[llvm-dev] [RFC] Introduction of the 'unknown_provenance' LLVM-IR constant

[Aaron Ballman via llvm-dev]

On Wed, Nov 17, 2021 at 9:17 AM Jeroen Dobbelaere via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> In the context of introducing the 'ptr_provenance' infrastructure (See [0], [1]),
> I am also introducing a new LLVM-IR constant: 'unknown_provenance' (See [2]).
>
> In [2], David (xbolva00) asked for a RFC. To be sure that the community is aware
> of those additions, I am posting this here.
>
> The 'unknown_provenance' constant is only to be used on the ptr_provenance path.
> It indicates that the actual provenance is unknown (and could be any valid object).
>
> Together with the ptr_provenance infrastructure, this provides a base infrastructure
> that allows us to:
> - implement the full restrict support[1]
> - provide a fix for PR44313 [3], while still making use of the p==q value equality
> - provide an implementation of N2676[4]
>
> Another reason for the new constant is that using any other value (null, undef, poison, 42, ...)
> would be ambiguous in some situation.
>
> On Thursday, I also give a talk[5] about the infrastructure needed for 'Full Restrict' support.
> That gives some extra rationale behind the need for the ptr_provenance related changes.
>
> Any thoughts ?

As a Clang community representative to WG14 (the C committee), I
greatly appreciate the aim for full support of C99's `restrict` as
well as investigating the efforts of N2676. I support these efforts!

Thanks!

~Aaron

>
> Jeroen Dobbelaere
>
> [0] https://lists.llvm.org/pipermail/llvm-dev/2021-June/151152.html [llvm-dev] pointer provenance introduction to load/store instructions
> [1] https://reviews.llvm.org/D68484 [PATCH 01/27] [noalias] LangRef: noalias intrinsics and ptr_provenance documentation.
> [2] https://reviews.llvm.org/D111160 [UnknownProvenance] Add LLVM-IR support for unknown_provenance
> [3] https://bugs.llvm.org/show_bug.cgi?id=44313
> [4] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2676.pdf A provenance aware memory object model for C
> [5] https://llvm.swoogo.com/2021DevMtg/session/705670/ptr_provenance-and-@llvm.noalias-the-tale-of-full-restrict
>
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