[CI] Build the Windows solution on more than one MSBuild node (#7379)

cmake --build only passes a job count to MSBuild when it is asked for
one, and .ci/compile.sh never asked, so every Visual Studio generator
build ran single-node: projects were built one after another and MTT
only ever parallelized the files of whichever project happened to be
building. Ask for one node per core, overridable with
BUILD_PARALLEL_LEVEL, and fall back to a single node when the core
count is not exposed.

Alongside it set EnforceProcessCountAcrossBuilds and CL_MPCount, the
knobs Microsoft's build throughput write-up pairs with MTT: without
them the node count and MTT multiply into more cl.exe processes than
the runner has cores for.

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

.ci/compile.sh

@@ -17,8 +17,9 @@
 # --dir <dir> sets the name of the build dir, default is "build"
 # --cmake-generator <generator> sets CMAKE_GENERATOR as used by cmake
 # --target-macos-version <version> sets the min os version - only used for macOS builds
-# uses env: BUILDTYPE MAKE_INSTALL MAKE_PACKAGE PACKAGE_TYPE PACKAGE_SUFFIX MAKE_SERVER MAKE_NO_CLIENT MAKE_TEST USE_CCACHE CCACHE_SIZE CCACHE_EVICTION_AGE BUILD_DIR CMAKE_GENERATOR TARGET_MACOS_VERSION MAKE_SANITIZE MAKE_COVERAGE VCPKG_TARGET_MACOS_VERSION
-# (correspond to args: --debug/--release --install --package <package type> --suffix <suffix> --server --test --sanitize --coverage --ccache <ccache_size> --dir <dir>)
+# --profile writes an MSBuild performance summary and binary log, Windows only
+# uses env: BUILDTYPE MAKE_INSTALL MAKE_PACKAGE PACKAGE_TYPE PACKAGE_SUFFIX MAKE_SERVER MAKE_NO_CLIENT MAKE_TEST USE_CCACHE CCACHE_SIZE CCACHE_EVICTION_AGE BUILD_DIR CMAKE_GENERATOR TARGET_MACOS_VERSION MAKE_SANITIZE MAKE_COVERAGE VCPKG_TARGET_MACOS_VERSION BUILD_PARALLEL_LEVEL BUILD_PROFILE
+# (correspond to args: --debug/--release --install --package <package type> --suffix <suffix> --server --test --sanitize --coverage --ccache <ccache_size> --dir <dir> --profile)
 # exitcode: 1 for failure, 3 for invalid arguments
 
 # Read arguments
@@ -115,6 +116,10 @@ while [[ $# != 0 ]]; do
       export CMAKE_GENERATOR=$1
       shift
       ;;
+    '--profile')
+      BUILD_PROFILE=1
+      shift
+      ;;
     '--target-macos-version')
       shift
       if [[ $# == 0 ]]; then
@@ -209,6 +214,22 @@ function ccachestatsverbose() {
   fi
 }
 
+function cpuinfo() {
+  # The number of cores decides how much build parallelism is useful; the hosted
+  # runners are not all sized alike, so read it off the machine instead of assuming.
+  case "$RUNNER_OS" in
+  Windows)
+    echo "cores: ${NUMBER_OF_PROCESSORS:-unknown}"
+    ;;
+  macOS)
+    echo "cores: $(sysctl -n hw.ncpu)"
+    ;;
+  *)
+    echo "cores: $(nproc 2>/dev/null || echo unknown)"
+    ;;
+  esac
+}
+
 # Compile
 if [[ $RUNNER_OS == macOS ]]; then
   # QTDIR is needed for macOS since we actually only use the cached thin Qt binaries instead of the install-qt-action,
@@ -293,9 +314,27 @@ if [[ $RUNNER_OS == macOS ]]; then
   fi
 
