[llvm-dev] RFC: Killing undef and spreading poison

[Sanjoy Das via llvm-dev]

Hi John,

John McCall wrote:
>>> Well, we could say non-nsw add and mul are actually "bitwise" operations, so "mul i32 poison, 2" is guaranteed to have its bottom bit zero (but "mul nsw i32 poison, 2" is poison).  Of course, there's a limit to how far we can go in that direction, or we just end up with the old notion of undef.  Off the top of my head, I'm not exactly sure where that line is.
>> Right, we could try to make it work.  I don't have a good model for bit-wise poison yet, but it might be possible.  One of the things it will break is rewrites into comparisons, since at least comparisons will need to return poison if any bit of the inputs is poison. So the direction of "arithmetic ->  comparison" gets broken (InstCombine has a few of these).
>>
>> But before we try to make bit-wise poison work, I would like to understand what are the deficiencies of value-wise poison. Is there any major issue?  I'm asking because value-wise poison seems to be a much simpler concept, so if it works ok it seems to be preferable.
>>
>> During the meeting last week, Richard Smith proposed that instead we add a "load not_poison %ptr" (like load atomic) instruction that would be equivalent to load+freeze+bitcast, because he is concerned that some C++ stuff needs to be lowered to such a load. This load then becomes tricky to move around (e.g., cannot be sank to inside loops), but there are options to attenuate that problem if necessary.
>> Perhaps this construct would make John McCall happy as well (re. his question during the talk last week)?
>
> I think the source of evil here is the existence of inconsistent values in the representation.  So no, if I understand your suggestion correctly, I don't think I'd be happy because it still admits the existence of an unfrozen poison value.
>
> It seems to me that the problem is that people are trying to specify the behavior of operations using the template "the operation succeeds normally if the conditions are met, and otherwise<something>".  I don't think it works.  I think the right template is simply "the result may be analyzed assuming that the conditions will be met", and we aren't really interested in what happens if the conditions aren't met.  Undefined behavior is valuable to a compiler when it allows us to simplify some operation assuming that the bad conditions never happen.

I don't think we can get away without assigning some sort of meaning
to the operations in the "abnormal case" for operations that don't
have immediate UB in abnormal cases.  Since the program can legally
continue executing after doing the "abnormal" thing, we still need
some sort of sane semantics for the values these abnormal operations
produce.

As a concrete example, say we have:

i32 sum = x *nsw y
i64 sum.sext = sext sum to i64
...
some use of sum.sext


Pretending "x +nsw 1" does not sign-overflow, we can commute the sext
into the arithmetic, but we still somehow need to capture the fact
that, depending on the optimizer's whims and fancies (i.e. whether it
does the commute or not), the high 32 bits of sum.sext can now be
something other than all zeroes or all ones.

> Actually optimizing code that we've proven has undefined behavior, in contrast, is basically uninteresting and leads us into these silly philosophical problems.

I don't think we care about optimizing code that we've _proven_ has
undefined behavior.  We care about optimizing code in ways that is
correct in the face of *other* transforms that we ourselves want to
do, where these "other transforms" pretend certain abnormal cases do
not exist.  Poison is a "stand-in" for these transforms, which are
sometimes non-local.

For instance, continuing the previous example, say we're interested in
the speculatibility of

t = K `udiv` (-1 + (sum.sext >> 32))

We don't _really_ care about doing something intelligent when sum.sext
is provably poison.  However, we do care about taking into the
_possibility_ of sum.sext being poison, which is really just a more
precise way of saying that we care about taking into the possibility
of sum.sext being commuted into (sext(x) * sext(y)) (in which case the
division is not speculatable).

And we want to do this with enough formalism in place so that we can
write correct checking interpreters, fuzzers etc.

> I would suggest:
> 1. Make undef an instruction which produces an arbitrary but consistent result of its type.
> 2. Revisit most of the transformations that actually try to introduce undef.  The consistency rule may make may of them questionable.  In particular, sinking undef into a loop may change semantics.
> 3. Understand that folding something to undef may actually lose information.  It's possible to prove that %x<  %x +nsw 1.  It's not possible to prove that %x<  undef.  Accept that this is ok because optimizing undefined behavior is not actually interesting.

As I said above, we don't really care about folding cases that we've
proved to have UB.

Moreover, in your scheme, we still won't be able to do optimizations
like:

  %y = ...
  %undef.0 = undefinst
  %val = select i1 %cond, %undef.0, %y
  print(%val)

  to

  %y = ...
  print(%val)

since it could have been that

  %x = INT_MAX at runtime, but unknown at compile time
  %y = %x +nsw 1
  %undef.0 = undefinst
  %val = select i1 %cond, %undef.0, %y
  print(%val) == %x s< %val

Folding %val to %y will cause us to go always printing false to
printing either true or false, depending on the will of the compiler.


I have to think a bit about this, but in your scheme I think we will
generally be only able to fold `undefinst` to constants since
non-constant values could have expressions like `<INT_MAX at
runtime> +nsw 1` as their source which would justify non-local
transforms that `undefinst` would not.

-- Sanjoy