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Faking a dependency, for consume-semantics

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Sometimes we want to fake a data-dependency in order
to use consume-semantics - we can use idioms like x & 0
or x ^ x, and by 1.10#9 the implementation must treat this
as carrying a dependency:

  x = ready.load(std::memory_order_consume);
  if (x)
     y = *(&buffer + (x & 0));   // fake dependency on x

I couldn't find a standard way to do this, i.e. a counterpart
to std::kill_dependency.

By my reading of 1.10#9, we can do this:

  template<typename P, typename V>
  P& with_dependency(P& p, V) { return p; }

  x = ready.load(std::memory_order_consume);
  if (x)
    y = with_dependency(buffer, x);

We could even define it as variadic:

  template<typename P>
  P& with_dependency(P& p, ...) { return p; }

Can we indeed create a data-dependency this way?

If so, it seems that compilers must not only avoid optimizing
away explicit fake data-dependencies (of the x&0 kind), ensuring
that they appear as data-dependent instruction sequences
on memory-models where consume-semantics needs this,
but actually synthesize such code to create data-dependencies
on values whose only "use" is to be passed as unused operands
to a function.


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Faking a dependency, for consume-semantics Al Grant <algrant@myrealbox.com> - 2013-06-05 23:13 -0700

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