[llvm-dev] RFC: Strong GC References in LLVM

Andrew Trick via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 15 17:32:12 PDT 2016

> On Jul 15, 2016, at 5:24 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> LLVM's design decision is one where everything has to explicitly care about implicit early exits to get correct answers (and not to harp too much, but "not everything does", years later).  If they don't, they will get wrong answers.
> So, ironically, while looking at this, i noticed it turns out LLVM's PRE in GVN is another place that does not do this correctly either.
> It will insert and hoist loads past may-throw calls depending on whether it thinks the call aliases the pointer or not (IE depending on what memdep tells it, and memdep only cares about aliasing here when coming up with deps), regardless of whether the call is nounwind or not.  This is rare but can happen.
> This is because memdep does this:
>        // If the call has no effect on the queried pointer, just ignore it.
> So it does not give a dep, and PRE then never does anything else to check whether there is a may-throw call in the way of the hoist.
> Testcase and patch coming.

At some point I stopped thinking about this as a bug and realized that you just need to think of LLVM as modeling speculative code barriers as memory dependence. In LLVM, it makes no sense to have a readonly may-throw call.

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