[libcxx] r274403 - Work around ABI break caused by C++17 inline variables.

[Eric Fiselier via cfe-commits]

The attribute is ignored unless it is placed on the in-class declaration of
the member. It has no effect when applied to the explicit definition.
I would rather keep the workaround internal than make the change in the
headers.

On Fri, Jul 1, 2016 at 5:34 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> On Fri, Jul 1, 2016 at 4:22 PM, Eric Fiselier via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>> Author: ericwf
>> Date: Fri Jul  1 18:22:25 2016
>> New Revision: 274403
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=274403&view=rev
>> Log:
>> Work around ABI break caused by C++17 inline variables.
>>
>> Modified:
>>     libcxx/trunk/src/chrono.cpp
>>
>> Modified: libcxx/trunk/src/chrono.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/chrono.cpp?rev=274403&r1=274402&r2=274403&view=diff
>>
>> ==============================================================================
>> --- libcxx/trunk/src/chrono.cpp (original)
>> +++ libcxx/trunk/src/chrono.cpp Fri Jul  1 18:22:25 2016
>> @@ -6,7 +6,7 @@
>>  // Source Licenses. See LICENSE.TXT for details.
>>  //
>>
>>  //===----------------------------------------------------------------------===//
>> -
>> +#define _LIBCPP_BUILDING_CHRONO
>>  #include "chrono"
>>  #include "cerrno"        // errno
>>  #include "system_error"  // __throw_system_error
>> @@ -32,6 +32,9 @@ namespace chrono
>>  // system_clock
>>
>>  const bool system_clock::is_steady;
>>
>
> Can you mark this as __attribute__((used)) instead of adding a separate
> use?
>
>
>> +// Make is_steady non-discardable in C++17
>> +// See PR28395 (https://llvm.org/bugs/show_bug.cgi?id=28395)
>> +static const bool& __is_steady_force_use1 __attribute__((used)) =
>> system_clock::is_steady;
>>
>>  system_clock::time_point
>>  system_clock::now() _NOEXCEPT
>> @@ -68,6 +71,10 @@ system_clock::from_time_t(time_t t) _NOE
>>  //  instead.
>>
>>  const bool steady_clock::is_steady;
>> +// Make is_steady non-discardable in C++17
>> +// See PR28395 (https://llvm.org/bugs/show_bug.cgi?id=28395)
>> +static const bool& __is_steady_force_use2 __attribute__((used)) =
>> steady_clock::is_steady;
>> +
>>
>>  #ifdef CLOCK_MONOTONIC
>>
>>
>>
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>
>
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