diff --git a/.ci/generate-buildkite-pipeline-premerge b/.ci/generate-buildkite-pipeline-premerge
--- a/.ci/generate-buildkite-pipeline-premerge
+++ b/.ci/generate-buildkite-pipeline-premerge
@@ -14,11 +14,6 @@
 # See https://buildkite.com/docs/agent/v3/cli-pipeline#pipeline-format.
 #
 
-if ! git diff --name-only HEAD~1 | grep -q -E "^libcxx/|^libcxxabi/|^libunwind/|^runtimes/|^cmake/|^clang/"; then
-  # libcxx/, libcxxabi/, libunwind/, runtimes/, cmake/ or clang/ are not affected
-  exit 0
-fi
-
 reviewID="$(git log --format=%B -n 1 | sed -nE 's/^Review-ID:[[:space:]]*(.+)$/\1/p')"
 if [[ "${reviewID}" != "" ]]; then
   buildMessage="https://llvm.org/${reviewID}"
@@ -51,3 +46,213 @@
       branch: "${BUILDKITE_BRANCH}"
 EOF
 fi
+
+#
+# If we're not running a more specific pipeline, generate a legacy monolithic pipeline
+# based on which subdirectories have been modified. We have some heuristics for this
+# to be reasonable.
+#
+# Individual projects should instead define the pre-commit CI tests that suits their
+# needs while letting them run on the infrastructure provided by LLVM.
+#
+function compute-projects-to-test() {
+  projects=${@}
+  for project in ${projects}; do
+    echo "${project}"
+    case ${project} in
+    lld)
+      for p in bolt cross-project-tests; do
+        echo $p
+      done
+    ;;
+    llvm)
+      for p in bolt clang clang-tools-extra flang lld lldb mlir polly; do
+        echo $p
+      done
+    ;;
+    clang)
+      for p in clang-tools-extra compiler-rt flang libc lldb openmp cross-project-tests; do
+        echo $p
+      done
+    ;;
+    clang-tools-extra)
+      echo libc
+    ;;
+    mlir)
+      echo flang
+    ;;
+    *)
+      # Nothing to do
+    ;;
+    esac
+  done
+}
+
+function add-dependencies() {
+  projects=${@}
+  for project in ${projects}; do
+    echo "${project}"
+    case ${project} in
+    bolt)
+      for p in lld llvm; do
+        echo $p
+      done
+    ;;
+    cross-project-tests)
+      for p in lld clang; do
+        echo $p
+      done
+    ;;
+    clang-tools-extra)
+      for p in llvm clang; do
+        echo $p
+      done
+    ;;
+    compiler-rt|libc|openmp)
+      echo clang
+    ;;
+    flang|lldb)
+      for p in llvm clang; do
+        echo $p
+      done
+    ;;
+    lld|mlir|polly)
+      echo llvm
+    ;;
+    *)
+      # Nothing to do
+    ;;
+    esac
+  done
+}
+
+function exclude-linux() {
+  projects=${@}
+  for project in ${projects}; do
+    case ${project} in
+    cross-project-tests) ;; # tests failing
+    lldb)                ;; # tests failing
+    openmp)              ;; # https://github.com/google/llvm-premerge-checks/issues/410
+    *)
+      echo "${project}"
+    ;;
+    esac
+  done
+}
+
+function exclude-windows() {
+  projects=${@}
+  for project in ${projects}; do
+    case ${project} in
+    cross-project-tests) ;; # tests failing
+    compiler-rt)         ;; # tests taking too long
+    openmp)              ;; # TODO: having trouble with the Perl installation
+    libc)                ;; # no Windows support
+    lldb)                ;; # tests failing
+    bolt)                ;; # tests are not supported yet
+    *)
+      echo "${project}"
+    ;;
+    esac
+  done
+}
+
+function keep-modified-projects() {
+  projects=${@}
+  git_diff="$(git diff --name-only HEAD~1)"
+  for project in ${projects}; do
+    if echo "${git_diff}" | grep -q -E "^${project}/"; then
+      echo "${project}"
+    fi
+  done
+}
+
+function check-targets() {
+  projects=${@}
+  for project in ${projects}; do
+    case ${project} in
+    clang-tools-extra)
+      echo "check-clang-tools"
+    ;;
+    compiler-rt)
+      echo "check-all"
+    ;;
+    cross-project-tests)
+      echo "check-cross-project"
+    ;;
+    lldb)
+      echo "check-all" # TODO: check-lldb may not include all the LLDB tests?
