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

[Manuel Klimek via cfe-dev]

On Wed, Nov 15, 2017 at 4:28 PM David Blaikie <dblaikie at gmail.com> wrote:

> On Wed, Nov 15, 2017 at 12:20 AM Manuel Klimek <klimek at google.com> wrote:
>
>> 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.
>>
>
> Right, yep yep (sorry I skipped over a few too many steps in my
> reduction/example). Calling a non-static member function is an ODR use,
> roughly equivalent to passing the value by reference to another function
> (much the same as returning by reference).
>

Even if the function never touches the reference? Bummer.


>
> - Dave
>
>
>>
>>
>>> };
>>>> 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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