Claire Xen  🏳️‍⚧️ 🏳️‍🌈 🧙🏻‍♀️ BLM  🏴 🚩

That Google hint seems to mostly be for C compilers; I assume you mean "the whole program becomes undefined if there's even one case of UB"?

No! I said don't google that! :) UB has nothing to do with C compilers specifically. But it's unsurprising that this get most hits. 1/

But yes, UB does not mean "we don't define what happens when X (with X being something like div by zero)". UB means we only define behavior

for programs/systems/whatever that do not do X. If this happens anywhere, all bets are off. Same with a CPU. This is different from things

that are "implementation defined", or "unspecified", or "unpredictable". Full blown UB means: X happens anywhere, even in the future, and

the entire spec goes out the window and the system can do whatever it wants. UB is part of the contract between system and user, and the

user must guarantee that the UB is never ever triggered by their program.

AIUI, UB is sometimes necessary for speed and to prevent state space explosion for realizable impls of software/hardware. So what I ended >>