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

[Sanjoy Das via llvm-dev]

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