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

[Richard Smith]

On Thu, Jan 17, 2013 at 2:35 PM, Michael Wong <michaelw at ca.ibm.com> wrote:

> 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 ...
>

For option 1, we would only need to disallow static local variables from
having types involving local lambdas with captures, right? All the other
problems I can think of would be handled by putting the lambda's symbols in
a COMDAT with the containing function.


> >
> > >
> > > I apologize if this issue has been dealt with.
> > >
> > > Regards, Michael
> > >
> > > OpenMP CEO:
> > > http://openmp.org/wp/about-openmp/
> > > My Blogs:
> > > http://ibm.co/pCvPHR
> > > C++11 status:
> > > http://tinyurl.com/43y8xgf
> > > Boost test results
> > > http://www.ibm.com/support/docview.wss?
> > rs=2239&context=SSJT9L&uid=swg27006911
> > >
> > > C/C++ Compilers Support/Feature Request Page
> > > http://www.ibm.com/software/awdtools/ccompilers/support/
> > > http://www.ibm.com/support/docview.wss?uid=swg27005811
> > > STM:
> > > https://sites.google.com/site/tmforcplusplus/
> > >
> > > Director & Vice President ISOCPP.org
> > > Canada and IBM C++Standard HoD
> > > Chair of WG21 SG5 Transactional Memory
> > >
> > > XL C++ Compiler kernel Development
> > > IBM Canada Ltd., C2/KD2/8200/MKM
> > > 8200 Warden Avenue
> > > Markham, Ontario  L6G 1C7
> > > W:905-413-3283 F:905-413-4839
> >
> >
> > --
> > Lawrence Crowl
> >
>
>
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