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Re: basic_string atomicity


On Thu, 5 Jan 2012, Jonathan Wakely wrote:

On 5 January 2012 21:28, Jonathan Wakely wrote:

What consistency model does libstdc++ require? ?__sync_fetch_and_xxx has become more heavy-weight and even the earlier memory model probably was unnecessarily expensive for its usage in libstdc++ based on our analysis for POWER.

std::string is required by C++11 to be not reference-counted, so changing the details of the ref-counted implementation isn't a high priority if we're going to replace it anyway (although we will retain a ref-counted COW string as __gnu_cxx::__rc_string.)

I'm not sure what the memory ordering requirements are for
std::string, I think decrementing the count to zero needs to
synchronise with deallocating the memory, but I'm not sure about
increments.

I think decrementing the refcount in _M_dispose() should be acq_rel, but as you suggested the increment in _M_ref_copy only needs to be acquire. I think. Check that with an expert though, I'm not confident!

Last time I tried asking experts about something related, it gave: https://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/cf19ee1e9d4086b9

Not sure it helps (and the exact requirements may differ).

--
Marc Glisse


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