llvm dev - May 2021 - [RFC] Introducing the opaque pointer type

On 5/3/21 11:01 PM, Nicolai Hähnle via llvm-dev wrote:
> Hi Arthur,
> 
> On Tue, May 4, 2021 at 2:39 AM Arthur Eubanks via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> 
>     For background on opaque pointer types, see [1] and many other
patches/threads searchable with "opaque pointers".
> 
>     While there's been lots of work around making opaque pointers work,
we don't actually have a type like that in LLVM yet.
https://reviews.llvm.org/D101704 <https://reviews.llvm.org/D101704>
introduces the opaque pointer type within LLVM so we can start playing around
with the opaque pointer type and see what goes wrong. Much of the patch above is
based on TNorthover's branch from a couple years ago [2].
> 
>     The opaque pointer type is essentially just a PointerType with a null
pointee type. Calling getElementType() on an opaque pointer asserts.
> 
>     Since the bitcode representation for non-opaque pointers contains the
pointee type, we need a new bitcode type code for opaque pointers, which only
contains the address space.
> 
>     For the textual IR representation, the current proposal is to represent
an opaque pointer type with "ptr" with an optional
"addrspace(N)". This seems consistent with existing uses of
"addrspace(N)" and "ptr" seems right.
>     There are a couple alternatives. TNorthover's version uses
"pN" where "N" is the address space, so most pointers would
be "p0", and a pointer in address space #5 would be "p5". I
initially attempted something like "ptr(N)", but the spelling is
slightly ambiguous with function types. We could also simply use a void pointer,
which LLVM currently does not allow [3].
> 
> 
> Thank you for doing this, and the approach seems largely good to me, except
for one important point: We've been moving steadily towards making addrspace
0 be non-special for a long time now, so I *strongly* prefer a spelling that
always has an address space. I don't care too much about the exact spelling,
pN and ptr(N) both seem fine to me assuming technical issues can be sorted out.
pN has the benefit of already being used in codegen contexts, so count that as a
*mild* preference for that spelling.
> 
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?

- Tom
> Cheers,
> Nicolai
> 
>     Feel free to bikeshed.
> 
>     [1]:
https://lists.llvm.org/pipermail/llvm-dev/2015-February/081822.html
<https://lists.llvm.org/pipermail/llvm-dev/2015-February/081822.html>
>     [2]:
https://lists.llvm.org/pipermail/llvm-dev/2019-December/137684.html
<https://lists.llvm.org/pipermail/llvm-dev/2019-December/137684.html>
>     [3]: https://llvm.org/docs/LangRef.html#pointer-type
<https://llvm.org/docs/LangRef.html#pointer-type>
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> 
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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?

David