[llvm-dev] SCCP is not always correct in presence of undef (+ proposed fix)

[Davide Italiano via llvm-dev]

On Sat, Dec 31, 2016 at 12:15 AM, Daniel Berlin <dberlin at dberlin.org> wrote:
>
>
> On Fri, Dec 30, 2016 at 11:55 PM, Sanjoy Das
> <sanjoy at playingwithpointers.com> wrote:
>>
>> Hi Daniel,
>>
>> On Fri, Dec 30, 2016 at 10:55 PM, Daniel Berlin <dberlin at dberlin.org>
>> wrote:
>> >> Right, but we are talking about "when, in the intermediate state, can i
>> >> transform an undef to a different value".
>> >>
>> >> Remember you can only go down the lattice. So you can't make undef
>> >> constant, and then discover it's wrong, and go back up :)
>> >> IE  can't change phi undef, undef to constant value 50, , discover 50
>> >> doesn't work, and say, well actually, let's make it undef again for an
>> >> iteration :)
>>
>> If the kind of simplification (I think) you've mentioned is allowed,
>> then I think even Davide's lattice is problematic.  Say we have:
>>
>> loop:
>>   X = phi(entry: undef, loop: T)
>>   ...
>>   T = undef
>>   if C then br loop else br outside
>>
>> When we first visit X then we'll compute its state as (Undef MEET
>> Unknown) = Undef.
>
> Yes
>
>>
>> My guess is that you're implying that the Undef
>> lattice state (or some other computation hanging off of it) can be
>> folded immediately to, say, 50, but once we mark the backedge as
>> executable we'll set it to undef (or mark it as 49)?
>
> Yes
>
>
>>   And both of
>> these are bad because you're making a non-monotonic transition?
>
> Yes
>
>>
>> Given
>> this, I'm not sure what prevents the bad transform from happening even
>> if we have separate Unknown and Undef states.
>
>
> You can always get *out* of the bad transform by dropping to overdefined if
> you discover it doesn't work.
> The question is more one of "what is the optimal time to  try to figure out
> a constant value that works".
> If you drop too early, you have to go overdefined when you may have been
> able to figure out a constant to use to replace the undef.
>>
>>
>>
>>
>> If the solution is to not fold X into undef (which, by assumption, can
>> "spontaneously decay" into any value) until you're sure every incoming
>> value is also undef then I suppose we'll need some special handling
>> for undef values to prevent breaking cases like (as shown earlier):
>>
>> loop:
>>   X = phi(entry: 10, loop: T)
>>   if X == 10 then br outside else br loop
>>
>> =>
>>
>> loop:
>>   X = phi(entry: 10, loop: T)
>>   br outside
>>
>> (i.e. to keep the algorithm properly optimistic)?
>
>
> Right.  This is precisely the unknown part, or at least, that was my
> thinking.
>
>>
>>
>> And if we do have a way of keeping undefs as undefs (i.e. prevent X
>> from decaying into 50 when we first visit the first loop) then why
>> can't we re-use the same mechanism to avoid spuriously decaying "phi
>> undef undef-but-actually-unknown" to "50"?
>
> That mechanism is the "all executable paths leading to our operands visited"
> bit, which is .. the unknown vs undef state.
>
> IE we only mark the operand undef when we visit it from a reachable edge.
>>
>>
>> Another way of stating my suggestion is that, if you agree that this
>> is a correct lattice (different from Davide's proposal) and pretend
>> the "spontaneous undef decay" problem does not exist, then:
>>
>> digraph G {
>>   Unknown -> Undef
>>   Undef -> Constant1
>>   Undef -> Constant2
>>   Undef -> Constant3
>>   Constant1 -> Bottom
>>   Constant2 -> Bottom
>>   Constant3-> Bottom
>> }
>>
>> then it should be legal / correct to first drop every lattice element
>> from "Unknown" to "Undef" before running the algorithm.  The only
>> cases where this would give us a conservative result is a place where
>> we'd have otherwise gotten "Unknown" for a used value; and the
>> algorithm should never have terminated in such a state.
>
>
> It is definitely *legal* to do this or the algorithm is broken otherwise
> (you should just end up in overdefined more).
>
> I'm not sure i believe the last part though.  I have to think about it.
>

Thanks for the comments Sanjoy, I'll have to think about it further.
My problem with this is when you put undef into the equation the
original Zadeck's algorithm becomes not entirely obvious. I mean, I
originally thought it was only my problem, but we're here discussing
(and have discussed this already in the past, so it seems it's not).

>
> FWIW: My initial proposal to all of this was "stop caring about trying to
> optimize undef here at all". Remove it from the lattice, and treat undef as
> overdefined if we see it. If we want to optimize undef, before SCCP solving,
> build SCCs of the parts of the SSA graph that contain undef, choose correct
> values for those subgraphs, then mark them constant *before* the solver sees
> it".
>

Although originally I wasn't, I'm increasingly convinced this is the
right path forward (at least in the beginning), i.e. strip undef
handling entirely. I tried to integrate undef in the solver to see how
that worked and it seems the proposed lattice has still some issues.
We have hard time reasoning about simple things like what's the
lattice structure and what should be the invariants we
maintain/enforce at the end of each run (something that's very nice
about the original algorithm is that you know at the end of the
algorithm nothing should have value TOP otherwise you forgot to lower
something/have a bug, but we can't currently assert this because of
`undef`).
I'm also becoming increasingly convinced that this problem is
something we (LLVM) created.
If optimizing `undef` (which is sometimes if not often symptom of
undefined behaviour/bugs in your program) should come at the expense
of correctness or complicated logic/potentially unsound algorithms I
don't think we should go that way.
What Danny proposed I think it's very appealing but I'm taking a more
extreme position here saying that we may consider not doing that in
the beginning. Today I ran SCCP over a bunch of codebases (completely
removing the undef optimization) and I found it never actually
resulting in a runtime performance hit. Of course this is not
representative at all, but still a datapoint.
I think we should take this opportunity as a way to see if we can make
things simpler/easier to consider correct instead of adding cases for
problems we discover.

>  This is pre-solve instead of post-solve or iterative solve. We already do
> this for arguments by solving them to overdefined. IPCP also works much the
> same way by pre-solving arguments constants
>
> This is precisely because this stuff is non-trivial to reason about, and
> unclear to me that it's really worth it.  If *it* is worth it, we can do
> better than we do now (and contain the mess *entirely* to the presolver),
> and if it's not, we shouldn't have complicated reasoning to try to make it
> work well.
>
> Right now we have the worst of all worlds.  We have a mess in both places
> (the resolver and the solver both step on each other and differ in
> behavior), it's hard to reason about (though provably not correct), and it
> doesn't even give you the best answers it can *anyway*
>
> (generally, at the point where we have a bunch of very smart people racking
> their heads about whether something is really going to work algorithmically
> or not, i take it as a sign that maybe we have the wrong path.  Obviously,
> there are some really complex problems, but ... this should not be one of
> them :P)
>

+1 on everything.

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare