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I mean that's what they did, and that's why there's that UB. All I'm saying is that this is the "weird platform behaviors exist and must be legalized by the standard" kind of UB and not the "we want a 0.5% win for benchmaxxing" kind of UB (the standard has plenty of both).

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No, they didn't. UB is a cop out and inserting yield is just plain bad. Locking up one or more threads in an implementation defined manner would be the outcome of least surprise (I already know it's going to lock up at least the one thread).

The standards intended interpretation of UB was always intended to be something like "implementation defined, no documentation required" to allow for implementation weirdness, even unpredictable ones. It was compiler authors who decided do abuse this allowance to do really unintuitive things instead of weird platform weirdness.

100% There are implementations that have sane behavior for "UB" instead of making it an excuse to misbehave.

The ISO standard is not the same as a language from a single vendor.

IIRC a lot of it was benchmark gaming between GCC and LLVM.

Because one can bleeping see that that's what would happen. Locking up a thread isn't a good thing, but it's a lot better than UB. There was never a need to make this UB.

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