carrying a dependency, then the compiler can break that dependency
by substituting a constant of that value.
+ Conversely, LKMM sometimes doesn't recognize that a particular
+ optimization is not allowed, and as a result, thinks that a
+ dependency is not present (because the optimization would break it).
+ The memory model misses some pretty obvious control dependencies
+ because of this limitation. A simple example is:
+
+ r1 = READ_ONCE(x);
+ if (r1 == 0)
+ smp_mb();
+ WRITE_ONCE(y, 1);
+
+ There is a control dependency from the READ_ONCE to the WRITE_ONCE,
+ even when r1 is nonzero, but LKMM doesn't realize this and thinks
+ that the write may execute before the read if r1 != 0. (Yes, that
+ doesn't make sense if you think about it, but the memory model's
+ intelligence is limited.)
+
2. Multiple access sizes for a single variable are not supported,
and neither are misaligned or partially overlapping accesses.