I'm sure a lot of work goes into it, but often features that are present in upstream clang seem to be removed or heavily delayed. Fom the perspective of a Linux "developer" whose software (C++ scientific tools) sometimes gets compiled with the Xcode clang++, I've seen (i.e. been told of by Apple users) all sorts of problems that don't happen with upstream clang or modern gcc. The fact that the versioning isn't coherent with upstream clang makes things even more confusing.
Re:LLVM 6.0.0 Release
by pertymcpert on Friday March 09, 2018 @06:42AM
(#16551119)
What features?
Well the one I'm familiar with is Apple's explicit disabling of thread-local storage for a long time. See e.g. https://stackoverflow.com/a/23850891, which thankfully has been fixed since then (but it took something like 2 years).
Thank you for the info/insight.
I assumed that the specified language features, especially ones also depending on core libraries like libc++, would most likely be released as part of the next major OS update (since these are very essential for the complete system and need to be tested along side it as one whole).
I didn't check the macOS 10.13.4 beta, but I'm assuming that the features I mentioned are coming as part of macOS 10.14 and Xcode 10.
You can have an idea about the integration exactly what version from upstream Apple is integrating by looking at Swift (they somehow have to be in sync). I guess the last Xcode update was: https://github.com/apple/swift-clang/commits/swift-4.1-branc... The next minor update will likely be this one: https://github.com/apple/swift-clang/commits/swift-4.2-branc... ; so judging at the history I bet you'll get a version of clang that is based of clang-5.0 upstream. And likely the next major Xcode (10?) would be based of this: https://github.com/apple/swift-llvm/tree/swift-5.0-branch ; which is based off clang-6.0. Note that this branch will very likely get many bug fixes in the next ~6 months. (the above are only guesses based on past release, I don't know anything about their current plan or schedule).
The Apple engineers will try to build all variants of the Apple OS for all the HW they ship with this new clang-6.0 based branch. The usual process is to fix upstream the development of clang-7.0 and cherry-pick in these branches.
So the compiler that you'll get with the next major Xcode, even if based of clang-6.0 will have many/most of the bug-fixes that will be in clang-7.0 (release in July), but not most of the new features (and new bugs!). Caveat: this apply only to ARM64 and X86, of course Apple does not try to backport bugfixes for platform they don't support.
(Note that libc++ is handled a bit differently.)
Disclaimer: I was part of the Apple clang team until ~1y ago.