[LLVMdev] [RFC] Scoped no-alias metadata
kparzysz at codeaurora.org
Mon Dec 3 06:29:00 PST 2012
On 12/2/2012 11:21 PM, Daniel Berlin wrote:
> "During each execution of B, let L be any lvalue that has &L based on
> P. If L is used to
> access the value of the object X that it designates, and X is also
> modiﬁed (by any means),then the following requirements apply:
> If these requirements are not met, then the behavior is undeﬁned
> (Note the last clause, requiring a modification for the requirements to apply).
> So you can come up with arbitrarily complex aliased restrict pointers
> and scopes, all of which share values,and as long as you don't modify
> any of the objects, it's all okay.
> Worse, none of the scoping requirements around restrict lifetimes
> apply *unless* you modify the objects, since those are inside the
> "..." part above.
If the object isn't modified, then none of this matters. Nobody cares
about aliasing between loads. What all this means is that is P is a
restrict pointer that points to X, then no stores can modify X, unless
they use addresses based on P. In other words, loads/stores to
locations based on P can be aliased with each other, but not with any
other loads or stores. This implies that we need grouping of loads and
stores to properly isolate those based on P from all the others.
Attaching information to loads and stores individually may not be
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