+    ;;
+    pstl)
+      echo "check-all"
+    ;;
+    libclc)
+      echo "check-all"
+    ;;
+    *)
+      echo "check-${project}"
+    ;;
+    esac
+  done
+}
+
+# Figure out which projects need to be built on each platform
+all_projects="bolt clang-tools-extra compiler-rt cross-project-tests flang libc libclc lld lldb llvm mlir openmp polly pstl"
+modified_projects="$(keep-modified-projects ${all_projects})"
+
+linux_projects_to_test=$(exclude-linux $(compute-projects-to-test ${modified_projects}))
+linux_check_targets=$(check-targets ${linux_projects_to_test} | sort | uniq)
+linux_projects=$(add-dependencies ${linux_projects_to_test} | sort | uniq)
+
+windows_projects_to_test=$(exclude-windows $(compute-projects-to-test ${modified_projects}))
+windows_check_targets=$(check-targets ${windows_projects_to_test} | sort | uniq)
+windows_projects=$(add-dependencies ${windows_projects_to_test} | sort | uniq)
+
+# Generate the appropriate pipeline
+if [[ "${linux_projects}" != "" ]]; then
+  cat <<EOF
+- label: ':linux: x64 Debian'
+  artifact_paths:
+    - '*_result.json'
+    - 'build/monolithic-linux/test-results.xml'
+  agents:
+    queue: 'linux'
+  retry:
+    automatic:
+      - exit_status: -1  # Agent was lost
+        limit: 2
+      - exit_status: 255
+        limit: 2 # Forced agent shutdown
+  timeout_in_minutes: 120
+  env:
+    CC: 'clang'
+    CXX: 'clang++'
+  commands:
+    - './.ci/monolithic-linux.sh "$(echo ${linux_projects} | tr ' ' ';')" "$(echo ${linux_check_targets})"'
+EOF
+fi
+
+if [[ "${windows_projects}" != "" ]]; then
+  cat <<EOF
+- label: ':windows: x64 Windows'
+  artifact_paths:
+    - '*_result.json'
+    - 'build/monolithic-windows/test-results.xml'
+  agents:
+    queue: 'windows'
+  retry:
+    automatic:
+      - exit_status: -1  # Agent was lost
+        limit: 2
+      - exit_status: 255
+        limit: 2 # Forced agent shutdown
+  timeout_in_minutes: 150
+  env:
+    CC: 'cl'
+    CXX: 'cl'
+    LD: 'link'
+  commands:
+    - 'C:\\BuildTools\\Common7\\Tools\\VsDevCmd.bat -arch=amd64 -host_arch=amd64'
+    - 'bash .ci/monolithic-windows.sh "$(echo ${windows_projects} | tr ' ' ';')" "$(echo ${windows_check_targets})"'
+EOF
+fi
diff --git a/.ci/generate-buildkite-pipeline-scheduled b/.ci/generate-buildkite-pipeline-scheduled
--- a/.ci/generate-buildkite-pipeline-scheduled
+++ b/.ci/generate-buildkite-pipeline-scheduled
@@ -27,4 +27,43 @@
       message: "${BUILDKITE_MESSAGE}"
       commit: "${BUILDKITE_COMMIT}"
       branch: "${BUILDKITE_BRANCH}"
-EOF
+
+  - label: ':linux: x64 Debian'
+    artifact_paths:
+      - '*_result.json'
+      - 'build/monolithic-linux/test-results.xml'
+    agents:
+      queue: 'linux'
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+        - exit_status: 255
+          limit: 2 # Forced agent shutdown
+    timeout_in_minutes: 120
+    env:
+      CC: 'clang'
+      CXX: 'clang++'
+    commands:
+      - './.ci/monolithic-linux.sh "bolt;clang-tools-extra;compiler-rt;flang;libc;libclc;lld;llvm;mlir;polly;pstl" "check-all"'
+
+  - label: ':windows: x64 Windows'
+    artifact_paths:
+      - '*_result.json'
+      - 'build/monolithic-windows/test-results.xml'
+    agents:
+      queue: 'windows'
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+        - exit_status: 255
+          limit: 2 # Forced agent shutdown
+    timeout_in_minutes: 150
+    env:
+      CC: 'cl'
+      CXX: 'cl'
+      LD: 'link'
+    commands:
+      - 'C:\\BuildTools\\Common7\\Tools\\VsDevCmd.bat -arch=amd64 -host_arch=amd64'
+      - 'bash .ci/monolithic-windows.sh "clang-tools-extra;flang;libclc;lld;llvm;mlir;polly;pstl" "check-all"'
diff --git a/.ci/monolithic-linux.sh b/.ci/monolithic-linux.sh
new file mode 100755
--- /dev/null
+++ b/.ci/monolithic-linux.sh
@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+#===----------------------------------------------------------------------===##
+#
+# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+#===----------------------------------------------------------------------===##
+
+#
+# This script performs a monolithic build of the monorepo and runs the tests of
+# most projects on Linux. This should be replaced by per-project scripts that
+# run only the relevant tests.
