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

[Johannes Doerfert via llvm-dev]

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
>>>
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