[llvm-dev] Compiler support in libc++

[Mehdi AMINI via llvm-dev]

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 at lists.llvm.org> wrote:

>
>
> > On Mar 1, 2021, at 15:41, Joerg Sonnenberger via llvm-dev <
> llvm-dev at 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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