diff --git a/.ci/compute_projects.py b/.ci/compute_projects.py
index fd97773e329e..42d1524d638e 100644
--- a/.ci/compute_projects.py
+++ b/.ci/compute_projects.py
@@ -132,11 +132,12 @@ EXCLUDE_DEPENDENTS_WINDOWS = {
 
 EXCLUDE_MAC = {
     "bolt",
-    "compiler-rt",
+    "CIR",  # Depends on mlir, which is excluded below.
     "cross-project-tests",
     "flang",
+    "flang-rt",
     "libc",
-    "lldb",
+    "mlir",
     "openmp",
     "polly",
     "libcxx",
@@ -145,6 +146,13 @@ EXCLUDE_MAC = {
     "offload",
 }
 
+# These projects are still built on the self-hosted macOS runners, but their
+# tests are temporarily skipped there.
+EXCLUDE_CHECK_TARGETS_MAC = {
+    "lldb",
+    "libclc",
+}
+
 PROJECT_CHECK_TARGETS = {
     "clang-tools-extra": "check-clang-tools",
     "compiler-rt": "check-compiler-rt",
@@ -262,6 +270,8 @@ def _compute_project_check_targets(
 ) -> Set[str]:
     check_targets = set()
     for project_to_test in projects_to_test:
+        if platform == "Darwin" and project_to_test in EXCLUDE_CHECK_TARGETS_MAC:
+            continue
         if project_to_test in PROJECT_CHECK_TARGETS:
             check_targets.add(PROJECT_CHECK_TARGETS[project_to_test])
     return check_targets
diff --git a/.ci/compute_projects_test.py b/.ci/compute_projects_test.py
index 3989c2d92026..c0c9cc100d91 100644
--- a/.ci/compute_projects_test.py
+++ b/.ci/compute_projects_test.py
@@ -62,11 +62,11 @@ class TestComputeProjects(unittest.TestCase):
         )
         self.assertEqual(
             env_variables["projects_to_build"],
-            "clang;clang-tools-extra;lld;llvm;mlir",
+            "clang;clang-tools-extra;lld;lldb;llvm",
         )
         self.assertEqual(
             env_variables["project_check_targets"],
-            "check-clang check-clang-python check-clang-tools check-lld check-llvm check-mlir",
+            "check-clang check-clang-python check-clang-tools check-lld check-llvm",
         )
         self.assertEqual(env_variables["runtimes_to_build"], "")
         self.assertEqual(
@@ -78,6 +78,54 @@ class TestComputeProjects(unittest.TestCase):
             "",
         )
 
+    def test_mlir_mac(self):
+        env_variables = compute_projects.get_env_variables(
+            ["mlir/CMakeLists.txt"], "Darwin"
+        )
+        self.assertEqual(env_variables["projects_to_build"], "")
+        self.assertEqual(env_variables["project_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_to_build"], "")
+        self.assertEqual(env_variables["runtimes_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_check_targets_needs_reconfig"], "")
+        self.assertEqual(env_variables["enable_cir"], "OFF")
+
+    def test_cir_mac(self):
+        env_variables = compute_projects.get_env_variables(
+            ["clang/lib/CIR/CMakeLists.txt"], "Darwin"
+        )
+        self.assertEqual(
+            env_variables["projects_to_build"],
+            "clang;clang-tools-extra;lld;lldb;llvm",
+        )
+        self.assertEqual(
+            env_variables["project_check_targets"],
+            "check-clang check-clang-python check-clang-tools",
+        )
+        self.assertEqual(env_variables["runtimes_to_build"], "compiler-rt")
+        self.assertEqual(env_variables["runtimes_check_targets"], "check-compiler-rt")
+        self.assertEqual(env_variables["runtimes_check_targets_needs_reconfig"], "")
+        self.assertEqual(env_variables["enable_cir"], "OFF")
+
+    def test_lldb_mac(self):
+        env_variables = compute_projects.get_env_variables(
+            ["lldb/CMakeLists.txt"], "Darwin"
+        )
+        self.assertEqual(env_variables["projects_to_build"], "clang;lld;lldb;llvm")
+        self.assertEqual(env_variables["project_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_to_build"], "compiler-rt")
+        self.assertEqual(env_variables["runtimes_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_check_targets_needs_reconfig"], "")
+
+    def test_libclc_mac(self):
+        env_variables = compute_projects.get_env_variables(
+            ["libclc/CMakeLists.txt"], "Darwin"
+        )
+        self.assertEqual(env_variables["projects_to_build"], "clang;llvm")
+        self.assertEqual(env_variables["project_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_to_build"], "")
+        self.assertEqual(env_variables["runtimes_check_targets"], "")
+        self.assertEqual(env_variables["runtimes_check_targets_needs_reconfig"], "")
+
     def test_clang(self):
         env_variables = compute_projects.get_env_variables(
             ["clang/CMakeLists.txt"], "Linux"
diff --git a/.github/workflows/premerge.yaml b/.github/workflows/premerge.yaml
index d506e631e2f2..2203084ae36d 100644
--- a/.github/workflows/premerge.yaml
+++ b/.github/workflows/premerge.yaml
@@ -199,52 +199,130 @@ jobs:
           path: |
             comments-Windows-AMD64
 
