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Message-ID: <bcacee1a2ce84123a664d783d108050e@huawei.com>
Date: Sat, 3 Dec 2022 11:47:06 +0000
From: Jonas Oberhauser <jonas.oberhauser@...wei.com>
To: Alan Stern <stern@...land.harvard.edu>
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Subject: RE: [PATCH] tools: memory-model: Make plain accesses carry
dependencies
-----Original Message-----
From: Alan Stern [mailto:stern@...land.harvard.edu]
Sent: Friday, December 2, 2022 9:22 PM
> > void *y[2];
> > void *x[2] = { (void*)&y[1], (void*)&y[0] };
> >
> > P0() {
> > void **t = (void**)(x[0]);
> Now t holds a pointer to y[1].
Unfortunately, this kind of inductive reasoning (arguing about what happens based on what happened "before") is not possible with memory models that allow OOTA; as you put it, one must allow for loads reading from stores that haven't happened yet.
One such store (I promise!(*)) is a store to x[0] which writes &x[1]. Let's consider the alternative universe where we read from this future store, so now t holds a pointer to x[1].
> > *t = (void*)(t-1);
> And now y[1] holds a pointer to y[0].
In our alternative universe, x[1] now holds a pointer to x[0].
> > }
> > P1() {
> > void **u = (void**)(x[1]);
> Now u holds a pointer to y[0].
In our alternative universe, u holds the pointer to x[0] stored by P0().
> > *u = (void*)(u+1);
> And now y[0] holds a pointer to y[1].
In our alternative universe, now x[0] holds a pointer to x[1]. Behold, the store I promised would happen!
> > }
> The contents of x[] never get changed, so there's no question about the values of t and u.
They might get changed, by the stores *t=... and *u=...
Have fun,
Jonas
(*= because this example is provided free of charge, there is no actual promise, to the extent permitted by applicable law)
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