LLVM Poison Values Defer Undefined Behavior Through the IR

Sat, 19 Sep 2026 00:00:00 +0000

LLVM IR does not turn every invalid arithmetic or pointer condition into immediate undefined behavior. Many instructions produce a poison value instead. That value can flow through later instructions, preserving the optimizer’s ability to rely on promises such as “this addition does not overflow” without forcing undefined behavior at the exact instruction where the promise is violated.

This is part of LLVM’s semantics, not an optimizer implementation detail. A frontend that emits nsw, nuw, inbounds, noundef, or related constraints is stating facts that later passes may trust.

Intermediate Representation on Nalar

LLVM Poison Values Defer Undefined Behavior Through the IR