[boost] [atomic] Help understanding consume order

Subject: [boost] [atomic] Help understanding consume order
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-06-01 06:58:03

Hi,

I'm reviewing (again) Boost.Atomic code and struggling to understand the 
consume order and in particular what should it mean on architectures other 
than DEC Alpha.

I read the explanation here:

http://en.cppreference.com/w/cpp/atomic/memory_order

but the point eludes me. Take ARM for example and the explanation in the 
"Release-Consume ordering" section. The producer thread allocates the string 
and stores the pointer with a release operation, so that the pointer, the 
string contents and the 'data' integer are visible to other threads.

Now the consumer thread reads the pointer with a consume operation. According 
to the explanation in the article, on ARM the consume operation need not issue 
any specific fences to be able to use the pointer and the string body. In that 
case, the consume operation becomes equivalent to relaxed (plus prohibiting 
compiler optimizations). But is there a guarantee that the string body will be 
visible to the consumer? Shouldn't the consume operation be promoted to 
acquire instead?

I guess, that's the ultimate question: how should consume ordering be handled 
on conventional architectures?