Can non-atomic-load be reordered after atomic-acquire-load?

Question

As known in since C++11 there are 6 memory orders, and in documentation written about std::memory_order_acquire:

• http://en.cppreference.com/w/cpp/

Answer 1

I believe this is the correct way to reason about your example within the C++ standard:

1. X.load(std::memory_order_acquire) (let's call it "operation (A)") may synchronize with a certain release operation on X (operation (R)) - roughly, the operation that assigned the value to X that (A) is reading.

[atomics.order]/2 An atomic operation A that performs a release operation on an atomic object M synchronizes with an atomic operation B that performs an acquire operation on M and takes its value from any side effect in the release sequence headed by A.

2. This synchronizes-with relationship may help establish a happens-before relationship between some modification of L and the assignment local2 = L. If that modification of L happens-before (R), then, due to the fact that (R) synchronizes-with (A) and (A) is sequenced-before the read of L, that modification of L happens-before this read of L.

3. But (A) has no effect whatsoever on the assignment local1 = L. It neither causes data races involving this assignment, nor helps prevent them. If the program is race-free, then it must necessarily employ some other mechanism to ensure that modifications of L are synchronized with this read (and if it's not race-free, then it exhibits undefined behavior and the standard has nothing further to say about it).

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It is meaningless to talk about "instruction reordering" within the four corners of the C++ standard. One may talk about machine instructions generated by a particular compiler, or the way those instructions are executed by a particular CPU. But from the standard's standpoint, these are merely irrelevant implementation details, as long as that compiler and that CPU produce observable behavior consistent with one possible execution path of an abstract machine described by the standard (the As-If rule).