[PATCH] D26432: [SCCP] Force unknown branches to overdefined

[Davide Italiano via llvm-commits]

On Wed, Nov 9, 2016 at 9:49 AM, Daniel Berlin via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Actually, digging further into it,  I don't understand how this fixes it at
> all, and your test does nothing because it's all CHECK's :-)
>
> The example given in the bug report is or true, undef
>
> ResolveUndefs already does the right thing for this, and does not consider
> the result undef.
>
> The propagation order makes it impossible for that to happen, because in
> reality, undef is a set of lattice values but not represented in the
> lattice, and resolveUndefs is trying to figure out the lattice values that
> work.
>
> That's ... never going to work, as you've discovered (and eli suspects on
> the bug report).
>
> I suspect pretty much every part of markForcedConstant is vulnerable to this
> undef different circumstances.
>
> Realistically, undef needs to be an actual lattice value lower than unknown
>
> That way, or undef, true comes out true during *solving*.
> The parts of resolveundefs handling this need to be made part of the meet
> operation
>
> This handles the case where undef must be constrained (IE or undef, true).
>
> After solving is finished, anything left as undef can then be filled in
> unconstrained, because you are guaranteed it is not forced to a certain
> value (or that your solver sucks :P)
>
> (note that in the freeze/poison proposal, this should just work because the
> freezes will be different ssa names)
>

Uh, thanks. Out of curiosity, do you happen to know why undef wasn't
represented as a lattice value from the beginning (and instead we end
up trying values that match up?) Is there a particular reason (e.g.
efficiency)? I can't think of one.
--
Davide