how the right thread should be reordered in order for the assertion to fail?(slide 30)

[Shynar88]

From slide 30:

"Assertion failure due to reordering in the left thread or in the right thread"
In the case of the left thread reordering, I can see why assertion fails(1)FLAG.store(1); 2)right thread 3)DATA = 42;). However, I cannot imagine how the right thread should be reordered in order for the assertion to fail?

Thank you!

[jeehoonkang]

Reading from DATA is reordered before reading from FLAG. It's actually observable in some architectures which perform branch speculation.

[Shynar88]

Does this mean that assert(DATA) == 42 can be executed irrespectively of the condition of FLAG.load()? Doesn't this kind of reordering break the logic of the program, and thus shouldn't be allowed by the compiler?

[jeehoonkang]

No, the loading from DATA alone can be reordered. Assertion on the loaded value, as you explained, cannot be reordered before the loading from FLAG.