[cfe-dev] Duplicate attributes

[Richard Smith]

On Mon, Oct 14, 2013 at 5:11 PM, Aaron Ballman <aaron at aaronballman.com>wrote:

> To resolve this in a more generic way, I am thinking of table
> generating some code on the parsed attributes that allows us to ask
> "given a list of Attr instances, are there any conflicts you can
> diagnose with this parsed attribute?"  Eg)
>
> bool AttributeList::conflictsWith(Sema &S, attr_iterator I,
> attr_iterator E) const;
>
> In this way, when SemaDeclAttr.cpp does common checking for a decl (or
> statement, type, etc), we can ask the parsed attribute to diagnose any
> problems it can, generically.
>
> There are two situations I am thinking of initially:
>
> 1) Duplicate attributes.  This applies pretty generally, and when
> opt-out is required, we can use table gen to specify the opt-out
> 2) Mutually exclusive attributes.  This doesn't apply as generally,
> but is something we already have hand-written support for (cold vs hot
> attributes), and is missing elsewhere (calling conventions immediately
> spring to mind).
>
> Before I do more concrete work in this direction, do you see any
> particular problems with this approach?  Or is there a better way you
> would prefer to see employed?
>

Two things:
1) What you're proposing sounds O(n^2). While that's unlikely to be a
problem in practice, it'd be nice to avoid.
2) The C++ restriction is only for attributes *in the same
attribute-specifier*. So:

[[noreturn, noreturn]] void f(); // ill-formed
[[noreturn]] [[noreturn]] void f(); // ok

IIRC, we don't preserve enough information into Sema to be able to diagnose
this there.


> Thanks!
>
> ~Aaron
>
> On Sun, Oct 6, 2013 at 9:01 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >
> > On 5 Oct 2013 16:15, "Aaron Ballman" <aaron at aaronballman.com> wrote:
> >>
> >> I noticed that we have a diagnostic "err_repeat_attribute", but
> >> currently only vecreturn is making use of it.
> >>
> >> This seems like it is something we could generalize across attributes
> >> relatively easily, but there appears to be no documentation (even in
> >> the C++ standard) as to whether duplicate attributes are acceptable or
> >> not for any given attribute.
> >
> > For a standard c++11 attribute, there is usually (always?) a restriction
> > that the attribute cannot be repeated in the same attribute-specifier.
> >
> >> As best I can tell, most attributes should diagnose a duplicate on the
> >> same subject.  The exceptions, as best I can gather, are:
> >>
> >> alloc_size
> >> format_arg
> >> nonnull
> >> ownership_holds
> >> ownership_returns
> >> ownership_takes
> >> argument_with_type_tag
> >> pointer_with_type_tag
> >>
> >> Basically, anything which allows you to specify an argument index as
> >> an attribute parameter seems like it could have a duplicate attribute.
> >>
> >> Would it make sense to add a generalized duplicate attribute test in
> >> SemaDeclAttr that emits an error when a duplicate is found, except for
> >> the above cases?
> >>
> >> ~Aaron
>
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