https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66848
--- Comment #14 from Jack Howarth <howarth.at.gcc at gmail dot com> --- I finally got around to rebuilding the Apple open source release of libunwind-35.3 from 10.10.5 under Xcode 7 on 10.10.5. The results are rather interesting as the default build is a Debug one compiled at -O0. The debug build of libunwind.dylib produces a binary which exhibits the same breakage in the boehm-gc test suite binaries built on darwin14 as is seen on darwin15 with the optimized system libunwind.dylib. This makes it much more likely that the issue isn't an optimization bug in Apple Clang 7.0 but rather a linker bug in Xcode 7. Unfortunately, it is impossible to test that by linking the Xcode 7 build under the Xcode 6 linker because the Xcode 7 build uses the 10.11 SDK on 10.10 which needs a linkage on libc++abi.tbd and thus requires the new linker with the .tbd support. FYI, The .tbd files are new "text-based stub libraries", that provide a much more compact version of the stub libraries for use in the SDK, and help to significantly reduce its download size.
