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

[John McCall via llvm-dev]

> On Nov 9, 2016, at 8:44 AM, Nuno Lopes <nuno.lopes at ist.utl.pt> wrote:
>> On 11/8/2016 3:32 PM, Sanjoy Das wrote:
>>> Hi Nuno, Chandler,
>>> 
>>> Nuno Lopes via llvm-dev wrote:
>>>> This program stores 8 bits, and leaves the remaining 24 bits 
>>>> uninitialized. It then loads 16 bits, half initialized to %v, half 
>>>> uninitialized. SROA transforms the above function to:
>>>> 
>>>> define i16 @g(i8 %in) {
>>>>   %v = add nsw i8 127, %in
>>>>   %1 = zext i8 %v to i16
>>>>   %2 = shl i16 %1, 8
>>>>   %3 = and i16 undef, 255
>>>>   %4 = or i16 %3, %2
>>>>   ret i16 %4
>>>> }
>>> 
>>> This program above returns i16 poison only if "shl i16 poison, 8" is a 
>>> full value poison.  Whether that is the case today or not is anybody's 
>>> guess (as Eli says, we should not rely too much on semantics that are 
>>> known to be broken), so the important question is whether "shl i16 
>>> poison, 8" _can_ be defined to be poison only on the higher 8 bits in 
>>> a world with bitwise poison.  If we could make that happen, we'd also 
>>> get some other fringe benefits, like ComputeKnownBits would be able to 
>>> fearlessly return "the low 8 bits of `shl i16 %val, 8` are 0" as 
>>> opposed to "the low 8 bits of `shl i16 %val, 8` are 0 or poison".
>>> 
>>> One problem with shl always generating non-poison zeroes in the lower 
>>> bits is that it then is not equivalent to multiplication.  This breaks 
>>> optimizations like:
>>> 
>>>  %t = ...
>>>  %q = shl i32 %t, 6
>>> 
>>> =/=>
>>> 
>>>  %t = ...
>>>  %q = mul i32 %t, 64
>>> 
>>> and (probably more meaningful)
>>> 
>>>  %t = ...
>>>  %q0 = shl i32 %t, 6
>>>  %q1 = mul i32 %q0, 3
>>> 
>>> =/=>
>>> 
>>>  %t = ...
>>>  ;; %q0 = shl i32 %t, 6
>>>  %q1 = mul i32 %q0, (3 * 64)
>>> 
>>> 
>>> and (probably even worse) it also breaks reassociation:
>>> 
>>>  %t = ...
>>>  %q0 = shl i32 %t, %P
>>>  %q1 = mul i32 %q0, %Q
>>> 
>>> (No other uses of %q0, but there are uses of %q1)
>>> 
>>> =/=>
>>> 
>>>  %t = ...
>>>  %q0 = mul i32 %t, %Q
>>>  %q1 = shl i32 %q1, %P
>>> 
>>> 
>>> I'm especially worried about this last re-association example since 
>>> SCEV does things like this internally when canonicalizing expressions, 
>>> and I'm not convinced that we can change it to tiptoe around these 
>>> problematic cases.
>> 
>> 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.  Actually optimizing code that we've proven has undefined behavior, in contrast, is basically uninteresting and leads us into these silly philosophical problems.

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.

John.