> Initially, x = y = v = 0.
> v is a volatile variable.
>
> Thread 1:
> r1 = x
> v = 0
> r2 = v
> y = 1
>
> Thread 2:
> r3 = y
> v = 0
> r4 = v
> x = 1
>
> Is the behavior r1 == r3 == 1 possible?
I think r1 == r3 == 1 being impossible is more intuitive.
I also believe volatile variables preventing compile time code motion is
a good idea, for the same reason. However, I also like to emit volatile
loads and writes as ld.acq and st.rel (half fence) at run time.
I know our C compiler emits volatile load & stores with ld.acq and st.rel.
Using the full fence can be orders of magnitude slower than the half fence.
This is important if you want to use volatiles to implement a spin lock
in Java. (assuming compare & exchange is available through some mechanism)
Thomas Wang
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