[cfe-dev] [RFC] New attribute `annotate_type`

[Aaron Ballman via cfe-dev]

On Mon, Oct 18, 2021 at 9:06 AM Martin Brænne <mboehme at google.com> wrote:
>
> Sorry, this is going to be a long reply again with lots of individual comments -- but to front-load the conclusion (which should also give some detail for all the detailed comments):
>
> If we can make [[clang::annotate]] work in type positions, then I would definitely find that palatable and probably even preferable! I think it may be non-trivial to implement though.

Excellent, I think we're on the same page with our conclusions.

> On Wed, 13 Oct 2021 at 17:46, Aaron Ballman <aaron at aaronballman.com> wrote:
>>
>> On Wed, Oct 13, 2021 at 9:19 AM Martin Brænne <mboehme at google.com> wrote:
>> >
>> >
>> >
>> > On Tue, 12 Oct 2021 at 22:16, Aaron Ballman <aaron at aaronballman.com> wrote:
>> >>
>> >> On Tue, Oct 12, 2021 at 7:49 AM Martin Brænne <mboehme at google.com> wrote:
>> >> >
>> >> >
>> >> >
>> >> > On Mon, 11 Oct 2021 at 18:54, Aaron Ballman <aaron at aaronballman.com> wrote:
>> >> >>
>> >> >> On Mon, Oct 11, 2021 at 10:28 AM Martin Brænne via cfe-dev
>> >> >> <cfe-dev at lists.llvm.org> wrote:
>> >> >> >
>> >> >> > I would like to propose a new attribute `annotate_type` that would be analogous to the existing `annotate` attribute but for use on types. As for `annotate`, the typical use case would be to add annotations to be used by static analysis tools.
>> >> >> >
>> >> >> > I have a draft patch implementing this attribute (https://reviews.llvm.org/D111548), but before it's reviewed I wanted to solicit some feedback more broadly on whether people feel this makes sense.
>> >> >>
>> >> >> Thanks for the proposal! One question I have is whether we need an
>> >> >> additional attribute for this instead of reworking [[clang::annotate]]
>> >> >> so that it applies to either a type or a declaration. Is a separate
>> >> >> attribute necessary because there may be unfortunate problems with
>> >> >> using __attribute__((annotate)) as the spelling?
>> >> >
>> >> >
>> >> > Yes, that's exactly the problem. Today, the GNU spelling can be applied both before and after the type name:
>> >> >
>> >> >   __attribute__((annotate("foo"))) int i1;
>> >> >   int __attribute__((annotate("foo"))) i2;
>> >> >
>> >> > We would need to reinterpret the second variant to refer (or "appertain") to the decl-specifier-seq, not to the declaration (which is, IIUC, what the C++ standard would prescribe if this was a C++ attribute). But as it's possible that this variant is already being used "in the wild" with the intent that it should refer to the declaration, I think this isn't something we can change.
>> >>
>> >> Agreed, thanks for the confirmation that this was the reason why we
>> >> need a second attribute.
>> >>
>> >> > Hence, the practical solution seems to be to introduce a separate attribute for types, for which it is then unambiguous what it should apply to.
>> >>
>> >> Another possible solution would be to not support
>> >> __attribute__((annotate)) as a type attribute (e.g., you can use
>> >> [[clang::annotate]] as a declaration or a type attribute, but we would
>> >> restrict __attribute__((annotate)) to only ever be a declaration
>> >> attribute.).
>> >
>> > I guess that would be an option too, though I think this would require some changes to Clang to allow C++ attributes to be used in this position:
>> >
>> > int [[clang::annotate]] foo;
>>
>> Clang already supports type attributes in that position today. e.g.,
>> https://godbolt.org/z/rTxoWWfdb
>
>
> Quoting the godbolt example to save having to follow the link:
>
> int * [[clang::address_space(126)]] ip = nullptr;
>
> This is different from my example above though because the attribute comes after the `*`, which makes it part of the declarator or, more specifically, the ptr-operator:
>
> http://eel.is/c++draft/dcl.dcl#nt:ptr-operator

Agreed.

