[cxx-abi-dev] lambda ABI inline function ODR compatibility issue

[Michael Wong]

Lawrence Crowl <crowl at googlers.com> wrote on 01/17/2013 04:50:52 PM:

> From:
>
> Lawrence Crowl <crowl at googlers.com>
>
> To:
>
> Michael Wong/Toronto/IBM at IBMCA
>
> Cc:
>
> cxx-abi-dev at codesourcery.com
>
> Date:
>
> 01/17/2013 04:50 PM
>
> Subject:
>
> Re: [cxx-abi-dev] lambda ABI inline function ODR compatibility issue
>
> On 1/16/13, Michael Wong <michaelw at ca.ibm.com> wrote:
> > Does the C++ Standard committee intend for the ODR to imply that
lambdas
> > need to have an ABI specified layout in order to deal with inline
> > functions.
>
> I believe that we thought it was not an issue.
>
> >
> > Consider the following with one object compiled with -DMAIN and another
> > without:
> > - in one case the layout needs to be compatible between different
> > implementations since the static local is shared between translation
units
> >
> > /data/a.o: In function `main':
> > a.cpp:(.text+0x18): undefined reference to `bar()'
> > collect2: error: ld returned 1 exit status
> >
> > - in the other case, the layout needs to be compatible between
different
> > implementations in order to satisfy the ODR requirement that the
program
> > behave as if there was only one definition of the inline function
> >
> > extern "C" int printf(const char *, ...);
> > extern long gz;
> >
> > inline void foo() {
> >    long x = 0, q = 0, &z = gz;
> >    static auto f = [=, &z]() mutable { q += ++x; gz = q + x; };
> >
> >    long a, b;
> >    auto ff = [=]{ sizeof(a /*not an odr-use*/), printf("%u\n", &b <
&a); };
> >    f();
> >    ff();
> > }
> >
> > void bar();
> >
> > #if ! MAIN
> > void bar() { foo(); }
> > #else
> > long gz;
> > int main() {
> >    foo();
> >    bar();
> >    foo();
> >    return gz;
> > }
> > #endif
>
> And this code demonstrates that it is an issue.
Agreed.

>
> Do you have a proposal?

Probably 2 thoughts:
1. Make a static in an inline a violation of the ODR rule in the C++ Std
(add in suitable wording covering all uses with external linkage)or
2. create an ABI binding in the C++ ABI such that all vendors follow the
same behavior in this case

I think I am OK with either solution.
There may be other solutions I have not entertained yet ...

>
> >
> > I apologize if this issue has been dealt with.
> >
> > Regards, Michael
> >
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> >
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>
>
> --
> Lawrence Crowl
>
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