[libc++][CI] Upgrades to LLVM 18 as HEAD version.

[Mordante]

Rebased to trigger CI.

[Mordante]

Rebased to test CI.

[philnik]

libcxx/utils/ci/buildkite-pipeline.yml

302

Is there a reason we keep 15 around? Also, could we make these changes automatic like we do for GCC?

339

[Mordante]

libcxx/utils/ci/buildkite-pipeline.yml

302

We still officially support clang-15. The main idea is to make sure we still work with clang-15 so we can be sure backported patches work on clang-15. Once LLVM 17 is released this can be removed.

What do you mean with automatically?

339

This will fail until we update to CMake 3.27, which is on my todo list.

[philnik]

libcxx/utils/ci/buildkite-pipeline.yml

302

This doesn't really make sense to me. This would require us to keep 15 until we are branching for LLVM 18, since there will be fixes backported until then. IMO the right thing to make this working properly is to run the CI on the release branch when a libc++ patch is backported.

I mean that we use label: "Clang ${LLVM_STABLE_VERSION}-1" (or however you have to write it to make it work) and then just update the version at the top of the file.

[Mordante]

libcxx/utils/ci/buildkite-pipeline.yml

302

The idea was to keep 15 until 17 is released to make backporting easier while the 17 branch is stabilized. Once 17 is released the number of backports, hopefully will slow down significantly. That is also the moment when we officially drop 15 support.

We hard-code the value for the older releases 16 and 17 too so I did the same here.

libcxx/utils/ci/buildkite-pipeline.yml

 # (Note the image isn't updated daily.) The LLVM_HEAD_VERSION contains that
 # version number. The Linux CI runners for GCC use the latest stable version.
 # Theses numbers are available in all runners, making it easier to update the
 # version number.
 env:
     # LLVM POST-BRANCH bump version
     # LLVM POST-BRANCH add compiler test for ToT - 1, e.g. "Clang 17"
     # LLVM RELEASE bump remove compiler ToT - 3, e.g. "Clang 15"
-    LLVM_STABLE_VERSION: "16"
-    LLVM_HEAD_VERSION: "17"
+    LLVM_STABLE_VERSION: "16" # Used for tooling, update after the RELEASE.
+    LLVM_HEAD_VERSION: "18"   # Used compiler, update POST-BRANCH.
     GCC_STABLE_VERSION: "12"
 steps:
   #
   # Light pre-commit tests for things like formatting or when people forget
   # to update generated files.
   #
   - label: "Format"
     command: "libcxx/utils/ci/run-buildbot check-format"
agents:
       queue: "libcxx-builders"
       os: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
   - label: "Clang 15"
     command: "libcxx/utils/ci/run-buildbot generic-cxx23"
     artifact_paths:
       - "**/test-results.xml"
       - "**/*.abilist"
     env:
         CC: "clang-15"
         CXX: "clang++-15"
         ENABLE_CLANG_TIDY: "On"
agents:
       queue: "libcxx-builders"
       os: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
+  - label: "Clang 17"
+    command: "libcxx/utils/ci/run-buildbot generic-cxx23"
+    artifact_paths:
+      - "**/test-results.xml"
+      - "**/*.abilist"
+    env:
+        CC: "clang-17"
+        CXX: "clang++-17"
+        ENABLE_CLANG_TIDY: "On"
+    agents:
+      queue: "libcxx-builders"
+      os: "linux"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    timeout_in_minutes: 120
+
   # Tests with the sanitizers.
   - group: "Sanitizers"
     steps:
     - label: "ASAN"
       command: "libcxx/utils/ci/run-buildbot generic-asan"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"