> In contrast, with the example `int [[clang::annotate]] foo` that I was referring to, the attribute is part of the decl-specifier-seq (specifically, it's an attribute on the defining-type-specifier, as you also discuss below).

Agreed; it's a bug that Clang currently rejects this. That is a valid
location for a type attribute to do, and we have type attributes. I
think nobody has run into this because our type attributes haven't
applied (as often) to this particular type location. They've been
pointer attributes like address_space and _Nullable or function type
attributes like calling conventions, etc.

>> The position Clang doesn't currently support is on decl specifiers.
>> e.g., https://godbolt.org/z/rG1aP7Tz5
>
>
> Agreed.
>
>> > See also discussion below.
>> >
>> >> > As a side note, IIUC, Clang currently more generally lumps GNU attributes that occur in these two positions together, right? I'm looking at Parser::ParseSimpleDeclaration(), specifically this line:
>> >> >
>> >> > DS.takeAttributesFrom(Attrs);
>> >> >
>> >> > For C++ attributes, I believe this would not be correct, but that's not (currently) a concern because AFAICT Clang currently doesn't allow C++ attributes to occur after a decl-specifier-seq at all (because, I presume, Clang doesn't yet support any C++ attributes that can occur in this position).
>> >>
>> >> We currently don't support any attributes that appertain to the
>> >> declaration specifiers (in any spelling mode).
>> >
>> >
>> > But one of the enumerators in TypeAttrLocation is this:
>> >
>> >   /// The attribute is in the decl-specifier-seq.
>> >   TAL_DeclSpec,
>> >
>> > And it looks as if it is used?
>>
>> Ah, yeah, this is confusing stuff. decl-specifier-seq can have
>> attributes (http://eel.is/c++draft/dcl.spec#nt:decl-specifier-seq) and
>> TAL_DeclSpec tracks that. However, we do not currently support
>> attributes on *all kinds of decl specifiers*. e.g., [[supported #1]]
>> static [[not supported #2]] int [[supported #3]] i [[supported #4]];
>
>
> Just to make sure we're on the same page: Not all of these attributes are part of the decl-specfier-seq -- right?

Correct.

> #1 is the attribute-specifier-seq in a simple-declaration: http://eel.is/c++draft/dcl.dcl#nt:simple-declaration

Correct.

> #4 is the attribute-specifier-seq in a noptr-declarator: http://eel.is/c++draft/dcl.dcl#nt:noptr-declarator

Correct.

>> We support the ones that are in the declaration position (#1), the
>> type position (#2),
>
>
> I think you mean #3?

Yup, sorry for that confusion! Also I meant to say we *intend* to
support #3 but we don't currently because of bugs. We support #3 more
generally in that we support an attribute after a type.

>> and the identifier position (#4) but do not
>> support ones on storage class specifiers, function specifiers, or
>> other kinds of decl specifiers other than a defining-type-specifier.
>> e.g., https://godbolt.org/z/8sq5Wo8sT
>
>
> I think [[]] is treated differently from known attribute (and unknown attributes may be treated differently yet again?). Anyway, I think we get into this more deeply below anyway.

[[]] is handled in two stages. There's the parsing stage (where can
[[]] appear *at all*) and the semantic stage (does the attribute
within the [[]] appertain to the entity it was specified on, is it
supported on this target, etc). Parsing will do special work for known
attributes to parse the expected arguments, etc; for unknown
attributes, parsing eats balanced tokens until the end of the
attribute argument list. The semantic stage is what diagnoses unknown
or ignored attributes.

