llvm dev - Sep 2020 - Is it valid to dereference a pointer that have undef bits in its offset?

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
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> %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>
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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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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