On Mon, Feb 10, 2014 at 03:04:43PM +0000, Paul E. McKenney wrote: > On Mon, Feb 10, 2014 at 11:49:29AM +0000, Will Deacon wrote: > > On Mon, Feb 10, 2014 at 11:48:13AM +0000, Peter Zijlstra wrote: > > > On Fri, Feb 07, 2014 at 10:02:16AM -0800, Paul E. McKenney wrote: > > > > 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. 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 [*]. ;-) > > > > > > Just tell them that because the hardware provides control dependencies > > > we actually use and rely on them. > > > > s/control/address/ ? > > Both are important, but as Peter's reply noted, it was control > dependencies under discussion. Data dependencies (which include the > ARM/PowerPC notion of address dependencies) are called out by the standard > already, but control dependencies are not. I am not all that satisified > by current implementations of data dependencies, admittedly. Should > be an interesting discussion. ;-) Ok, but since you can't use control dependencies to order LOAD -> LOAD, it's a pretty big ask of the compiler to make use of them for things like consume, where a data dependency will suffice for any combination of accesses. Will -- 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
