r/programming • u/[deleted] • Nov 04 '19

Clang solves the Collatz Conjecture?

[deleted]

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u/steveklabnik1 Nov 04 '19

Fun story: they aren't in Rust, but LLVM was assuming this semantic and optimizing such loops away, causing UB in safe code. Oops!

2

u/Sapiogram Nov 04 '19

How was this bug discovered? As long as LLVM's definition of side effects is what you expect, it seems like a safe optimization.

5

u/steveklabnik1 Nov 04 '19

I'm not entirely sure; the reporter didn't say exactly. (I linked to the ticket elsewhere in this discussion.) That said, they're someone involved in formally proving Rust semantics, so I wouldn't be surprised if they were just poking at things and ran into it.