-  premerge-check-macos:
-    name: MacOS Premerge Checks
-    runs-on: macos-14
+  # Computes which projects/check-targets to build and test on the
+  # self-hosted macOS runners. This job intentionally runs on a
+  # GitHub-hosted runner rather than a self-hosted one, since self-hosted
+  # macOS capacity is limited and shouldn't be spent just to compute what
+  # to build. Its outputs feed premerge-check-macos below, which does the
+  # actual build/test on the self-hosted runners, so changes here directly
+  # affect what runs there.
+  premerge-compute-macos:
+    name: Compute macOS Projects
     if: >-
-      github.repository_owner == 'llvm' &&
-      (startswith(github.ref_name, 'release/') ||
-       startswith(github.base_ref, 'release/')) &&
-      (github.event_name != 'pull_request' || github.event.action != 'closed')
+        github.repository_owner == 'llvm' &&
+        (github.event_name != 'pull_request' || github.event.action != 'closed')
+    runs-on: ubuntu-22.04
+    permissions:
+      contents: read
+      pull-requests: read
+    outputs:
+      macos-projects: ${{ steps.vars.outputs.macos-projects }}
+      macos-check-targets: ${{ steps.vars.outputs.macos-check-targets }}
+    steps:
+      # For pull requests we get the changed files from the GitHub API, so we
+      # only need a sparse checkout of .ci/ to run compute_projects.py. For
+      # pushes (e.g. release branches) there is no PR to query, so we fall
+      # back to a git diff, which needs real history.
+      - name: Checkout LLVM (sparse)
+        if: github.event_name == 'pull_request'
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+          sparse-checkout: .ci
+          sparse-checkout-cone-mode: false
+      - name: Checkout LLVM
+        if: github.event_name != 'pull_request'
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+          fetch-depth: 2
+      - name: Get Changed Files
+        env:
+          GH_TOKEN: ${{ github.token }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          REPO: ${{ github.repository }}
+        run: |
+          if [[ "${{ github.event_name }}" == "pull_request" ]]; then
+            gh api "repos/${REPO}/pulls/${PR_NUMBER}/files" --paginate --jq '.[].filename' > /tmp/changed_files.txt
+          else
+            git diff --name-only HEAD~1...HEAD > /tmp/changed_files.txt
+          fi
+      - name: Compute Projects
+        id: vars
+        run: |
+          python3 .ci/compute_projects.py Darwin < /tmp/changed_files.txt > /tmp/env_vars.sh
+          source /tmp/env_vars.sh
+
+          echo "Building projects: ${projects_to_build}"
+          echo "Running project checks targets: ${project_check_targets}"
+          echo "Building runtimes: ${runtimes_to_build}"
+          echo "Running runtimes checks targets: ${runtimes_check_targets}"
+          echo "Running runtimes checks requiring reconfiguring targets: ${runtimes_check_targets_needs_reconfig}"
+
+          echo "macos-projects=${projects_to_build}" >> $GITHUB_OUTPUT
+          echo "macos-check-targets=${project_check_targets}" >> $GITHUB_OUTPUT
+
+  premerge-check-macos:
+    name: Build and Test macOS arm64
+    needs: premerge-compute-macos
+    runs-on: [self-hosted, macos, apple-runners]
+    # Skip the self-hosted runner entirely if there is nothing to build,
+    # rather than tying up a runner slot for a no-op run.
+    if: >-
+        github.repository_owner == 'llvm' &&
+        (github.event_name != 'pull_request' || github.event.action != 'closed') &&
+        needs.premerge-compute-macos.outputs.macos-projects != ''
     steps:
       - name: Checkout LLVM
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           persist-credentials: false
           fetch-depth: 2
-      - name: Setup ccache
-        uses: hendrikmuhs/ccache-action@33522472633dbd32578e909b315f5ee43ba878ce # v1.2.22
-        with:
-          max-size: "2000M"
-      - name: Install Ninja
+      - name: Install Ninja, CMake, and sccache
         run: |
-          brew install ninja
+          brew install ninja cmake
+          command -v sccache >/dev/null || brew install sccache
       - name: Build and Test
+        timeout-minutes: 240
+        env:
+          SCCACHE_BUCKET: sccache-bucket
+          SCCACHE_REGION: us-west-2
+          PROJECTS: ${{ needs.premerge-compute-macos.outputs.macos-projects }}
+          CHECK_TARGETS: ${{ needs.premerge-compute-macos.outputs.macos-check-targets }}
+          CLANG_TOOLCHAIN_PROGRAM_TIMEOUT: 120
         run: |
-          source <(git diff --name-only HEAD~1...HEAD | python3 .ci/compute_projects.py)
+          export SCCACHE_IDLE_TIMEOUT=0
+          mkdir -p artifacts
+          SCCACHE_LOG=info SCCACHE_ERROR_LOG=$(pwd)/artifacts/sccache.log sccache --start-server
 
