[Bug boehm-gc/66848] boehm-gc fails test suite on x86_64-apple-darwin15

["howarth.at.gcc at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66848

--- Comment #13 from Jack Howarth <howarth.at.gcc at gmail dot com> ---
(In reply to mrs@gcc.gnu.org from comment #12)
> Is this just a partial import from upstream?  If so, I think we should just
> check it in and call the issue solved.

   No, the patch shown is an ugly hack of reducing the alignment to 2 (or 1)
which merely suppresses the breakage of boehm-gc (and thus gcj) under the Apple
Clang 7.0 compiled libunwind.dylib on darwin15. The newer upstream boehm-gc
uses the same alignments as gcc's boehm-gc but doesn't suffer the test suite
failures.
   All I can say at the moment is that the bug seems to be alignment related.
However it is unclear to me if the bug actually lies in gcc's current boehm-gc
or whether the breakage really lies in the Apple Clang 7.0 optimizations to the
system unwinder and that the newer upstream boehm-gc merely makes the bug go
latent. Apple claims these new optimizations to the unwinder are valid but I
really doubt that the bug has been truly examined in depth to validate that .