[llvm-dev] Is it valid to dereference a pointer that have undef bits in its offset?

[Johannes Doerfert via llvm-dev]

On 9/21/20 9:41 PM, Juneyoung Lee wrote:
> Thank you for the infos; it seems making it raise UB is problematic.
>
> Would clarifying it in LangRef be good? I can update the patch to contain
> the information instead.

Yes, please.


> Another concern is then, how can we efficiently encode an assumption that a
> pointer variable in IR does not have undef bits?
> Certainly, in the front-end language, (most of) pointers won't have undef
> bits, and it would be great if the information is still available in IR.
> A pointer argument can be encoded using noundef, but, e.g., for a pointer
> that is loaded from memory, such information disappears.
> I think this information is helpful reducing the cost of fixing existing
> undef/poison-related optimizations, because we can conclude that we don't
> need to insert freeze in more cases.

I thought we solved that already:

     `call void llvm.assume(i1 true) ["noundef"(type* %ptr), 
"noundef"(type2* %ptr2)]`

See http://lists.llvm.org/pipermail/llvm-dev/2019-December/137632.html

Is that enough for your needs?


~ Johannes


> Juneyoung
>
>
> On Tue, Sep 22, 2020 at 5:51 AM Johannes Doerfert <
> johannesdoerfert at gmail.com> wrote:
>
>> To be fair, if the address has to be `noundef` the example would just be
>> UB. That said, I still believe it "is not".
>>
>>
>> On 9/21/20 1:41 PM, Philip Reames wrote:
>>> I think we need to allow this.  Otherwise, we have to prove that
>>> addresses are non-undef before we can hoist or sink a memory
>>> instruction.  Today, aliasing can use things like known bits, and if
>>> we imposed a no-undef in address requirement, we'd either need to
>>> replace such reasoning in AA, or have passes which wish to hoist/sink
>>> check the property afterwards.
>>>
>>> Or to say it differently, I think it's reasonable for %p2 and %p3 to
>>> be provably no alias and dereferenceable, and for %v and %v2 to be
>>> safe to speculate.
>>>
>>> %p = alloca [16 x i8]
>>> %p2 = gep %p, (undef & 7)
>>> %v = load %p2
>>> %p3 = gep %p, 8
>>> %v2 = load %p3
>>>
>>> Keep in mind that the undef doesn't have to be literal and can be
>>> arbitrarily obscured (e.g. behind a function call).  The alternative
>>> interpretation is extremely limiting.
>>>
>>> Philip
>>>
>>>
>>> On 9/21/20 10:41 AM, Johannes Doerfert via llvm-dev wrote:
>>>> My feeling tells me we should allows this.
>>>> No proper justification handy but your example doesn't strike me as UB.
>>>>
>>>> ~ Johannes
>>>>
>>>>
>>>> On 9/21/20 12:32 PM, Eli Friedman via llvm-dev wrote:
>>>>> I think it’s reasonable to expect that IR generated by frontends
>>>>> doesn’t do this.
>>>>>
>>>>> Not sure about transforms; I can imagine that we might speculate a
>>>>> load without proving all the bits are well-defined.
>>>>>
>>>>> -Eli
>>>>>
>>>>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of
>>>>> Juneyoung Lee via llvm-dev
>>>>> Sent: Sunday, September 20, 2020 3:54 PM
>>>>> To: llvm-dev <llvm-dev at lists.llvm.org>
>>>>> Subject: [EXT] Re: [llvm-dev] Is it valid to dereference a pointer
>>>>> that have undef bits in its offset?
>>>>>
>>>>>> %p2 = gep %p, (undef & 8)
>>>>> A silly typo: undef & 8 -> undef & 7
>>>>>
>>>>> On Mon, Sep 21, 2020 at 7:47 AM Juneyoung Lee
>>>>> <juneyoung.lee at sf.snu.ac.kr<mailto:juneyoung.lee at sf.snu.ac.kr>> wrote:
>>>>> Hello all,
>>>>>
>>>>> Is it valid to dereference a pointer that has undef bits in its offset?
>>>>>
>>>>> For example,
>>>>>
>>>>> %p = alloca [8 x i8]
>>>>> %p2 = gep %p, (undef & 8)
>>>>> store 0, %p2
>>>>>
>>>>> undef & 8 is always less than 8, so technically it will store zero
>>>>> to one of the array's elements.
>>>>>
>>>>> The reason is that I want to improve no-undef analysis by suggesting
>>>>> that a pointer that is passed to load/store is well-defined, by
>>>>> making it raise UB when a pointer with undef bits is given.
>>>>>
>>>>> A suggested patch is here: https://reviews.llvm.org/D87994
>>>>>
>>>>> I wonder whether there is a case using this to do something that I'm
>>>>> not aware.
>>>>>
>>>>> Thanks,
>>>>> Juneyoung
>>>>>
>>>>>
>>>>> --
>>>>>
>>>>> Juneyoung Lee
>>>>> Software Foundation Lab, Seoul National University
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
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