08.12.2015 12:26, Dimitry Sibiryakov wrote:
> Hello, All.
>
> Currently to do subj, code like this is used:
>
>> while (true)
>> {
>> AtomicCounter::counter_type old = dbb_flags;
>> if ((old & (DBB_sweep_in_progress |
>> DBB_sweep_starting)) || (dbb_ast_flags & DBB_shutdown))
>> return false;
>>
>> if (dbb_flags.compareExchange(old, old |
>> DBB_sweep_starting))
>> break;
>> }
>
> Is there a reason not to use _InterlockedOr() intrinsic with MSVC and
> __sync_or_and_fetch() with GCC for such cases?
Here we need to know if flag was set by our thread or another one, therefore
we must use compare_exchange.
Note, with MSVC, when return value of _InterlockedOr() intrinsic is used by
the caller, compiler will
generate almost the same code as above (with a loop and compare_exchange).
Read, for example:
http://blogs.msdn.com/b/doronh/archive/2006/11/10/the-case-of-amazingly-morphing-intrinsic-function.aspx
With GCC, you suggest to use wrong function : __sync_or_and_fetch() will
return *new* value, while
we need an old value. Correct function is __sync_fetch_and_or(), see:
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html
But __sync_fetch_and_or() give us no guarantee because it perform two actions
which is not atomic together.
Regards,
Vlad
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