researchers in processor modelization (most likely to hang out on Theoretical Computer Science, though I haven't seen any), but that site only accepts research-level questions anyway. The largest target audience is compiler writers, and they're more likely to hang out on Stack Overflow. On this basis, I recommend Stack Overflow.
(And not Super User, which is for people who take processors and put them into computers, which is not your problem at all. SU would be appropriate for a question such as which CPU models have what features.)