I think that one of the main reasons why AMD is not a big fan of SMT is because it's very hard and complex to validate. SMT doesn't add much die size to the core, but to validate if it actually properly works is a tough job. This complexity means that it costs quite a bit of money to properly implement and AMD has to choose for the most economically efficient way of designing its CPUs. AMD wants the best performing CPU design for the least amount of money it can spend and that means that SMT is not a priority for AMD. That's just my guess though, I'm not sure if there's any truth to it.
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