I'm a Rust compiler author, and I'm fully in favor of "UB exploitation". In fact, LLVM should be doing more of it. LLVM shouldn't be holding back optimizations in memory-safe languages for edge cases that don't really matter in practice.
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I don't see any surprised compiler authors in that thread. The reporter immediately suggests the correct underlying reason for the bug and another compiler author even says that they wondered how long it would take for someone to notice this.
replyEven if you read any surprise into their messages they wouldn't be surprised that C does something completely unreasonable, they would be surprised that LLVM does something unreasonable (by default).
Wait, that's not even linked in your post AFAICT. It's also about an LLVM bug and not in fact exploiting UB.
reply"LLVM shouldn't miscompile programs" is uncontroversial, but claiming that these miscompilations are somehow "Exploiting Undefined Behaviour" is either incompetent or an attempt to sell your position as something it isn't.