[LLVMdev] Addressing const reference in ArrayRef

[David Blaikie]

I seem to recall discussing this before - is there an existing llvm bug
filed, another email thread or something (or perhaps it was just an IRC
conversation)? It would be good to keep all the discussion together or at
least reference the prior (llvm community) discussion.

Have you tried applying your suggested patch? I assume you meant to suggest
replacing/modifying the existing ctor (to take non-const ref) rather than
adding another?

I'd assume that causes a lot of build failures as we probably rely on
binding temporaries to ArrayRef's in many places correctly (most ArrayRef's
are temporaries, not local variables).

I think in the previous discussion I suggested we might just want to make a
practice of treating named/local (not parameter) ArrayRef's with greater
suspicion, the same way we do for twine, but I'm not sure.

We could move this ctor into a factory function (and/or just make the CTor
private and friend the makeArrayRef helper for this case) to make it more
obvious/easier to find bad call sites. But it would be helpful to have the
context of the prior discussion to continue from there.
On Aug 19, 2014 11:18 PM, "Joey Ye" <joey.ye.cc at gmail.com> wrote:

> Analyzing why GCC failed to build LLVM recently, one root cause lies
> in definition of ArrayRef:
> // ArrayRef.h:
> ArrayRef(const T &OneElt) : Data(&OneElt), Length(1) {}
>
> Here address of const reference is taken and stored to an object. It
> is believed that live range of const reference is only at the function
> call site, escaping of its address to an object with a longer live
> range is invalid. Referring to the case and discussion here:
> https://gcc.gnu.org/ml/gcc/2014-08/msg00173.html
>
> So I would suggest to fix ArrayRef. Adding a non-const version of
> constructor should work, but it still leaves the vulnerability in
> const version, which I'd expect on people in the community to work out
> a solution.
> ArrayRef(T &OneElt) : Data(&OneElt), Length(1) {}
>
>
> Thanks,
> Joey
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