On Mon, Feb 10, 2014 at 01:27:51AM +0100, Torvald Riegel wrote: > On Fri, 2014-02-07 at 10:02 -0800, Paul E. McKenney wrote: > > On Fri, Feb 07, 2014 at 04:55:48PM +0000, Will Deacon wrote: [ . . . ] > > And then it is a short and uncontroversial step to the following: > > > > Initial state: x == y == 0 > > > > T1: atomic_store_explicit(42, y, memory_order_relaxed); > > r1 = atomic_load_explicit(x, memory_order_relaxed); > > if (r1 != 42) > > atomic_store_explicit(r1, y, memory_order_relaxed); > > > > T2: r2 = atomic_load_explicit(y, memory_order_relaxed); > > atomic_store_explicit(r2, x, memory_order_relaxed); > > > > This can of course result in r1 == r2 == 42, even though the constant > > 42 never appeared in the original code. This is one way to generate > > an out-of-thin-air value. > > > > As near as I can tell, compiler writers hate the idea of prohibiting > > speculative-store optimizations because it requires them to introduce > > both control and data dependency tracking into their compilers. > > I wouldn't characterize the situation like this (although I can't speak > for others, obviously). IMHO, it's perfectly fine on sequential / > non-synchronizing code, because we know the difference isn't observable > by a correct program. For synchronizing code, compilers just shouldn't > do it, or they would have to truly prove that speculation is harmless. > That will be hard, so I think it should just be avoided. > > Synchronization code will likely have been tuned anyway (especially if > it uses relaxed MO), so I don't see a large need for trying to optimize > using speculative atomic stores. > > Thus, I think there's an easy and practical solution. I like this approach, but there has been resistance to it in the past. Definitely worth a good try, though! > > Many of > > them seem to hate dependency tracking with a purple passion. At least, > > such a hatred would go a long way towards explaining the incomplete > > and high-overhead implementations of memory_order_consume, the long > > and successful use of idioms based on the memory_order_consume pattern > > notwithstanding [*]. ;-) > > I still think that's different because it blurs the difference between > sequential code and synchronizing code (ie, atomic accesses). With > consume MO, the simple solution above doesn't work anymore, because > suddenly synchronizing code does affect optimizations in sequential > code, even if that wouldn't reorder across the synchronizing code (which > would be clearly "visible" to the implementation of the optimization). I understand that memory_order_consume is a bit harder on compiler writers than the other memory orders, but it is also pretty valuable. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
