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    SubjectRe: [RFC][PATCH 0/5] arch: atomic rework
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    On Tue, Feb 18, 2014 at 8:17 AM, Torvald Riegel <triegel@redhat.com> wrote:
    >>
    >> "Consume operation: no reads in the current thread dependent on the
    >> value currently loaded can be reordered before this load"
    >
    > I can't remember seeing that language in the standard (ie, C or C++).
    > Where is this from?

    That's just for googling for explanations. I do have some old standard
    draft, but that doesn't have any concise definitions anywhere that I
    could find.

    >> and it could make a compiler writer say that value speculation is
    >> still valid, if you do it like this (with "ptr" being the atomic
    >> variable):
    >>
    >> value = ptr->val;
    >
    > I assume the load from ptr has mo_consume ordering?

    Yes.

    >> into
    >>
    >> tmp = ptr;
    >> value = speculated.value;
    >> if (unlikely(tmp != &speculated))
    >> value = tmp->value;
    >>
    >> which is still bogus. The load of "ptr" does happen before the load of
    >> "value = speculated->value" in the instruction stream, but it would
    >> still result in the CPU possibly moving the value read before the
    >> pointer read at least on ARM and power.
    >
    > And surprise, in the C/C++ model the load from ptr is sequenced-before
    > the load from speculated, but there's no ordering constraint on the
    > reads-from relation for the value load if you use mo_consume on the ptr
    > load. Thus, the transformed code has less ordering constraints than the
    > original code, and we arrive at the same outcome.

    Ok, good.

    > The standard is clear on what's required. I strongly suggest reading
    > the formalization of the memory model by Batty et al.

    Can you point to it? Because I can find a draft standard, and it sure
    as hell does *not* contain any clarity of the model. It has a *lot* of
    verbiage, but it's pretty much impossible to actually understand, even
    for somebody who really understands memory ordering.

    Linus


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