>> >> GNU attributes will
>> >> "slide" to the type or the declaration based on the name of the
>> >> attribute when doing semantic processing for them (see the
>> >> moveAttrFromListToList() calls in SemaType.cpp for some examples of
>> >> this).
>> >
>> >
>> > Thanks, that's an interesting pointer.
>> >
>> > I wondering now though: What do attributes attach to when the "sliding" logic doesn't apply to them (because it doesn't know them)?
>>
>> Heh, now you're seeing why GNU attributes were replaced by what was
>> standardized in C and C++. The answer is: we don't know what to do and
>> this can sometimes even impact our ability to parse properly (for
>> example, if it's a type attribute in C, it may be the difference
>> between ANSI "implicit int" being allowed or not). However, in Clang,
>> we do not retain unknown attributes in the AST, so we parse as best we
>> can and then drop what we parsed onto the floor if the attribute is
>> unknown.
>
>
> Hm. Got it.
>
> This does seem to make a pretty strong argument for what you're suggesting, namely that we shouldn't support the GNU syntax of the `annotate` attribute for annotations on types and instead should support only the C++ syntax (where, thankfully, the standard makes it pretty clear what the attribute appertains to).
>
>> > Maybe I should clarify my use case (which will probably benefit the whole discusssion). What I'm looking to be able to do is to attach attributes to different levels of a potentially multi-level pointer. For example:
>> >
>> > int __attribute__((annotate_type("A"))) * __attribute__((annotate_type("B"))) * __attribute__((annotate_type("C"))) pp;
>> >
>> > The "A" annotation is what I mean by being able to attach an attribute to the decl specifiers (maybe my terminology isn't accurate here?).
>> >
>> > One use case for this is being able to specify which of the levels a nullability annotation refers to and not just have it always implicitly refer to the outermost pointer, as I believe the existing nullability annotations do. (See the Clang fork here for an example that does something very similar: https://github.com/sampsyo/quala)
>> >
>> > Staying with the nullability example, it's important to be able to attach an annotation to the base type itself -- imagine the `int` in the example above was instead a `unique_ptr`.
>> >
>> > This does seem to be possible to some extent with Clang today because if I feed the example above to the code in my draft change, I get `AttributedTypeLoc`s for the types `int`, `int *`, and `int **`, with annotations "A", "B", and "C" respectively.
>>
>> It is possible today, but I note that we might have a bug with the
>> [[]] implementation: https://godbolt.org/z/vr81KzG33 -- I'd have to
>> dig around more to see what's going on there.
>
>
> Yes, I think this example makes the problem pretty clear, namely that C++ attributes can't be applied to the defining-type-specifier today.

Yup! They are allowed by the standard, but Clang has an implementation bug here.

> There's an additional wrinkle here in that, IIUC, the `address_space` attribute only makes sense to apply to a ptr-declarator, not a defining-type-specifier. I think the code that is causing the attribute to be rejected here isn't specific to `address_space` though -- I think it's just the general code we discuss below.

Agreed.

>> >> > This also means that if we want to use the proposed `annotate_type` attribute in this position, we have to use the GNU spelling.
>> >>
>> >> I don't think that's correct. We certainly support type attributes
>> >> using a C++ spelling already.
>> >
>> > I think this only works though if the attribute is applied to a declarator, but not to the decl specifiers? Here's an excerpt from the test in my draft change (https://reviews.llvm.org/D111548):
>> >
>> >   int __attribute__((annotate_type("bar"))) w;
>> >   int [[clang::annotate_type("bar")]] w2; // expected-error {{'annotate_type' attribute cannot be applied to types}}
>> >
>> >   int *__attribute__((annotate_type("bar"))) x;
>> >   int *[[clang::annotate_type("bar")]] x2;
>> >
>> > The error seems to be coming from the following chunk of code in Parser::ParseDeclarationSpecifiers(), which is specific to C++ attributes:
>> >
>> >         // Reject C++11 attributes that appertain to decl specifiers as
>> >         // we don't support any C++11 attributes that appertain to decl
>> >         // specifiers. This also conforms to what g++ 4.8 is doing.
>> >         ProhibitCXX11Attributes(attrs, diag::err_attribute_not_type_attr);
>>
>> Oh hey, this is the possible bug I just noted above. :-D I think we
>> should not be rejecting if the decl specifier is a defining type
>> specifier because that has utility,
>
>
> Agree -- I'd like to add support for this in Clang.

Fantastic!

> I suspect the reason this isn't supported today is because none of the existing type attributes need to be applied to the declaring-type-specifier. As noted above, `address_space` only really makes sense on ptr-declarators. And without having checked, I suspect the other attributes also refer to properties of pointers (which is where I think most of the practical reasons for attributing types comes from).

Yup, agreed.

