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

Tue May 11 00:20:38 PDT 2021

On Mon, May 10, 2021 at 11:59 PM pawel k. via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I am very much beginner in opaque pointers but I am also minimalist too in
> a sense entities shouldnt be multiplied but rather divided where applicable.
>
> Can someone point me to article(s) describing what problems opaque
> pointers solve that cant be solved with forward declaractions and typed
> pointers etc?
>
> My first gutfeeling was when learning on idea of opaque pointers, theyre
> not much more than void*
>

Yep, that's basically what they are. Though this is only relative to the IR
design, not source language design.

> with all its issues from static analysis, compiler design, code
> readability, code quality, code security perspective. Can someone correct a
> newbie? Very open to change my mind.
>

LLVM doesn't provide any guarantees about pointer types (unlike, say, C++
that has type based aliasing guarantees about pointers - if you have an
int* you know it can't hold the same value as a float* in C++, but this
property isn't true in LLVM IR (this information can be carried separately
in type based alias analysis metadata - but it's not inherent in the LLVM
IR of pointers themselves)) - so the type information provides limited
value (somewhat useful for frontends generating IR to be able to have some
intended type information carried around in the IR as it's being
constructed) and inhibits optimizations - converting between pointer types
involves instructions (geps or bitcasts) - instructions that optimizations
have to know to skip over/look through.

So instead, we're moving to a model where pointers don't have a type (since
it's not informative to optimizations anyway) - and operations carry type
information (instead of "load from this int pointer" it'll be "load an
integer from this opaque pointer").

If you look at the LLVM IR today, you'll see these explicit types on
operations (eg: the load instruction has an explicit type parameter to it,
which currently looks redundant with the type of the pointer parameter
that's passed to the load instruction - but in the future that pointer
parameter won't carry any pointee type information and the load will rely
entirely on the explicit type parameter it has).

- Dave

> -Pawel
>
> wt., 11.05.2021, 02:35 użytkownik Duncan P. N. Exon Smith via llvm-dev <
> llvm-dev at lists.llvm.org> napisał:
>
>> 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> wrote:
>>
>>> On Fri, May 7, 2021 at 11:20 AM Arthur Eubanks via llvm-dev
>>> <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> 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
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210511/453da756/attachment.html>