[patch libstdc++] Optimize synchronization in std::future if futexes are available.

[Doug Gilmore]

On 01/18/2015 05:19 AM, Jonathan Wakely wrote:
> On 17/01/15 19:51 -0700, Sandra Loosemore wrote:
>> On 01/17/2015 03:58 PM, Jonathan Wakely wrote:
>>>
>>> My fault, this additional chunk is needed alongside the patch I sent
>>> earlier:
>>>
>>> --- a/libstdc++-v3/include/bits/atomic_futex.h
>>> +++ b/libstdc++-v3/include/bits/atomic_futex.h
>>> @@ -35,7 +35,7 @@
>>> #include <bits/c++config.h>
>>> #include <atomic>
>>> #include <chrono>
>>> -#if !defined(_GLIBCXX_HAVE_LINUX_FUTEX)
>>> +#if ! (defined(_GLIBCXX_HAVE_LINUX_FUTEX) && ATOMIC_INT_LOCK_FREE > 1)
>>> #include <mutex>
>>> #include <condition_variable>
>>> #endif
>>>
>>> What I sent earlier causes your target to use std::mutex and
>>> std::condition_variable, but without the bit above the headers aren't
>>> included.
>>
>> Still no joy:
>> /scratch/sandra/arm-fsf2/src/gcc-mainline/libstdc++-v3/src/c++11/futex.cc:45:3: error: '__atomic_futex_unsigned_base' has not been declared
>>   __atomic_futex_unsigned_base::_M_futex_wait_until(unsigned *__addr,
>>   ^
>> /scratch/sandra/arm-fsf2/src/gcc-mainline/libstdc++-v3/src/c++11/futex.cc:88:3: error: '__atomic_futex_unsigned_base' has not been declared
>>   __atomic_futex_unsigned_base::_M_futex_notify_all(unsigned* __addr)
>>   ^
> 
> futex.cc needs the same change ...
I am still noticing a problem building a native X86_64 ToT compiler on
Ubuntu 12.04.5 LTS.

Attached is a patch where the CPP guards in atomic_futex.h are
reflected in futex.cc, which fixes my build problem.

OK to commit?

Thanks,

Doug

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