[llvm-dev] Improving SCEV's behavior around IR level no-wrap flags

[Andrew Trick via llvm-dev]

> On Sep 23, 2016, at 10:32 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
> Hi Andy,
> 
> Andrew Trick wrote:
> >> On Sep 23, 2016, at 7:50 AM, Christof Douma <Christof.Douma at arm.com <mailto:Christof.Douma at arm.com>> wrote:
> >>
> >> I can’t help to ask. Why not define a wrapping nsw instruction as UB, instead of “delayed UB” aka poison? I believe we
> >> have the notion of poison solely to ease the movement of instructions. In my example above line 6 can be moved to line
> >> 2 due to poison. If the wrapping was UB such a move is still possible but would require dropping the nsw flag, nothing
> >> more. If this information loss is considered a problem, the use of llvm.assume can counter that.
> >>
> >> Has somebody ever looked at not having poison at all? What are the downsides of that?
> >
> > That’s basically incompatible with transformation of the IR. I’m sure that’s been explained by DanG in the past, but I
> > don’t have a link. e.g. the optimizer doesn’t think that an ‘add’ has side effects and wants to freely hoist and combine
> > these operations. If the operation has an ‘nsw’ flag that would change the UB semantics.
> 
> +1.
> 
> > I have to say, my heart is not really into optimizing for UB. Is this relevant to any scenario other than legacy
> > C-without-overflow/bounds checks?
> 
> I don't think we'll be able to give up on C and C++ inspired
> optimizations anytime soon.  Given what I've seen on llvm-dev and
> bugzilla, it is pretty clear to me that we have to do _something_ to
> address the missing pieces here.
> 
> -- Sanjoy

Maybe. Or the answer might be to insert a check outside the loop. This is what I’m really getting at: do you see the Instruction nsw flags as useful apart from UB? Are you flagging them now after proving nsw based on some @guard intrinsics or other dominating checks?

-Andy