Building clang with clang

incripshin · Posted: Sun Sep 17, 2017 4:13 am Post subject: Building clang with clang

I've done a lot of investigation, and it appears the culprit is that my gcc (5.4.0) installed libLLVMSupport.so (among others) with the cxx11 ABI tag. What I'm wondering about:

Install llvm-ld with GCC, use clang+llvm-ld however that works. Maybe that linker will understand? [edit I decided this would never work as per my edit below; I htink I really need a single-ABI system.]
Downgrade GCC to 4.9, build llvm, clang, etc and retry building clang with clang
Upgrade GCC to something modern. 6.4? 7.1?
Some USE flag tweaks?

It seems this should have been fixed in LLVM 3.9. See LLVM bug 23529. But maybe I don't understand the issue.

What I did

First, I have gcc 5.4.0. My libc is musl, but that should be irrelevant. I installed llvm with the following USE flags (which I'm copying manually

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ct85711 · Veteran Joined: 27 Sep 2005 Posts: 1791

incripshin · Posted: Sun Sep 17, 2017 5:56 am Post subject:

Yeah, this is a new installation. I stopped using Gentoo for too long that my installation was severely out of date. This is also why I took the opportunity to switch to musl (which got me to thinking about LLVM and the BSD/MIT/ISC life).

If all I use GCC-4.9 for is to build a working LLVM, it's worth it. I can then proceed to rebuild anything built with that broken ABI.

I've also been thinking... what I really need is a statically linked clang. It would have everything it needs to generate all LLVM libraries & binaries without any pesky GCC ABI dependency. LLVM has an option BUILD_SHARED_LIBS, which is OFF by default but forced ON in the ebuild. I bet if I changed my ebuild to *not* build them shared (which is probably, honestly, a better approach), then I get my mostly-statically-linked clang.

I'm building gcc-4.9.4 right now, but I think I like this new angle because maybe I can fix this in the future through a Gentoo bug report.

ct85711 · Veteran Joined: 27 Sep 2005 Posts: 1791

Honestly, I don't think you are going to have much luck getting a patch accepted because you don't like the ABI change that upstream intentionally did. From what I have seen, gcc devs have been fairly good to keeping the ABI changes so that they are backwards compatible. Gcc-5 is the one exception in that it was too big of change that they couldn't keep it backwards compatible and still be reasonable on the extra amount of code needed.

The big thing you need to keep in mind, is if upstream will accept the patch or not. From my experience Gentoo's devs are less likely to carry and maintain a patch when they know upstream won't accept it. Now if it is a patch, that you can show that upstream accepts it and will/already is committed; you'd have a decent chance to the patch being accepted as it is only a temporary thing and they don't have to maintain it when the version changes breaks it.

Beyond that, gcc-5 change over is a known breakage point; hence why there is a news item about this and we bring it up numerous times.
https://www.gentoo.org/support/news-items/2015-10-22-gcc-5-new-c++11-abi.html

Hu · Administrator Joined: 06 Mar 2007 Posts: 23366

The cxx11 ABI tag was added because the generated code isn't compatible, so without that tag, your link would succeed, but you would get undefined results at runtime. You can turn off the new ABI with a preprocessor flag, but you need to keep the system consistent. I am not aware of any plans by the gcc developers to revert this, so upgrading to newer gcc will not eliminate this issue.

Please explain why you consider this multi-ABI design to be a bad idea. It is a bit ugly, but the only other alternatives I see are either to retain the old ABI indefinitely (and all problems it had) or to cause an extremely painful flag day where any pre-update files incorrectly link with post-update files, then die horribly when the ABI incompatibility causes problems. Do you have a better solution?

As for Markdown: please never suggest such a horrible language. Markdown is a plague upon the Internet. There are trivial constructs that common Markdown parsers cannot get right that are easily expressed even in the relatively limited BBcode available on this forum.

incripshin · Posted: Sun Sep 17, 2017 8:20 pm Post subject:

Hu · Administrator Joined: 06 Mar 2007 Posts: 23366

That disgust is not exaggerated. I really do dislike Markdown.

I'm not saying it's good that LLVM can't build itself. I'm just not saying it's the fault of anything other than LLVM that it is in this halfway state that it wants to use files not built by it, but assumes files not built by it will be built the same way it builds itself. It needs either to be properly self-hosting and not rely on details of how gcc builds it, or it needs to be compatible with how gcc builds it.

incripshin · Posted: Mon Sep 18, 2017 12:02 am Post subject:

clang-built-clang is at 49%!

Is it really LLVM's fault when Gentoo is overriding defaults to values that only LLVM developers are supposed to use? Given LLVM's architecture, Gentoo should be using static libraries [edit i.e. BUILD_SHARED_LIBS=NO].

edit aaaaand that did it! And now to rebuild everything that depends on libstdc++...

mv · Watchman Joined: 20 Apr 2005 Posts: 6780

Dr.Willy · Guru Joined: 15 Jul 2007 Posts: 547 Location: NRW, Germany

Dumb question: Can't you just use gcc to build llvm+clang:4 and then use llvm+clang:4 to build llvm+clang:5?

incripshin · Posted: Wed Sep 27, 2017 7:55 pm Post subject:

Hu · Administrator Joined: 06 Mar 2007 Posts: 23366