[llvm-dev] [RFC] A vision for building the runtimes
Dominik Montada via llvm-dev
llvm-dev at lists.llvm.org
Fri Oct 23 09:36:32 PDT 2020
Am 23.10.20 um 16:30 schrieb Louis Dionne:
>
>
>> On Oct 23, 2020, at 02:25, Dominik Montada
>> <dominik.montada at hightec-rt.com
>> <mailto:dominik.montada at hightec-rt.com>> wrote:
>>
>> Hi Louis,
>>
>> big +1 from me. I recently set up the runtimes for our downstream
>> toolchain and to be honest it was quite a pain to get everything
>> working. Due to the complexity we ended up creating our own CMake
>> cache to make the configuration easier. We also tried to use the
>> existing toolchain build. But since it tries to build the runtimes
>> immediately after it built the compiler we cannot use it, as we have
>> to build our C library first. So having a "unified standalone"
>> approach sounds just like something that would make this use-case a
>> lot easier.
>
> Glad to see support!
>
>>
>> Do you already have an idea of how a multilib build would look like
>> with your proposed setup?
>
> Can you define what you mean by a multilib build? Do you mean building
> for several architectures at once? Do you produce the libs for each
> architecture in different directories?
Yes, but also more than that: our target has different architectures, so
we build each library for each architecture. But on top of that, we also
build the libraries with different configurations, e.g. with or without
exceptions. The Fuchsia toolchain is doing something similar IIRC. We
produce these libraries in different directories per architecture and
per configuration, so for example:
target/arch1/except
target/arch1/noexcept
target/arch2/except
target/arch2/noexcept
>
> If you're talking about what I'm thinking of, I believe the simplest
> and most CMake-friendly way of doing it would be to have multiple
> CMake invocations. We can hide those behind a "driver" build like what
> the Runtimes build does, for example, but the
> libc++/libc++abi/libunwind builds themselves wouldn't be aware they're
> being built for multiple archs.
With the use-case described above, we simply do one CMake invocation for
each library configuration.
>
>> Also, are you planning of including compiler-rt in this as well or is
>> this strictly meant for libc++, libc++abi and libunwind?
>
> For now, libc++, libc++abi and libunwind. Those are different because
> it makes sense to ship them alongside the compiler, or not. For
> example, at Apple we ship those as system libraries, not alongside our
> compiler. The compiler-rt build is also significantly more complicated.
>
> Louis
>
>>
>> Thanks for doing this!
>>
>> Dominik
>>
>> Am 23.10.20 um 00:31 schrieb Louis Dionne via llvm-dev:
>>> Hi folks,
>>>
>>> The topic of how to build the runtimes has been brought up several
>>> times in the past, sometimes by me, sometimes by others. All in all,
>>> building the runtimes is fairly complex: there's several ways of
>>> doing it, and they don't always work. This complexity leads to
>>> several problems, the first one being that I, as a maintainer, can't
>>> currently guarantee a good experience for people trying to build the
>>> library. What follows is a concrete proposal to make things better
>>> within a reasonable time frame.
>>>
>>>
>>> The current state of things
>>> -------------------------------------
>>>
>>> The runtimes (libc++, libc++abi and libunwind) currently support the
>>> following ways of being built:
>>>
>>> - Monorepo
>>> This is the "easy" and most common way to build the runtimes. It
>>> builds the runtimes as subprojects of LLVM (with
>>> LLVM_ENABLE_PROJECTS), with the same compiler that's used to build LLVM.
>>>
>>> However, this is mostly wrong, since it encourages users to
>>> build libc++ using the system compiler, not the just-built compiler.
>>> Since a recent compiler is necessary for correctness (e.g. the RTTI
>>> for fundamental types generated in libc++abi), this is a real issue.
>>>
>>> This also requires the whole LLVM CMake machinery to work on the
>>> platform where one wants to build the runtimes. This doesn't really
>>> work on most embedded platforms, so they just can't use the monorepo
>>> build.
>>>
>>> This also suffers from issues like the fact that LLVM sets
>>> several global variables/flags that subprojects inherit. While it
>>> may make sense for some subprojects to use these defaults (e.g. the
>>> default C++ Standard), it is actively harmful for the runtimes,
>>> which need to have a tight control over these things.
>>>
>>> - Standalone
>>> Each runtime project also supports a Standalone build. This is a
>>> build where the root CMakeLists.txt is the one from the project
>>> itself. This is nice because it's lightweight, and it doesn't
>>> require all the LLVM CMake setup to work, which solves problems for
>>> embedded platforms.
>>>
>>> Before the monorepo era, this type of build also made sense
>>> cause you could build one runtime without checking out the other
>>> ones, however that is not true anymore (the runtimes all share code
>>> that requires them to be co-located in the monorepo even if you're
>>> just building one of them).
>>>
>>> This type of build has the significant downside that we need to
>>> tie together the various runtime projects using CMake variables. For
>>> example, we have to tell libc++abi where to find the libc++ headers,
>>> and we have to tell libc++ where to find the library for libc++abi.
