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I would hope that this paradigm requires all participating accesses to be annotated:

  int i = load(p, memory_order_consume);
  if (i == 3) {
    int y = load(x[i], memory_order_consume);
  }

it seems like bad design to require ordering between plain loads and annotated loads. We want to identify the two locations that are important, so the compiler can reorder the others however it sees fit, without regard as to their placement w.r.t. those two.

No, loads from x need not be consume nor even need to be atomic. If the thread that last wrote to x and then i never touches x again, there is no possible data race. This is for example the case of for initialization guards.

OK, so p is here like a lock, and x[] is protected data. Indeed; we don't want to have to do anything special with the data protected by the lock. But accesses to the data must not cross the point where the lock is manipulated; e.g. if i == 3 means we own the lock, so that we may access x, x must not be accessed before that check by an optimization.

If p is like by a lock, the compiler won't reorder the access outside of the lock but it can still take into account that i is a known constant to simplify the address computation of x[i] into x[3]. This is perfectly safe for a mutex protected variable as the mutex acquire will, in addition to the compiler barrier, provide the required hardware barriers.

This is is catastrophic, if, instead of relying on an hardware barrier, we rely on a load-load dependency chain originating from i; rewriting x[i] into x[3] break the chain and now, while the compiler might still not reorder the access, the hardware very much can.

> the hardware very much can.

If its name is DEC Alpha? ;)

No, alpha will reorder even if you have a dependency chain (so it will need a barrier no matter what). Power and ARM will reorder only if the chain is broken.

For another empirical example, the linux kernel requires marking all address-dependent accesses via rcu_dereference or LOAD_ONCE. In particular rcu_dereference has an entire section on all the restrictions on what you can do with the result to prevent the compiler from messing with it. For example only deal with pointers, avoid cancellations, etc. Only the things that have been empirically been shown to be safe are allowed.

Of course "do what gcc x.y does" is not a good spec so the standard tried to specify exactly the behavior of consume is, but it failed to be implementable.

> For another empirical example, the linux kernel requires marking all address-dependent accesses via rcu_dereference

Right. Solely so it can support DEC Alpha! I'm not saying data dependence has never been an ordering constraint, I'm saying it was only an ordering constraint on an ancient platform that thus doesn't belong in the standard.

And by extension I'm saying that you people talking about standards instead of hardware are needlessly (frankly: deliberately and obfuscatorily) obscuring that clear truth.

Absolutely not solely for DEC Alpha; From the source [1]:

        You must use one of the rcu_dereference() family of primitives
 to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
 will complain.  Worse yet, your code can see random memory-corruption
 bugs due to games that compilers and DEC Alpha can play.
 Without one of the rcu_dereference() primitives, compilers
 can reload the value, and won't your code have fun with two
 different values for a single pointer!  Without rcu_dereference(),
 DEC Alpha can load a pointer, dereference that pointer, and
 return data preceding initialization that preceded the store of
 the pointer.

The whole document then goes in great details on the all the ways the compiler can screw you over.

If the kernel were to drop Alpha support tomorrow, rcu_dereference and all the warnings would still be relevant.

Similarly, if only x86, SPARC and Alpha existed, memory order relaxed, acquire/release and seq-cst would be sufficient. consume only exists to cater to architectures with memory models stronger than Alpha but weaker than TSO.

[1]https://www.kernel.org/doc/Documentation/RCU/rcu_dereference...

Your focus on hardware misses the big picture. Atomic operations also restrict what the compiler can reorder.

Just because the simplest translation to assembly yields a data dependency on ARM et al doesn't mean the compiler is obligated to honor that.

The focus on standards is precisely because that's the only contract the compiler gives you. If you write code expecting to also get your target architecture's guarantees for free, you're writing software that is at best only incidentally correct until the next compiler update.

I look around this thread and I don't see a group of people trolling you - I genuinely think you're missing the point.

That doesn't show anything incorrect though! You're just stating a requirement that the dereference to x[] happen after the load through p, which is something that can be trivially expressed using 1980's syntax by marking *p volatile. You don't need consume for that. Show me something that explains why the standard needs consume. It doesn't need consume, and I think you agree but you won't admit it.

You keep saying that somehow this isn't about alpha, but it's clear this is about alpha, because alpha needs a barrier there and volatile can't do barriers. You just... won't admit it. And per my point WAY upthread, I contend you won't admit it because you view the "standard" as the important thing and not the hardware.

They did, elsewhere: https://news.ycombinator.com/item?id=36068685

Even if volatile was absolutely all you needed, why would the committee build a memory model with a nice API and then force you to sprinkle in volatile? It makes absolutely no sense.

Volatile is what you would use when you don't care about the DEC Alpha, and ignore the API. When using the API, you wouldn't require volatile.

You can build yourself an access API if you have volatile. E.g.

  #define volatile_lvalue(TYPE, LOC) (*(volatile TYPE *) &(LOC))

With typeof or _Generic, we can eliminate the TYPE argument.

Then you can do:

  if (volatile_lvalue(int, i) == 3) {
    y = x[i]; // don't care if i replaced with 3 here
  }

The user of volatile_lvalue doesn't have to declare anything volatile.

That's the crux of the matter: if i is replaced with 3 on the second line, what do you think prevents an ARM CPU from hoisting the x[3] above the i load first line?

In practice the volatile cast might prevent current GCCs from performing constant propagation, so this exact code sequence might work, but it is extremely fragile and can break for some variations:

  if (int i2 = volatile_lvalue(int, i); i2 == 3) {
    y = x[i2]; // i2 is replaced, no data dependency
  }

or

   if (volatile_lvalue(int, i)) {
     y = x[i-i]; // data dependency becomes a control dependency
   }

You also have to be careful to what you do with y and with any value derived form it.

As currently there is no way to efficiently implement consume you have to do with what you have, so for example the Linux kernel does use volatile casts and compilation barriers to implement LOAD_ONCE, but then has a large manual that documents what operations are currently considered safe and what are known to break.

That's hardly a specification that can be put in a standard and you can't really can reason about the semantics of it. Linus himself has considered multiple time if it is worth the hassle and whether the kernel should just switch to acquire everywhere now that arm and power have relevant cheap barriers.

Ah, for that problem, I'd prefer to have read_barrier() and write_barrier() type primitives that take no arguments.