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...ntended to make branching on poison values OK. If branching on poison wasn't OK, then we couldn't go from select to -> br/phi. > > > There's another caveat: it *does* seem to fix the problem described by Dan > in http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-December/046152.html > > However, it introduces a potential performance penalty: we won't be able > to speculate instructions with undefined behavior whose input may be poison. > > > > For example, take the following code: > > loop: > > %add = add nsw %x, %y > > %div...

Hello, What follows is my attempt to describe how poison works. Let me know what you think. -- David # LLVM Poison Semantics Poison is an LLVM concept which exists solely to enable further optimization of LLVM IR. The exact behavior of poison has been, to say the least, confusing for users, researchers and engineers working with LLVM. This document hopes to clear up some of the confusion