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Hi,

It seems to me that this would require one extra stage of bootstrap in CI for many buildbots.

For example, today I have a Linux bot with a clang-8 host compiler and libstdc++. The goal is to ensure that MLIR (but it is applicable to any project) builds with clang and libc++ at the top of the main branch.

So the setup is:

- stage1: build clang/libc++ with host clang-8/libstdc++

- stage2: build test "anything" using stage1 (\`ninja check-all\` in the monorepo for example, but applicable to any other external project)

With this proposal, the setup would be:

- stage1: build just clang with host clang-8/libstdc++

- stage2: build clang/libc++ with stage1 clang and host libstdc++

- stage3: build test "anything" using stage2 (\`ninja check-all\` in the monorepo for example, but applicable to any other external project)

The only way to avoid adding a stage in the bootstrap is to keep updating the bots with a very recent host clang (I'm not convinced that increasing the cost of maintenance for CI / infra is good in general).

We should aim for a better balance: it is possible that clang-5 is too old (I don't know?), but there are people (like me, and possibly others) who are testing HEAD with older compiler (clang-8 here) and it does not seem broken at the moment (or the recent years), I feel there should be a strong motivation to break it.

Could we find something more intermediate here? Like a time-based support (2 years?) or something based on the latest Ubuntu release or something like that. That would at least keep the cost of upgrading bots a bit more controlled (and avoid a costly extra stage of bootstrap).

Thanks,

Mehdi

On Tue, Mar 2, 2021 at 7:10 AM Louis Dionne via llvm-dev <llvm-dev@lists.llvm.org> wrote:

\> On Mar 1, 2021, at 15:41, Joerg Sonnenberger via llvm-dev <llvm-dev@lists.llvm.org> wrote:
\>
\> On Mon, Mar 01, 2021 at 12:40:36PM -0500, Louis Dionne via llvm-dev wrote:
\>> However, for a library like libc++, things are a bit different.
\>
\> So how does this prevent the libstdc++ mess that you need to lock step
\> the RTL with the compiler and more importantly, get constantly screwed
\> over when you need to upgrade or downgrade the compiler in a complex
\> environment like an actual Operating System?

Could you please elaborate on what issue you’re thinking about here? As someone who ships libc++ as part of an operating system and SDK (which isn’t necessarily in perfect lockstep with the compiler), I don’t see any issues. The guarantee that you can still use a \~6 months old Clang is specifically intended to allow for that use case, i.e. shipping libc++ as part of an OS instead of a toolchain.

\> I consider this proposal a major step backwards...

To be clear, we only want to make official the level of support that we already provide in reality. As I explained in my original email, if you’ve been relying on libc++ working on much older compilers, I would suggest that you stop doing so because nobody is testing that and we don’t really support it, despite what the documentation says. So IMO this can’t be a step backwards, since we already don’t support these compilers, we just pretend that we do.

Louis

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