[dundarious]

True, when writing my comment I wanted to qualify it to the same effect, but thought it would be an unnecessary subtlety to the general thrust of my point. That is, we can ignore this kind of "undefined behavior in the machine itself" for the purposes of this particular discussion.

[tialaramex]

I don't see how to ignore it though. If we're defining the behaviour but then our "definition" just doesn't specify the actual behaviour because it's specified in terms of hardware with no clearly defined behaviour for that situation then it's just word play, we're not really doing what I set out.

[dundarious]

If for the purposes of this discussion we can't ignore it at the machine level (because we're assuming higher level languages, crappy or otherwise, are unlikely to generate machine instructions that exhibit undefined behavior), then why were we discussing higher level languages and their crappiness at all? I'm not saying this to be snarky, I just mean that I really think the likelihood of machine undefined behavior being an issue is on the order of likelihood for cosmic rays to flip bits -- happens, and can't be ignored (buy ECC memory), but more interesting to talk about the things that we are many orders of magnitude more likely to experience, e.g., bugs in C programs, bugs in unsafe Rust, bugs in managed language runtimes, etc. I think those things are not all equally likely, but could all benefit from endbr64 type mechanisms, including in JIT output.

To be clear, unlike the comment root, I don't think this particular hardware mechanism obviates the need/benefits of related software mechanisms. But in terms of cost/benefit/applicability, endbr64 type mechanisms look pretty good all around.