[cfe-dev] Adding a new attribute: no_extern_template

[Richard Smith via cfe-dev]

On Tue, 21 Aug 2018 at 14:44, Louis Dionne via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi,
>
> This message is related to solving the issue explained at [1], and
> indirectly, the usage of __always_inline__ in libc++ as explained at [2].
>
> I've been looking to add an attribute (tentatively called
> no_extern_template) which would prevent member functions in a template from
> getting available_externally linkage even when there is a matching extern
> template declaration. In other words, given the following code:
>
>     template <class T>
>     struct Foo { void func() { } };
>     extern template struct Foo<int>;
>
>     void use() {
>       Foo<int> f;
>       f.func();
>     }
>
> it is possible for Clang to emit a call to the extern function
> `Foo<int>::func()`, because the extern template declaration promises that
> it exists somewhere else in the program. Clang could also decide to inline
> the function and generate no extern call, but it does not _have_ to. What I
> want is an attribute that inhibits generating an external call, i.e. that
> makes sure `Foo<int>::func()` is either emitted in this TU despite the
> extern template declaration, or inlined (in which case a definition is
> obviously not needed). If the function is emitted, it should be emitted
> with proper linkage, in this case linkonce_odr (IIUC). The attribute would
> be applied to the member function that should be opted-out of the extern
> template declaration, like so:
>
>     template <class T>
>     struct Foo { __attribute__((no_extern_template)) void func() { } };
>     extern template struct Foo<int>;
>
> I'd like to have some feedback on this idea, specifically:
>
> - What entities does it make sense to apply this attribute to? Currently,
> I'm thinking only static and non-static member functions of templates, for
> lack of imagination and other use cases. Does it make sense to apply it to
> static data members? To the vtable and RTTI somehow?
>
> - What other attributes would be mutually exclusive with this one? Right
> now I can only think of `internal_linkage`, but is there anything else?
>
> Finally, I'd also welcome any guidance on how to best achieve this in
> Clang. So far, I've achieved something that "works" by forcing declarations
> with `GVA_AvailableExternally` linkage to have `GVA_DiscardableODR` linkage
> (in `adjustGVALinkageForAttributes`). I think this is probably the wrong
> approach but I’m a Clang beginner, so I’m looking for advice if somebody
> has some. I'll soon publish an early code review on Phabricator to ease the
> process of getting feedback on the code itself.
>

FWIW, I'd implement this by skipping members that are declared with the
attribute in this loop:
https://github.com/llvm-mirror/clang/blob/master/lib/Sema/SemaTemplateInstantiate.cpp#L2561


> Thanks,
> Louis
>
> [1]: http://lists.llvm.org/pipermail/cfe-dev/2018-July/058460.html
> [2]: http://lists.llvm.org/pipermail/cfe-dev/2018-July/058419.html
>
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