On Thu, 2012-05-03 at 11:44 -0600, Jeff Law wrote: > On 05/03/2012 10:47 AM, William J. Schmidt wrote: > >> > > Yes and no. What's important is that you don't want to introduce page > > faults (or less urgently, cache misses) by speculating the load. So the > > patch is currently extremely constrained, and likely will always stay > > that way. Only fields that are pointers and that are strictly adjacent > > are hoisted, and only if they're in the same 16-byte block. (The number > > 16 is a parameter that can be adjusted.) > > > > Hopefully I didn't miss your point -- let me know if I did and I'll try > > again. :) > You missed the point :-) > > Under the C++11 memory model you can't introduce new data races on > objects which might be visible to multiple threads. This requirement > can restrict speculation in many cases. Furthermore, it sounds like C11 > will have similar constraints. > > I believe there's a wiki page which touches on these kinds of issues. > > That doesn't mean we can't ever do the optimization, just that we have > to be more careful than we have in the past when mucking around with > memory optimizations.
OK, thanks! Looks like I have some reading to do about the new memory models. However, from the wiki page I see: "A speculative load which has its results thrown away are considered to not have changed the semantics of the program, and are therefore allowed." That seems to cover the case here: the load is hoisted, but if the path where it was originally loaded is not executed, its result is discarded. If needed, though, what flags/detection mechanisms are available for determining that the load speculation should be disabled? Thanks, Bill > > jeff >