>>> This leads to a plethora of CMake options that are brittle and add a
>>> lot of complexity (LIBCXX_CXX_ABI_INTREE, LIBCXXABI_LIBCXX_INCLUDES,
>>> etc.).
>>>
>>> - The llvm/runtimes build
>>> I like to call this the Toolchain build instead, cause that's
>>> really what it does. It builds the runtimes using the just-built
>>> toolchain, and with the goal of including those runtimes in the
>>> toolchain. It's more of a driver for the individual builds than a
>>> build configuration itself. It's currently built on top of the
>>> Standalone builds -- it builds the toolchain and then builds the
>>> various runtimes individually, stringing them together as required.
>>>
>>>
>>> Proposal
>>> --------------
>>>
>>> My goal with this proposal is to achieve:
>>> 1. Decoupling from the top-level LLVM CMake setup (which doesn't
>>> work, see above)
>>> 2. A simple build that works everywhere, including embedded platforms
>>> 3. Remove the need to manually tie together the various runtimes (as
>>> in the Standalone builds)
>>>
>>> My proposal is basically to have a "Unified Standalone" build for
>>> all the runtimes. It would look similar to a Monorepo build in
>>> essence (i.e. you'd have a single CMake invocation where you would
>>> specify the flags for all runtime projects at once), but it wouldn't
>>> be using the top-level LLVM CMake setup [1]. Specifically:
>>>
>>> 1. Add a `runtimes/CMakeLists.txt` file that includes the runtimes
>>> subprojects that are requested through -DLLVM_ENABLE_PROJECTS (open
>>> to bikeshed), and sets up minimal stuff like the `llvm-lit` wrapper
>>> and Python, but none of the harmful stuff that's done by the
>>> top-level LLVM CMake.
>>> 2. Deprecate the old individual Standalone builds for this new
>>> "Unified Standalone build".
>>> 3. Users migrate to the new Unified Standalone build. Users include
>>> the current "Runtimes" build, some places in compiler-rt, and
>>> various organizations.
>>> 4. Remove support for the old individual Standalone builds.
>>>
>>> As a second step, we should also:
>>> 5. Deprecate the current Monorepo build in favor of either the
>>> Unified Standalone build (for those who just wish to build the
>>> runtimes), or the current Runtimes (aka Toolchain) build, for those
>>> who wish to build a toolchain.
>>> 6. Let users migrate to either
>>> 7. Remove support for the Monorepo build (e.g. make it an error to
>>> try and build one of the runtimes through the current Monorepo build).
>>>
>>> At the end of this transition, we'd hence have a single way to build
>>> all the runtimes, and a "driver" to build them with the just-built
>>> toolchain.
>>>
>>> Moving towards a single CMake invocation for the Standalone builds
>>> is the key element of this proposal that will make everything
>>> simpler, and remove the need to setup a bunch of things manually. It
>>> will also make it easier to start sharing more code across the
>>> various runtimes [2].
>>>
>>> I have already written the CMakeLists.txt for the "Unified
>>> Standalone" build, and I've been using it to build libc++ and
>>> libc++abi at Apple. It is incredibly simple, and it works well so far.
>>>
>>> I'm looking forward to your comments,
>>> Louis
>>>
>>> [1] If you're wondering what that would look like:
>>>
>>> $ mkdir <monorepo-root>/build
>>> $ cd <monorepo-root>/build
>>> $ cmake ../runtimes
>>> -DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi;libunwind" \
>>> -C <path-to-your-cache-if-desired> \
>>> -DLIBCXX_ENABLE_XXX \
>>> -DLIBCXXABI_ENABLE_XXX \
>>> <other options>
>>> $ ninja install-cxx install-cxxabi
>>>
>>> [2] If you're wondering, I'm not proposing to remove being able to
>>> build libc++ against other ABI libraries, or any such thing. The
>>> Unified Standalone build would retain the same amount of flexibility
>>> as today.
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> --
>> ----------------------------------------------------------------------
>> Dominik Montada Email: dominik.montada at hightec-rt.com
>> <mailto:dominik.montada at hightec-rt.com>
>> HighTec EDV-Systeme GmbH Phone: +49 681 92613 19
>> Europaallee 19 Fax: +49-681-92613-26
>> D-66113 Saarbrücken WWW:http://www.hightec-rt.com
>> <http://www.hightec-rt.com/>
>>
>> Managing Director: Vera Strothmann
>> Register Court: Saarbrücken, HRB 10445, VAT ID: DE 138344222
>>
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--
----------------------------------------------------------------------
Dominik Montada Email: dominik.montada at hightec-rt.com
HighTec EDV-Systeme GmbH Phone: +49 681 92613 19
Europaallee 19 Fax: +49-681-92613-26
D-66113 Saarbrücken WWW: http://www.hightec-rt.com
Managing Director: Vera Strothmann
Register Court: Saarbrücken, HRB 10445, VAT ID: DE 138344222
This e-mail may contain confidential and/or privileged information. If
you are not the intended recipient please notify the sender immediately
and destroy this e-mail. Any unauthorised copying, disclosure or
distribution of the material in this e-mail is strictly forbidden.
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