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

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Tue Nov 14 12:25:48 PST 2017


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();
};
const foo f; // this has internal linkage
const foo &x() { return f; }

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.


>
> 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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