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

[Juneyoung Lee via llvm-dev]

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.

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.

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
> >>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
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>


-- 

Juneyoung Lee
Software Foundation Lab, Seoul National University
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