[cfe-dev] Demystifying the byte type

[George Mitenkov via cfe-dev]

Hi James,


> If what you describe is correct, doesn't that imply that the
> opaque pointer work is a fools errand ?
> I.e. If memory needs to be typed, surely pointers to that memory also need
> to be typed?

Not at all, the issue described in the post is orthogonal to opaque
pointers. The main problem that the post talks about is that LLVM does not
have a type that represents a raw sequence of bits (i.e. memory).
Currently, integers are used for that, which makes them carry pointers
sometimes (as described in first parts of the post). This creates a problem
for optimizations on integers, since we do not know that the values that we
load are "pure" real integers or pointers casted to ints (and different
LLVM optimizations make different assumptions about that).

So it is not about making memory "typed", but rather creating a universal
type that can be used to load from memory something that carries raw data
(integers and pointers) and preserves provenance.

Hope this helps!

Thanks,
George

On Sat, Oct 16, 2021 at 1:25 PM James Dutton <james.dutton at gmail.com> wrote:

> Hi,
>
> The gist post, seems to imply that one needs memory to be typed.
> If what you describe is correct, doesn't that imply that the opaque
> pointer work is a fools errand ?
> I.e. If memory needs to be typed, surely pointers to that memory also
> need to be typed?
>
> Kind Regards
>
> James
>
>
> On Fri, 15 Oct 2021 at 19:41, George Mitenkov via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> >
> > Hi all,
> >
> > In May 2021, together with Nuno Lopes and Juneyoung Lee, we proposed to
> add a byte type in LLVM to fix load type punning issues. Initial RFC
> touched some subtle aspects of LLVM IR and its semantics, and sparked a lot
> of questions, concerns, and discussions.
> >
> > We decided to write a post that would summarise the thread and the
> complicated topic:
> >
> > https://gist.github.com/georgemitenkov/3def898b8845c2cc161bd216cbbdb81f
> >
> > We hope that our post clarifies initial concerns raised on the mailing
> list. As always, any questions, suggestions and advice are welcome!
> >
> > Thanks,
> > George
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
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