Build ROOT6 without builtin_llvm?

I was not aware of intentional instability of llvm and clang APIs. That certainly causes havoc.

Thanks for the suggestion about building separately ROOT’s llvm and clang. I suppose I can count on ROOT build system checking compatibility between given build of llvm and clang and the current version of ROOT, right? So that I will hit cmake error if there is some development in llvm or clang that new ROOT version needs, as opposite to hitting a compilation error few minutes in the build process.

You need to switch both repositories to the ROOT-patches branch and you are good to go.

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But when I switch to ROOT-patches, how compatibility of given ROOT version and a checked-out commit of ROOT-patches is asserted?

@vvassilev @Axel @pcanal @amadio
I didn’t get an answer on how ROOT build system controls compatibility with ROOT-patches of LLVM and CLANG. Supposing that the head of ROOT-patches is right now compatible with the recent ROOT pro (e.g. v6-16-00) and supposing in 2 days 3 commits are added to ROOT-patches that are then required by ROOT v6-18-00. Now let’s suppose, that I try to build ROOT v6-18-00 against the current ROOT-patches which doesn’t contain these 3 commits. What will happen?

@amarcine I don’t think that there is any mechanism in the build system to ensure you are using a compatible LLVM/Clang version other than just the version check for 5.0.x. That said, the LLVM/Clang in ROOT has not changed much recently, so it probably remains compatible, although the external one based on ROOT-patches may not support C++ runtime modules yet.

Thank you.

So ROOT-patches is not updated along with LLVM inside ROOT?

As for the 5.0.x version check, is version incremented in ROOT-patches to allow for these compatibility checks or is this version just the one which is the base for ROOT-patches? Looking at tags of ROOT-patches, they do not seem to correspond to 5.0.x scheme of LLVM/CLANG versioning nor to ROOT versioning.

While I understand that pushing upstream patches to LLVM/CLANG made for ROOT is problematic and one cannot really assure compatibility of new LLVM releases with ROOT, ROOT-patches is fully under ROOT control, so I thought it should not be a major effort to assure that it is updated and that there is matching between versions there and ROOT versions.

On a best effort basis.

What’s the background of this?

Cheers, Axel.

Do you mean why do I ask the question? If so, let me summarize the discussion up to now: In a situation when I build ROOT from scratch, I wanted to reduce build time by disabling builtin llvm and clang. I was told that I cannot use official clang, so the only option is to build ROOT-patches llvm with clang. So I wanted to know to what extent this is safe. Now it looks like this is not really safe, because ROOT-patches are not synchronized with ROOT and there is no mechanism of matching ROOT versions with ROOT-patches versions.


I see. Yes indeed - for the time being and esp regarding efficiency I’d recommend to “simply” build llvm + clang with ROOT. This will improve in the future.


We cannot do it beyond checking the version is compatible. How do you know a compiled code has a patch applied or not? It’s not a simple check.