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

Chris Lattner via llvm-dev llvm-dev at lists.llvm.org
Sat Oct 9 18:15:32 PDT 2021

> On Sep 30, 2021, at 10:07 AM, Mircea Trofin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> TL;DR; When either of LLVM_BUILD_BENCHMARKS or LIBCXX_INCLUDE_BENCHMARKS are enabled, as well as for llvm-test-suite, a dependency to abseil would either be auto-downloaded by the build system, or need to be user-specifiable, or provided in the source tree.

Hi Mircea,

As others have pointed out, this seems like a fairly problematic dependency to take on.

> There are (afaik) 3 copies of the google/benchmark <https://github.com/google/benchmark> project in the llvm tree: in llvm-test-suite, in llvm/utils, and in libcxx/utils/.

Ok, that sounds bad.  It seems like an intermediately good step is to have a single copy of this in the monorepo (e.g.) in llvm-project/utils, and have livcxx and llvm-test-suite use that copy.  Is there any downside to consolidating these?

> The benchmark code uses some functionality otherwise offered by abseil <https://abseil.io/>. Over time, this is inconvenient: continued need for duplication for some features; integration issues in projects using abseil due to macro conflicts; and overall bit rot / maintenance overhead.

I’m not sure what you mean here - I think you are saying that there is code from abseil that was copied into the google benchmark library, and downstream code that uses both has issues?  Or are you saying it is similar-but-different functionality that happens to use the same macro names?

Because I don’t understand the benefit, it seems like introducing a new dependency is just a negative - can you explain the benefit more?


> We want <https://github.com/google/benchmark/pull/1183> to add a dependency to abseil to the benchmarks project.
> 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. 
> Naturally, projects snap to whichever version of benchmark they want to; but this new dependency would add an extra consideration when considering updating the version of benchmarks; and the need to handle the extra dependency (either by being OK with it being auto-downloaded, or via the other means described above)
> Are there any other issues that we're missing? Would anyone be hindered by this adoption of abseil in google/benchmarks?

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