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    SubjectRe: [RFC][PATCH 0/5] arch: atomic rework
    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


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