> Having said that, it would be great if the nullability attributes could, for example, be applied to a unique_ptr, but it seems that this isn't the case (https://godbolt.org/z/qEn68xMz9).
>
>>
>> but I'd have to go stare at the
>> standard a bit more to be sure. Declarator parsing is not the easiest
>> thing to reason about.
>
>
> Let me know what you conclude.
>
> From what I've seen in the standard (see links that you and I posted above), I think it's pretty clear where this is supposed to slot into the grammar. But I assume what you're referring to is more the problem of how to make this parse unambiguously in Clang?

Correct. I think we're both convinced that the standard allows
specifying a type attribute in this position. I've not looked into how
hard it is to correct that in our implementation -- declarator parsing
is super dense code.

>> >> > This is an unfortunate niggle, but teaching Clang to correctly process C++ attributes in this position seems like a non-trivial undertaking because it would require us to treat GNU spellings and C++ spellings differently in Parser::ParseSimpleDeclaration(), which seems like it would be a bit of a headache.
>> >>
>> >> I don't think we'll have to go down that route, hopefully.
>> >>
>> >> >> What kind of type semantic impacts does this attribute have on the
>> >> >> type identity for things like overloading, name mangling, type
>> >> >> conversion, etc?
>> >> >
>> >> > Good point. The intent is that the attribute should have no impact. Anything else would be pretty problematic from a portability point of view because other compilers will simply ignore the attribute -- so if it had an effect on the type system in Clang, that could cause other compilers to generate different code.
>> >>
>> >> Vendor-specific attributes are inherently not portable or safe to
>> >> ignore.
>> >
>> >
>> > But for C++ attributes at least, the standard says that compilers should do exactly this (i.e. ignore unknown attributes)?
>>
>> Same for C. The standard says that vendor-specific attributes are
>> fully implementation-defined (so they're permitted to have whatever
>> semantics the implementation wants, including necessary side effects).
>> The standard also says that if the implementation doesn't know about
>> an attribute, the implementation should ignore the attribute. The
>> combination of these rules is what makes vendor-specific attributes
>> inherently nonportable or safe to ignore.
>>
>> As an example: https://godbolt.org/z/vsv3Wo6En
>
>
> Thanks, makes sense!
>
>> >> Generally speaking, if a type attribute has no type semantics,
>> >> it probably shouldn't be a type attribute. However, because this
>> >> attribute doesn't really have any semantics beyond "keep this
>> >> information associated with the type", it's a bit more complex.
>> >> Consider:
>> >>
>> >> void func(int i);
>> >> void func(int [[clang::annotate("hoo boy")]] i);
>> >>
>> >> I can see an argument that this is not a valid overload set because
>> >> both functions accept an integer.
>> >
>> >
>> > I think nullability attributes are a good analogy here (and essentially "prior art"). Here's an example:
>> >
>> > void func(int* i) {}
>> > void func(int* _Nonnull i) {}
>> >
>> > (https://godbolt.org/z/sP6za4WEE)
>> >
>> > Clang doesn't consider these to be overloads and instead complains about a redefinition.
>>
>> I think modeling on the nullability attributes may not be a bad idea,
>> but note that those are surfaced to the user as keywords and not
>> attributes at all. I believe that was done specifically because the
>> nullability attributes had type semantics, but I could be remembering
>> incorrectly.
>
>
> Do you think you can dig out this discussion again?

Sure!

https://lists.llvm.org/pipermail/cfe-dev/2015-March/041779.html

> I don't think it's really important though -- for the [[clang::annotate]] case (assuming that's what we'll go with in preference to `annotate_type`), we certainly want to specify that the annotation does not create a new type.

It's a bit weird to have a type attribute that has no type semantics,
but I think that design makes sense here for what you are trying to
accomplish. I think the best solution here is for Clang to have a
pluggable type system (https://bracha.org/pluggableTypesPosition.pdf),
but that's a huge, experimental request that's orthogonal to your RFC.

