[llvm] allow building with builtin_llvm=OFF in Ubuntu26

[ferdymercury]

builtin_llvm=OFF option is not tested anywhere in the CI.

find_package(LLVM REQUIRED CONFIG) does not work otherwise, because the Find-script is rather strict, it only returns true if major AND minor versions are the same, meaning that API compatibility is ensured.

Without this change, in Ubuntu26 I get:

couldn't find LLVM, candidate version 20.1.8 does not match required version. Setting a range 20...21 does not help either, it needs exact match of minor and major.

depends on root-project/root-ci-images#131

[github-actions]

Test Results

    22 files      22 suites   3d 15h 25m 27s ⏱️
 3 859 tests  3 848 ✅   0 💤 11 ❌
76 253 runs  76 124 ✅ 118 💤 11 ❌

For more details on these failures, see this check.

Results for commit c766d9e.

♻️ This comment has been updated with latest results.

[dpiparo]

this is a good initiative. Once ubu26 failed, I stopped the build. Maybe, for the tests done in this PR, would it make sense to reduce the number of builds to the ones necessary to check the change? Then, when happy, the full battery can be used before the actual merge.

[ferdymercury]

Thanks!

I have tested the Ubuntu26 build like this locally by downloading the docker image registry.cern.ch/root-ci/ubuntu2604 and it seems to build fine with -Dminimal=ON -Dbuiltin_llvm=OFF after installing those three missing packages. So, my suggestion would be:

• first merge packages needed for building with builtin_llvm=OFF root-ci-images#131
• then restart this build
• If there are failures in Ubu26, then do what you say: disable all platforms but Ubuntu26 to save CI resources and debug further. Side note: it would be really great to have a Github label "only-ubuntu" or "only-mac" or "only-windows" where this can be controlled more easily/automated than having to modify the .github yml scripts

[dpiparo]

So far, we did not implement any mechanism to restrict the number of platforms to keep the system simple and maximise coverage. Once upon at time, this was possible, and it did not help development much: in order to save time not all platforms were tested, and that led to breakage.

Said that, I think that if this works well, it well could, it ought to become a special build. It is a big change and we might not want (yet) to affect the binaries created for ubuntu 26 nor add the builtin_llvm as customisation for the PRs.

[dpiparo]

we'll also have to remember that that hypothetical build, will cease to function once LLVM22 is merged, for obvious reasons, and perhaps we want it also in the CI of the 6.40 branch, again for obvious reasons :-)

[ferdymercury]

Indeed A special build sounds great! It's just for testing not for changing binaries