On Fri, Nov 06, 2020 at 01:04:13PM -0800, Paul E. McKenney wrote: > On Fri, Nov 06, 2020 at 03:40:08PM -0500, Alan Stern wrote: > > Is it really true that data dependencies are so easily destroyed? I > > would expect that a true "semantic" dependency (i.e., one where the > > value written really does vary according to the value read) would be > > rather hard to second guess. > > The usual optimizations apply, for but one example: > > r1 = READ_ONCE(x); > WRITE_ONCE(y, (r1 + 1) % MAX_ELEMENTS); > > If MAX_ELEMENTS is 1, so long, data dependency! Sure, but if MAX_ELEMENTS is 1 then the value written will always be 0 no matter what value r1 has, so it isn't a semantic dependency. Presumably a semantic data dependency would be much more robust. I wonder if it's worth pointing out this distinction to the reader. > With pointers, the compiler has fewer optimization opportunities, > but there are still cases where it can break the dependency. > Or transform it to a control dependency. Transforming a data dependency into a control dependency wouldn't make any important difference; the hardware would still provide the desired ordering. Alan
