[llvm-dev] [RFC] Introducing the opaque pointer type

[Duncan P. N. Exon Smith via llvm-dev]

I agree. I think it would be a mistake to add an unnecessary difference vs. typed pointers along some other axis (address space, or otherwise). Opaque pointers have enough of their own challenges to solve.

> On 2021 May  10, at 15:28, Arthur Eubanks via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> If there's a larger effort to make address spaces then I'd be happy to change the representation since mass updating tests once is better than twice, but I'm worried that this may start becoming intertwined with more address space work, and the opaque pointers project has gone on long enough (like many other LLVM projects).
> 
> And of course, there's always time before we do mass test updates to easily change the textual representation.
> 
> On Fri, May 7, 2021 at 11:27 AM David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
> On Fri, May 7, 2021 at 11:20 AM Arthur Eubanks via llvm-dev
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >
> >
> >
> > On Fri, May 7, 2021 at 8:40 AM David Chisnall via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >>
> >> On 04/05/2021 19:32, Tom Stellard via llvm-dev wrote:
> >> > I think requiring an address space would be too confusing for a majority
> >> > of use
> >> > cases.  Would it help if instead of defaulting to 0, the default address
> >> > space
> >> > was target dependent?
> >>
> >> For CHERI targets, the default address space is ABI dependent: AS0 is a
> >> 64-bit integer that's relative to the default data capability, AS200 is
> >> a 128-bit capability (on 64-bit platforms).  It can also differ between
> >> code, heap, and stack.
> >>
> >> If this is purely a syntactic thing in the text serialisation, would it
> >> be possible to put something in the DataLayout that is ignored by
> >> everything except the pretty-printer / parser?
> >
> > Could you give an example?
> >
> >
> > Also, perhaps we should separate the opaque pointer types transition from any changes to address spaces. Currently the proposal is basically unchanged from the current status quo in terms of pointer address spaces. We definitely should have a "default" pointer type in some shape or form which is represented by "ptr", or else writing IR tests is too cumbersome. Currently that means AS0, but we can change that in the future if we want independently of opaque pointers.
> 
> +1 to this - pointers already carry their address space with explicit
> syntax and I think it's OK to do that for this transition. Though I
> wouldn't be opposed to a change in the future to roll it into the
> pointer type name if that seems suitable.
> 
> - Dave
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