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

[Philip Reames via llvm-dev]

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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