On Nov 15, 2012, at 3:30 PM, David Nadlinger <s...@klickverbot.at> wrote:
> On Thursday, 15 November 2012 at 23:22:32 UTC, Sean Kelly wrote: >> On Nov 15, 2012, at 3:05 PM, David Nadlinger <s...@klickverbot.at> wrote: >>> Well, to be picky, that depends on what kind of memory operation you mean – >>> moving non-volatile loads/stores across volatile ones is typically >>> considered acceptable. >> >> Usually not, really. Like if you implement a mutex, you don't want >> non-volatile operations to be hoisted above the mutex acquire or sunk below >> the mutex release. However, it's safe to move additional operations into >> the block where the mutex is held. > > Oh well, I was just being stupid when typing up my response: What I meant to > say is that you _can_ reorder a set of memory operations involving > atomic/volatile ones unless you violate the guarantees of the chosen memory > order option. > > So, for Andrei's statement to be true, shared needs to be defined as making > all memory operations sequentially consistent. Walter doesn't seem to think > this is the way to go, at least if that is what he is referring to as »memory > barriers«. I think because of the as-if rule, the compiler can continue to optimize all it wants between volatile operations. Just not across them.