+#
+
+set -ex
+set -o pipefail
+
+MONOREPO_ROOT="${MONOREPO_ROOT:="$(git rev-parse --show-toplevel)"}"
+BUILD_DIR="${BUILD_DIR:=${MONOREPO_ROOT}/build/monolithic-linux}"
+
+rm -rf ${BUILD_DIR}
+
+ccache --zero-stats
+ccache --show-config
+function show-stats {
+  ccache --print-stats
+}
+trap show-stats EXIT
+
+projects="${1}"
+targets="${2}"
+
+echo "--- cmake"
+pip install -q -r ${MONOREPO_ROOT}/mlir/python/requirements.txt
+cmake -S ${MONOREPO_ROOT}/llvm -B ${BUILD_DIR} \
+      -D LLVM_ENABLE_PROJECTS="${projects}" \
+      -G Ninja \
+      -D CMAKE_BUILD_TYPE=Release \
+      -D LLVM_ENABLE_ASSERTIONS=ON \
+      -D LLVM_BUILD_EXAMPLES=ON \
+      -D COMPILER_RT_BUILD_LIBFUZZER=OFF \
+      -D LLVM_LIT_ARGS="-v --xunit-xml-output ${BUILD_DIR}/test-results.xml" \
+      -D LLVM_ENABLE_LLD=ON \
+      -D CMAKE_CXX_FLAGS=-gmlt \
+      -D BOLT_CLANG_EXE=/usr/bin/clang \
+      -D LLVM_CCACHE_BUILD=ON
+
+echo "--- ninja"
+ninja -C ${BUILD_DIR} ${targets}
diff --git a/.ci/monolithic-windows.sh b/.ci/monolithic-windows.sh
new file mode 100755
--- /dev/null
+++ b/.ci/monolithic-windows.sh
@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+#===----------------------------------------------------------------------===##
+#
+# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+#===----------------------------------------------------------------------===##
+
+#
+# This script performs a monolithic build of the monorepo and runs the tests of
+# most projects on Windows. This should be replaced by per-project scripts that
+# run only the relevant tests.
+#
+
+set -ex
+set -o pipefail
+
+MONOREPO_ROOT="${MONOREPO_ROOT:="$(git rev-parse --show-toplevel)"}"
+BUILD_DIR="${BUILD_DIR:=${MONOREPO_ROOT}/build/monolithic-windows}"
+
+rm -rf ${BUILD_DIR}
+
+sccache --zero-stats
+function show-stats {
+  sccache --show-stats
+}
+trap show-stats EXIT
+
+projects="${1}"
+targets="${2}"
+
+echo "--- cmake"
+pip install -q -r ${MONOREPO_ROOT}/mlir/python/requirements.txt
+cmake -S ${MONOREPO_ROOT}/llvm -B ${BUILD_DIR} \
+      -D LLVM_ENABLE_PROJECTS="${projects}" \
+      -G Ninja \
+      -D CMAKE_BUILD_TYPE=Release \
+      -D LLVM_ENABLE_ASSERTIONS=ON \
+      -D LLVM_BUILD_EXAMPLES=ON \
+      -D COMPILER_RT_BUILD_LIBFUZZER=OFF \
+      -D LLVM_LIT_ARGS="-v --xunit-xml-output ${BUILD_DIR}/test-results.xml" \
+      -D COMPILER_RT_BUILD_ORC=OFF \
+      -D CMAKE_C_COMPILER_LAUNCHER=sccache \
+      -D CMAKE_CXX_COMPILER_LAUNCHER=sccache
+
+echo "--- ninja"
+ninja -C ${BUILD_DIR} ${targets}