[llvm-dev] [RFC] Toolchain update policy (migrating LLVM past C++11)

JF Bastien via llvm-dev llvm-dev at lists.llvm.org
Wed Jan 16 15:35:18 PST 2019

Hi C++ enthusiasts!

It’s a new year, so let’s try a new approach in getting LLVM’s codebase past C++11. Instead of discussing toolchain versions and whether C++14 or 17 is best, let’s just focus on one baby step: agreeing on a policy. This policy will be used to update our toolchain, hopefully warning people in LLVM 8 and actually moving past C++11 for LLVM 9.

Good news! I believe we already have agreement on this policy. I went through all the discussions (again) and I think I captured everyone’s points of view and concerns. Here are the discussions: 
LLVM dev meeting 2018 BoF "Migrating to C++14, and beyond!" <http://llvm.org/devmtg/2018-10/talk-abstracts.html#bof3>
A Short Policy Proposal Regarding Host Compilers <http://lists.llvm.org/pipermail/llvm-dev/2018-May/123238.html>
Using C++14 code in LLVM (2018) <http://lists.llvm.org/pipermail/llvm-dev/2018-May/123182.html>
Using C++14 code in LLVM (2017) <http://lists.llvm.org/pipermail/llvm-dev/2017-October/118673.html>
Using C++14 code in LLVM (2016) <http://lists.llvm.org/pipermail/llvm-dev/2016-October/105483.html>
Document and Enforce new Host Compiler Policy <http://llvm.org/D47073>
Require GCC 5.1 and LLVM 3.5 at a minimum <http://llvm.org/D46723>When replying to this email, please avoid having the same discussions again. Please provide references to anything I might have missed. If you’re making a new point, say so. And don’t assume ill-will, I’m just trying to get us off C++11.

I have a patch for you to review: https://reviews.llvm.org/D56819 <https://reviews.llvm.org/D56819>

Here’s what it currently says our policy should be:

+We intend to require newer toolchains as time goes by. This means LLVM's
+codebase can use newer versions of C++ as they get standardized. Requiring newer
+toolchains to build LLVM can be painful for those building LLVM, it will
+therefore only be done through the following process:
+  * Generally, try to support LLVM and GCC versions from the last 3 years at a
+    minimum. This time-based guideline is not strict: we may support much older
+    compilers, or decide to support fewer ones.
+  * An RFC is sent to the `llvm-dev mailing list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_
+    - Detail upsides of the version increase (e.g. allow LLVM to use newer C++
+      language or library features; avoid miscompiles in particular compiler
+      versions, etc).
+    - Detail downsides on important platforms (e.g. Ubuntu LTS status).
+  * Once the RFC reaches consensus, update the CMake toolchain version checks
+    and this document. We want to soft-error when developers compile LLVM. We
+    say "soft-error" because the error can be turned into a warning using a
+    CMake flag. This is an important step: LLVM still doesn't have code which
+    requires the new toolchains, but it soon will. If you compile LLVM but don't
+    read the mailing list, we should tell you!
+  * Ensure that at least one LLVM release has had this soft-error. Not all
+    developers compile LLVM tip-of-tree. These release-bound developers should
+    also be told about upcoming changes.
+  * Turn the soft-error into a hard-error after said LLVM release has branched.
+  * Update the :doc:`coding standards<CodingStandards>` to explicitly allow the
+    new features we've now unlocked.
+  * Start using the new features in LLVM's codebase.


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