[cfe-dev] ASTMatchers.h and its internal linkage variable definitions

[Manuel Klimek via cfe-dev]

On Wed, Nov 15, 2017 at 9:18 AM Manuel Klimek <klimek at google.com> wrote:

> On Tue, Nov 14, 2017 at 9:26 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>> On Tue, Nov 14, 2017 at 1:04 AM Manuel Klimek <klimek at google.com> wrote:
>>
>>> On Tue, Nov 14, 2017, 12:53 AM David Blaikie <dblaikie at gmail.com> wrote:
>>>
>>>> Ping?
>>>>
>>>> Richard & I decided that supporting this particular case of
>>>> namespace-scope static variables in modular headers using modular codegen.
>>>>
>>>
>>> Part of the sentence is missing?
>>>
>>
>> Huh, right you are...
>>
>> What I was going to say (but lost track through all the
>> caveats/criteria): Richard & I decided that <stuff> is invalid (it's a
>> legit ODR violation) & doesn't need to be supported.
>>
>> (though there may still be issues with the non-ODR violation - such as
>> the use of internal linkage namespace-scope reference variables... :/ )
>>
>>
>>>
>>> (specifically the case where the namespace-scope static variable is
>>>> referenced from an inline function - thus making a true ODR violation (in
>>>> classic C++ this'd be an ODR violation if more than one TU used that header
>>>> - so it seems OK to not support this in modular codegen))
>>>>
>>>
>>> Calling is not an odr use, right? And I thought referencing alone
>>> doesn't lead to an odr violation?
>>>
>>
>> This is what the ASTMatcher code boils down to:
>>
>> struct foo {
>>   foo();
>>
>
Ah, here's the detail I missed: it actually boils down to
struct foo {
  ExpressionTemplateMatcherType operator()(...);
}
The only use of foo is by calling the operator on it.


> };
>> const foo f; // this has internal linkage
>> const foo &x() { return f; }
>>
>
> Where do we return matcher reference types?
> This should just return foo by value?
>
> if 'x' appears in more than one TU, it's an ODR violation because 'f'
>> resolves to different entities in each TU so there are multiple distinct
>> definitions of 'x'.
>>
>> In this case, for example, 'f' is 'callExpr' (ASTMatchers.h:1138) and 'x'
>> is the AST_POLYMORPHIC_MATCHER_P2 on ASTMatchers.h:3491 that uses
>> 'callExpr' in an inline function in the header.
>>
>
> That just calls callExpr() though, and never ODR uses it?
>
>
>> If the namespace-scoped static variable is unreferenced from inline
>>>> functions, that's fine - such as the iostreams global initializer.
>>>>
>>>> I'll send a CR to move these definitions out of line.
>>>>
>>>
>>> I'm happy with that, too. I assume once c++17 hits the shelves we'll be
>>> able to replace it anyway?
>>>
>>
>> With inline variables? Yeah, I think so. Though I should test/figure out
>> whether/how modular codegen will work with inline variables...
>>
>>
>>>
>>> On Thu, Nov 9, 2017 at 12:30 AM Manuel Klimek <klimek at google.com> wrote:
>>>>
>>>>> On Thu, Nov 9, 2017 at 12:43 AM David Blaikie <dblaikie at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Hey Manuel,
>>>>>>
>>>>>> In an effort to apply Modular Code Generation to Clang internally, I
>>>>>> came across the fact that this header contains a bunch of internal linkage
>>>>>> variables. (now, maybe modular code generation should be compatible with
>>>>>> internal linkage namespace-scoped variables... I'm looking into that)
>>>>>>
>>>>>> These variables technically create ODR violations any time they're
>>>>>> ODR-used in another inline function used in more than one translation unit.
>>>>>>
>>>>>> Is there a good way we could fix this header so it doesn't create ODR
>>>>>> violations (hopefully by not containing internal linkage entities)? I mean
>>>>>> one simple solution is to declare all these things in a header and define
>>>>>> them out of line. Since they have no actual state, that's probably pretty
>>>>>> low-cost except for a lot of duplication. I'm happy to use a .def/.inc file
>>>>>> to help remove a bunch of that duplication, if useful.
>>>>>>
>>>>>
>>>>> I remember having that argument a long time ago with somebody (perhaps
>>>>> Sam (cc'ed)?)
>>>>>
>>>>>>
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