[llvm-dev] Compiler support in libc++ (original) (raw)

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 3 10:32:37 PST 2021

On Wed, Mar 3, 2021 at 9:31 AM Mehdi AMINI via llvm-dev < llvm-dev at lists.llvm.org> wrote:

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)

Would it be possible to change the build system so that libc++ can be built like compiler-rt, using the just-built clang? That would then avoid the need for the extra stage? (though it would bottleneck the usual build a bit

& again, this isn't so much a proposal of change, but one of documenting the current state of things - which reveals the current situations are sort of unsupported? (though it also goes against the claim that they're untested) - so I'll be curious to hear from the libc++ folks about this for sure.

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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