[LLVMdev] RFC: Proposal for Poison Semantics

[Sanjoy Das]

> I don't think your example is actually problematic.  The original program
> before your transformation *executed* undefined behavior in the form of '%x
> = add nuw i32 %m, %n' with "%m = %n = 2^32-1 (a.k.a INT_MAX)".  If I

I was trying to show why the rule "signed overflow is undefined
behavior" is problematic w.r.t. hoisting arithmetic by repeating an
argument David made in the first email on this thread.  That
particular example is not a counterexample to the RFC.

The RFC has issues with icmps, sexts and zexts; but they're distinct
from the hoisting problem.

>  %x1 = add nuw i32 %m, %n
>  %x  = select i1 %cnd x1, 0
>  %y = zext i32 %x to i64
>  %s = lshr i64 %y, 32
>  %addr = gep %some_global, %s
>  store i32 42, i32* %addr
>
> This is well defined as per the spec David sent out.  %x is not poison.  %x1
> is, but that's fine.

AFAICT, going by the RFC, %x is poison iff %x1 is poison and %cond is true.

-- Sanjoy