[llvm-dev] Proposal: introduce dependency on abseil when building benchmarks

James Y Knight via llvm-dev llvm-dev at lists.llvm.org
Thu Oct 7 12:51:02 PDT 2021

On Thu, Oct 7, 2021 at 11:40 AM Renato Golin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Thu, 30 Sept 2021 at 18:08, Mircea Trofin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Abseil has some requirements
>> <https://abseil.io/docs/cpp/platforms/platforms#:~:text=Abseil%20requires%20a%20code%20base,14%20through%20C%2B%2B20).>
>> that may not perfectly match those of the impacted projects. For example,
>> abseil stopped supporting Ubuntu 14.04 before its TLS.
> The list of supported platforms is *definitely* too small for LLVM users.
> Half of their support is "best effort", which really isn't going to cut it
> once we forcefully depend on it.
> We definitely run benchmarks on X86_64, Arm32/64, MIPS, PowerPC (Linux,
> Mac and Windows on a mix of those), and there are probably people running
> on SystemZ, RISCV and other less known architectures.

I think this isn't actually as big a deal as it sounds. The primary
difference between "supported" and "best-effort" is whether they have
working CI for that platform. Furthermore, many unlisted platforms do work.
In practice, abseil has a history of accepting patches to add portability
for "unsupported" platforms.

E.g. to take two examples:
https://github.com/abseil/abseil-cpp/pull/1021 adding support for AIX
https://github.com/abseil/abseil-cpp/pull/904 adding support for Linux on
HPPA, Alpha, IA64, and SH.

> What is your plan for all the other platforms where abseil isn't supported?

The plan should simply be: submit patches if it's broken on an architecture
or OS that someone cares about.

Old versions of the toolchains is a different matter, however -- Abseil's
general promise is to support an old release of a C++ compiler for 5 years
after it has been superseded by a newer version. And supporting old
compilers tends to be a significant burden, so unlike porting to a new CPU
or OS, *that* isn't simply a "patches welcome" situation -- there would
have to be a compelling case made that preserving support for an compiler
older than that was important enough to be worth the hassle.

See also: https://abseil.io/blog/20201001-platforms
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