http://lists.llvm.org/pipermail/llvm-dev/2013-April/061768.html is 404. Based on context http://lists.llvm.org/pipermail/llvm-dev/2013-April/061737.html seems like the intended list posting.
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It looks to me like you are probably correct about the intended mailing list post, but it's still not particularly helpful. It would be nice to have a reference to a primary source, but this is the best I could find:
https://opensource.apple.com/source/libunwind/libunwind-35.3/src/UnwindLevel1-gcc-ext.c
In any event, your change seems like a step forward so lgtm.
Yes, I couldn't find a primary source. It seems the expected behaviour is not documented by GCC, but GCC's API is the de-facto correct one.
My guess is that the Apple API is just a bug. I've modified libunwind in FreeBSD to implement GCC's libgcc API: https://svnweb.freebsd.org/changeset/base/310365
this is the best I could find:
https://opensource.apple.com/source/libunwind/libunwind-35.3/src/UnwindLevel1-gcc-ext.c
Oh, good point on this file: I also added a reference to its canonical location in the llvm tree.