 elif [[ $RUNNER_OS == Windows ]]; then
+  # cmake only hands MSBuild a job count when one is asked for, and without it
+  # MSBuild builds a single node: one project at a time, whatever MTT does inside
+  # it. Ask for a node per core.
+  if [[ ! $BUILD_PARALLEL_LEVEL ]]; then
+    BUILD_PARALLEL_LEVEL="${NUMBER_OF_PROCESSORS:-1}"
+  fi
+  buildflags+=(--parallel "$BUILD_PARALLEL_LEVEL")
+
   # Enable MTT, see https://devblogs.microsoft.com/cppblog/improved-parallelism-in-msbuild/
   # and https://devblogs.microsoft.com/cppblog/cpp-build-throughput-investigation-and-tune-up/#multitooltask-mtt
-  buildflags+=(-- -p:UseMultiToolTask=true -p:EnableClServerMode=true)
+  # EnforceProcessCountAcrossBuilds makes the CL_MPCount ceiling apply across all
+  # the projects the nodes are building, instead of each node running its own
+  # cl.exe processes and /m and MTT multiplying into an oversubscribed machine.
+  buildflags+=(-- -p:UseMultiToolTask=true -p:EnableClServerMode=true -p:EnforceProcessCountAcrossBuilds=true -p:CL_MPCount="$BUILD_PARALLEL_LEVEL")
+  if [[ $BUILD_PROFILE ]]; then
+    # A performance summary says which projects and tasks cost what, the binary log
+    # keeps the full timeline; both are far too noisy to always produce. The files
+    # land next to the solution, i.e. in the build dir the workflow uploads. These
+    # join the MTT options above, which already carried the "--" separator.
+    buildflags+=("-flp:PerformanceSummary;v=q;LogFile=msbuild-perf.log" -bl:msbuild.binlog)
+  fi
 fi
 
 if [[ $USE_CCACHE ]]; then
@@ -311,10 +350,19 @@ cmake .. "${flags[@]}"
 echo "::endgroup::"
 
 echo "::group::Build project"
+cpuinfo
 echo "Running cmake --build with flags: ${buildflags[*]}"
 cmake --build . "${buildflags[@]}"
 echo "::endgroup::"
 
+if [[ $BUILD_PROFILE && $RUNNER_OS == Windows && -f msbuild-perf.log ]]; then
+  # Repeat the summary in the job log so the numbers are readable without
+  # downloading the binary log, which is hundreds of megabytes.
+  echo "::group::MSBuild performance summary"
+  cat msbuild-perf.log
+  echo "::endgroup::"
+fi
+
 if [[ $USE_CCACHE ]]; then
   if [[ $CCACHE_EVICTION_AGE ]]; then
     echo "::group::evict ccache files older than $CCACHE_EVICTION_AGE"

.github/workflows/desktop-build.yml

@@ -385,6 +385,10 @@ jobs:
             target: 10
             runner: windows-2025    # https://github.com/actions/runner-images/blob/main/images/windows/Windows2025-VS2026-Readme.md
 
+            # Set build_profile: 1 to have the build write an MSBuild performance
+            # summary and a binary log; empty on every entry that runs by default.
+            build_profile: ''
+
             cmake_generator: "Visual Studio 18 2026"
             cmake_generator_platform: x64
             make_package: 1
@@ -400,6 +404,7 @@ jobs:
     runs-on: ${{ matrix.runner }}
     timeout-minutes: 100
     env:
+      BUILD_PROFILE: '${{ matrix.build_profile }}'
       CCACHE_DIR: ${{ github.workspace }}/.cache/
       CCACHE_SIZE: 600M    # space of all repo is 10Gi: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy
 
@@ -701,6 +706,17 @@ jobs:
             build/oracle/Release/*.pdb
             build/servatrice/Release/*.pdb
 
+      # Written only when the matrix entry sets build_profile, so the artifact
+      # exists exactly on the runs that were asked to produce one.
+      - name: "[Windows] Upload MSBuild binary log"
+        if: always() && matrix.os == 'Windows' && env.BUILD_PROFILE == '1'
+        uses: actions/upload-artifact@v7
+        with:
+          if-no-files-found: error
+          name: ${{ steps.build.outputs.name }}-msbuild
+          path: build/msbuild.binlog
+          retention-days: 7
+
       - name: "Upload to release"
         if: needs.configure.outputs.tag != null && matrix.make_package == '1'
         id: upload_release