[PATCH] D38680: [libunwind] Fix handling of DW_CFA_GNU_args_size

[Reid Kleckner via Phabricator via cfe-commits]

rnk added a comment.

In https://reviews.llvm.org/D38680#903203, @joerg wrote:

> I've looked at this in some detail now. I'm not exactly sure yet why it is broken. The patch seems quite wrong to me. DW_CFA_GNU_args_size should be applied only when unwinding a call instruction and that regard, the commit message of the original change is quite correct. What I am still trying to understand is how the precise unwind frame disagrees with the unwinder.


If you look at Clang's output here https://godbolt.org/g/jFcSxz, you can see that we emit precise CFA adjustments for each push. We don't need to adjust the CFA by gnu arg size in UnwindCursor::step, which unwinds through frames. We only apply it when setting up the register context before transitioning to the landingpad. That's why unw_set_reg UNW_REG_IP is at least approximately the right place to do this SP adjustment, IMO.

Basically, a general purpose unwinder that collects return addresses can ignore gnu args size because we already have CFA adjustments, but an unwinder that implements landingpad transitions must adjust SP by gnu arg size as part of that context switch.

At least, that's how I understand this change. Maybe older (VAX?) compilers didn't implement gnu arg size this way, but at least with this patch, we handshake with ourselves correctly.


https://reviews.llvm.org/D38680