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...om> > > wrote: > >> > >> Hi David, > >> > >> I spent some time thinking about poison semantics this way, but here > >> is where I always get stuck: > >> > >> Consider the IR fragment > >> > >> %x = zext i32 %maybe_poison to i64 > >> %y = lshr i64 %x 32 > >> %ptr = gep %global, %y > >> store 42 to %ptr > >> > >> If %maybe_poison is poison, then is %y poison? For all i32 values of > >> %maybe_poison, %y is i64 0, so in some sense you can determine the >...

On Tue, Jan 27, 2015 at 7:23 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote: > Hi David, > > I spent some time thinking about poison semantics this way, but here > is where I always get stuck: > > Consider the IR fragment > > %x = zext i32 %maybe_poison to i64 > %y = lshr i64 %x 32 > %ptr = gep %global, %y > store 42 to %ptr > > If %maybe_poison is poison, then is %y poison? For all i32 values of > %maybe_poison, %y is i64 0, so in some sense you can determine the > value %y without looking at %x. I agree, this makes...

On Tue, Jan 27, 2015 at 8:32 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote: > > > > Correct me if I am wrong but we are talking about transforming: > > %maybe_poison = add nuw i32 %a, %b > > %x = zext i32 %maybe_poison to i64 > > %y = lshr i64 %x 32 > > > > To: > > %za = zext i32 %a to i64 > > %zb = zext i32 %b to i64 > > %x = add nuw i64 %za, %zb > > %y = lshr i64 %x 32 > > > > ? &gt...

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