"LLVM considers infinite recursion to be UB, so the only valid way for this function to execute is to return true"

The Collatz conjecture is saying that the programmed algorithm will eventually terminate and return true. Here the compiler optimized the code to always return true. Therefore the author jokingly assumed that the compiler must have proven the Collatz conjecture and the authors should be rewarded a million dollars.

In reality, the reason is that out of the two outcomes: true or infinite loop, the infinite loop violates the standard, and therefore, the result must be true. Another way to see it is that the only way for the code to be valid is if the programmer knows that the Collatz conjecture is true, and the compiler trusts the programmer, saying that the compiler knows the answer to the Collatz conjecture is a kind of circular reasoning.

There are other issues here: the numbers are bounded (a 128 bits integer), but the conjecture is not. And no million dollar prize have been assigned to this problem (it is not one of the millenium problems). Also, an infinite loop is not a result, it is actually the textbook example of an undecidable problem.

Here's the relevant part of the C++ standard:

> 6.9.2.2 Forward progress [intro.progress]

> The implementation may assume that any thread will eventually do one of the following:

> — terminate,

> — make a call to a library I/O function,

> — perform an access through a volatile glvalue, or

> — perform a synchronization operation or an atomic operation.

That loop doesn't ever do any of the second through fourth things, and if it didn't return true, then it would never terminate either, so Clang is allowed to assume that it will eventually return true. And since the loop doesn't have any other effects visible outside the function, it can be optimized away entirely.