>> > I can certainly see how for different use cases you _would_ want an attribute to create a different type -- but I would say those would then require a second, separate attribute.
>> >
>> >>
>> >> However:
>> >>
>> >> void func(int i) [[clang::annotate("ABI=v1")]];
>> >> void func(int i) [[clang::annotate("ABI=v2")]];
>> >>
>> >> I can see an argument that this is a valid overload set because
>> >> something else (the linker, whatever) is using that annotated
>> >> information to provide additional semantics beyond what the compiler
>> >> cares about.
>> >
>> >
>> > As a data point that my be useful, I looked at what Clang does when I move those attributes to a position where they are declaration attributes (so that I can run the current Clang on them), i.e.:
>> >
>> >  [[clang::annotate("ABI=v1")]] void func(int i) {}
>> >  [[clang::annotate("ABI=v2")]] void func(int i) {}
>> >
>> > (https://godbolt.org/z/fo5vrso6d)
>> >
>> > Again, Clang doesn't consider these to be overloads and instead complains about a redefinition.
>>
>> FWIW, that's because declaration attributes do not impact the type of
>> the function. We do have a declaration attribute that says "this
>> declaration is allowed to be overloaded" in C, but that's subtly a
>> different thing.
>>
>> > So it seems that the existing semantics of the `annotate` attribute are similar to what I'm proposing for the `annotate_type` attribute.
>> >
>> >> >> Also, one problem here is -- [[clang::annotate]] isn't just used for
>> >> >> the static analyzer, it's also something that can be used by backend
>> >> >> tools (for example, when using attribute plugins).
>> >> >
>> >> >
>> >> > It's good that you bring this up because I'm actually also thinking about extending ParsedAttrInfo to handle type attributes (with a new handleTypeAttribute() function as an analog to handleDeclAttribute()). The proposed annotate_type attribute would then also be a natural candidate for an attribute that the new handleTypeAttribute() could add to types.
>> >>
>> >> Oh, neat! Unifying type/stmt/decl attributes more is definitely a great goal.
>> >>
>> >> > Is this the concern that you had here or were you thinking about something different?
>> >>
>> >> Not really, it was more along the lines of lowering to LLVM IR (below)
>> >> so that the backend can also take advantage of this.
>> >>
>> >> >> Do we need to
>> >> >> consider what information should be lowered to LLVM IR when this new
>> >> >> attribute appears on a type?
>> >> >>
>> >> >> Thanks!
>> >> >
>> >> > My intent was that this should have no effect on code generation.
>> >> >
>> >> > Lowering annotations on declarations to LLVM IR has some obvious use cases, but for annotations on types it's less obvious how it would be useful to lower these to IR (and also less obvious to me how one would represent these). (Did you have anything specific in mind here?)
>> >>
>> >> LLVM currently has the ability to describe some types (e.g.,
>> >> `%struct.X = type { i32 }`), so I was envisioning attaching this
>> >> attribute to the type (either as an LLVM attribute or metadata, I have
>> >> no idea what the distinction really is between them). LLVM currently
>> >> supports plugins as does Clang, and Clang's support for attributes in
>> >> plugins still strongly encourages users to use existing attributes to
>> >> surface functionality, so I can imagine users wanting to use the
>> >> annotate type attribute in the same way they already use the annotate
>> >> declaration attribute for this sort of collusion. However, I don't
>> >> think the codegen needs to happen up front -- mostly trying to
>> >> understand the shape of the design still.
>> >
>> >
>> > Makes sense.
>> >
>> > Sorry that this has become such a long email, but I hope I've been able to give you a clearer picture of what I'm trying to achieve.
>>
>> No worries, I appreciate the detailed discussion of the design!
>>
>> I think we should double-check that I'm correct about the bug with
>> Clang accepting:
>>
>> static int [[]] i;
>>
>> but rejecting:
>>
>> int [[]] i;
>
>
> I don't think that's the distinction:
>
> https://godbolt.org/z/MWcojK8n4
>
> (Noting that it accepts both of these.)
>
> Rather, this seems to be a case of [[]] being treated specially (probably earlier in the parser) and therefore being accepted in this position, while named, known attributes are not accepted here.

<nods> that sounds plausible too.

