std::mem_order_consume

Introduction

That is for two reasons that std::memory_order_consume is the most legendary of the six memory models: first, std::memory_order_consume is extremely hard to understand, and second - which may change in the future - no compiler currently supports it. With C++17 the situation gets even worse. Here is the official wording: “The specification of release-consume ordering is being revised, and the use of memory_order_consume is temporarily discouraged.”

How can it be that a compiler that implements the C++11 standard doesn’t support the memory model std::memory_order_consume? The answer is that the compiler maps std::memory_order_consume to std::memory_order_acquire. This is acceptable because both are load or acquire operations; std::memory_order_consume requires weaker synchronization and ordering constraints than std::memory_order_acquire. Therefore, the release-acquire ordering is potentially slower than the release-consume ordering, but - and this is the key point - it’s well-defined.

To get an understanding of the release-consume ordering, it is a good idea to compare it with the release-acquire ordering. I speak in the following subsection explicitly about the release-acquire ordering (not about the acquire-release semantic) to emphasize the strong relationship of std::memory_order_consume and std::memory_order_acquire.

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Release-acquire Ordering

Let’s use the following program with two threads t1 and t2 as a starting point. t1 plays the role of the producer, t2 the role of the consumer. The atomic variable ptr helps to synchronize the producer and consumer.