The meaning of the“dereferenceable” attribute in LLVM
we have this attribute, using it to model C++ references, but realized that it is not what people need
the intended semantics was vaguely“this thing will be dereferenceable through the function call”
but the C++ spec doesn’t say that, the function can deallocate the memory backing a T& during course of the function, it just has to be“dereferenceable initially”
they intend to fix this
there has been discussion of how best to fix, so that you can express that something is dereferenceable initially
What changes are being considered in the future from LLVM’s perspective?
a“nofree” function indicates that it doesn’t free anything
so maintains dereferenceability(if you ignore concurrency)
with concurrency, you have more problems(c.f. Arc)
a“nosync” function doesn’t“interact with other threads”(no atomic ops, no loads/stores, no synchronizes-with edges introduced)
“nofree” can also be put on parameters, perhaps, to indicate that data behind a particular pointer is not freed
dereferenceable will change to have the weaker semantics to(ret-con) compatibility
dereference_globally means it will never be derefered ever? maybe useful, but not for Rust
How do we justify“derefenceable” in stacked borrows?
Context: We intend to justify emitting derefenceable because of UB caused under stacked borrows. If we were going to try to stop emitting dereferenceable, we’d presumably also have to adjust stacked borrows to make the pattern not UB. Right?
Answer: we justify by introducing these“protectors” on entry/exit from function
when you push something onto the stack, there’s an optional flag to say“this is protected by that function item”
when you pop an item that is“protected by” a fn that is ongoing, that is UB
this is“per item” — i.e., per element on the“stacked borrows stack”
a side effect of protectors is that having two`&mut` arguments that wind up being given the same value by caller, that is UB due to adding the protectors
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struct MyRegisters { reg1: ReadOnly, reg2: WriteOnly,