>> If we're agreed that this is a bug, then it seems like once we fix
>> that bug, you could support [[clang::annotate]] in type positions
>> (with no type semantics) as well as declarations, but only if you were
>> willing to define __attribute__((annotate)) as applying to
>> declarations when there's an ambiguity between type vs decl. Would
>> that be a palatable approach for you?
>
>
> Yes, I think that would definitely work for me, and there would be an attraction in not having to introduce an additional `annotate_type` attribute that otherwise works mostly the same as `annotate`.
>
> I think making this work may not be as simple as lifting the restriction in Parser::ParseDeclarationSpecifiers() that we discussed above. I think there's another issue that I touched on previously (https://lists.llvm.org/pipermail/cfe-dev/2021-October/069094.html), namely that Clang currently lumps attributes for the declaration and the decl-specifier-seq together.
>
> This happens in Parser::ParseSimpleDeclaration(), specifically this line
> <https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDecl.cpp#L1818>
> :
>
> DS.takeAttributesFrom(Attrs);
>
> To quote the conclusion I reached at the time:
>
>> For C++ attributes, I believe this would not be correct, but that's not
>> (currently) a concern because AFAICT Clang currently doesn't allow C++
>> attributes to occur after a decl-specifier-seq at all (because, I presume,
>> Clang doesn't yet support any C++ attributes that can occur in this
>> position). This also means that if we want to use the proposed
>> `annotate_type` attribute in this position, we have to use the GNU
>> spelling. This is an unfortunate niggle, but teaching Clang to correctly
>> process C++ attributes in this position seems like a non-trivial
>> undertaking because it would require us to treat GNU spellings and C++
>> spellings differently in Parser::ParseSimpleDeclaration(), which seems like
>> it would be a bit of a headache.
>
>
> And I believe this is still true?
>
> Do you have any other ideas on what we could do about this?

I believe all of that is still true, and my only ideas boil down to
digging in and trying to solve the headaches, unfortunately. Hower, I
think that work will pay dividends in the long run beyond just
enabling [[clang::annotate]] on types; I think we'll see additional
type attributes in the future that will benefit from these fixes as
well.

~Aaron

>
> Thanks,
>
> Martin
>
>> ~Aaron
>>
>> >
>> > Cheers,
>> >
>> > Martin
>> >
>> >>
>> >>
>> >> ~Aaron
>> >>
>> >> > This could actually make another case for why we have two different attributes, because that would make the distinction clearer that we're doing code generation for one but not the other.
>> >> >
>> >> > Thanks,
>> >> >
>> >> > Martin
>> >> >
>> >> >>
>> >> >> ~Aaron
>> >> >>
>> >> >> >
>> >> >> > Thanks,
>> >> >> >
>> >> >> > Martin
>> >> >> >
>> >> >> > --
>> >> >> >
>> >> >> > Martin Brænne
>> >> >> >
>> >> >> > Software Engineer
>> >> >> >
>> >> >> > mboehme at google.com
>> >> >> >
>> >> >> >
>> >> >> > Google Germany GmbH
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>> >> >> >
>> >> >> >
>> >> >> >
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>> >> >> >
>> >> >> > _______________________________________________
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>> >> >
>> >> >
>> >> >
>> >> > --
>> >> >
>> >> > Martin Brænne
>> >> >
>> >> > Software Engineer
>> >> >
>> >> > mboehme at google.com
>> >> >
>> >> >
>> >> > Google Germany GmbH
>> >> > Erika-Mann-Straße 33
>> >> > 80363 München
>> >> >
>> >> > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
>> >> >
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>> >> >
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>> >> >
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>> >> >
>> >> >
>> >> >
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>> >
>> >
>> >
>> > --
>> >
>> > Martin Brænne
>> >
>> > Software Engineer
>> >
>> > mboehme at google.com
>> >
>> >
>> > Google Germany GmbH
>> > Erika-Mann-Straße 33
>> > 80363 München
>> >
>> > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
>> >
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>> >
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>> >
>> >
>> >
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>
>
>
> --
>
> Martin Brænne
>
> Software Engineer
>
> mboehme at google.com
>
>
> Google Germany GmbH
> Erika-Mann-Straße 33
> 80363 München
>
> Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
>
> Registergericht und -nummer: Hamburg, HRB 86891
>
> Sitz der Gesellschaft: Hamburg
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