-          if [[ "${projects_to_build}" == "" ]]; then
-            echo "No projects to build"
-            exit 0
-          fi
-
-          echo "Building projects: ${projects_to_build}"
-          echo "Running project checks targets: ${project_check_targets}"
-
-          # -DLLVM_DISABLE_ASSEMBLY_FILES=ON is for
-          # https://github.com/llvm/llvm-project/issues/81967
-          # Disable sharding in lit so that the LIT_XFAIL environment var works.
-          cmake -G Ninja \
+          xcrun cmake -G Ninja \
                 -B build \
                 -S llvm \
-                -DLLVM_ENABLE_PROJECTS="${projects_to_build}" \
+                -DLLVM_ENABLE_PROJECTS="${PROJECTS}" \
                 -DLLVM_DISABLE_ASSEMBLY_FILES=ON \
                 -DCMAKE_BUILD_TYPE=Release \
                 -DLLDB_INCLUDE_TESTS=OFF \
                 -DLLVM_ENABLE_ASSERTIONS=ON \
-                -DCMAKE_C_COMPILER_LAUNCHER=ccache \
-                -DCMAKE_CXX_COMPILER_LAUNCHER=ccache
+                -DCMAKE_C_COMPILER_LAUNCHER=sccache \
+                -DCMAKE_CXX_COMPILER_LAUNCHER=sccache
 
-          # The libcxx tests fail, so we are skipping the runtime targets.
-          ninja -C build ${project_check_targets}
+          ninja -C build ${CHECK_TARGETS}
+      - name: Show sccache stats
+        if: always()
+        run: |
+          mkdir -p artifacts
+          sccache --show-stats
+          sccache --show-stats >> artifacts/sccache_stats.txt
+      - name: Upload Artifacts
+        # In some cases, Github will fail to upload the artifact. We want to
+        # continue anyways as a failed artifact upload is an infra failure, not
+        # a checks failure.
+        # https://github.com/actions/upload-artifact/issues/569
+        continue-on-error: true
+        if: '!cancelled()'
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: Premerge Artifacts (macOS arm64)
+          path: artifacts/
+          retention-days: 5
+          include-hidden-files: 'true'