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

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Wed Nov 15 08:53:58 PST 2017


On Wed, Nov 15, 2017 at 7:48 AM Manuel Klimek <klimek at google.com> wrote:

> 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.
>

*nod*

Committed r318304 (and tested that this is the final change needed to
successfully link a modular code generation enabled build of the clang
binary - which is awesome :) ). Happy to iterate on exactly how that's
fixed, refactor things into macros, etc. (I did introduce an alias template
to help simplify some things - but happy if a more (consistent with
existing code) macro solution would be preferred, etc)

Thanks!
- Dave


>
>
>>
>> - 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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