[CI] Cache the Windows compile results with sccache

A pull request that touches one file rebuilds it, but sccache can hand
back the objects for everything else, and the GitHub Actions cache keeps
them between runs, so the second build of a branch costs about what the
first one stored and nothing more.

sccache has to wrap the compiler, and cmake only wraps it for the
generators that run the compiler themselves: the Visual Studio
generator drives MSBuild, which would drop a launcher on the floor
without a word. The cache therefore goes on the Ninja job, and
compile.sh now refuses --sccache on any other generator instead of
pretending to cache, and prints the sccache statistics next to the
existing ccache ones so the hit rate is visible in the log.

For MSVC, a cacheable object has to carry its debug information
inside, which is /Z7 rather than /Zi; sccache cannot restore the
latter, it can only restore a whole object file. Ask CMake for the
format by name through CMP0141 and CMAKE_MSVC_DEBUG_INFORMATION_FORMAT
instead of hardcoding /Zi in the flags, where it would end up next to
/Z7, and keep the format as it was for every build that does not
cache. The linker still gets /DEBUG, so the release PDBs are written
either way, which is what that flag is there for.

The policy is set before project(), since that is when it takes
effect, and behind an if(POLICY) guard because it does not exist
before CMake 3.25 - where the old defaults remain the only option.

.ci/compile.sh

@@ -14,11 +14,13 @@
 # --debug or --release sets the build type ie CMAKE_BUILD_TYPE
 # --ccache [<size>] uses ccache and shows stats, optionally provide size
 # --evict-ccache <age> runs ccache eviction based on given age after build
+# --sccache uses sccache instead, needs a generator that runs the compiler itself
 # --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 USE_SCCACHE
+# (correspond to args: --debug/--release --install --package <package type> --suffix <suffix> --server --test --sanitize --coverage --ccache <ccache_size> --dir <dir> --profile --sccache)
 # exitcode: 1 for failure, 3 for invalid arguments
 
 # Read arguments
@@ -93,6 +95,10 @@ while [[ $# != 0 ]]; do
       CCACHE_EVICTION_AGE=$1
       shift
       ;;
+    '--sccache')
+      USE_SCCACHE=1
+      shift
+      ;;
     '--vcpkg')
       USE_VCPKG=1
       shift
@@ -115,6 +121,10 @@ while [[ $# != 0 ]]; do
       export CMAKE_GENERATOR=$1
       shift
       ;;
+    '--profile')
+      BUILD_PROFILE=1
+      shift
+      ;;
     '--target-macos-version')
       shift
       if [[ $# == 0 ]]; then
@@ -133,6 +143,12 @@ done
 
 set -e
 
+# cmake reads the generator platform from the environment, so a leftover platform
+# follows the generator around; Ninja has no platform to select.
+if [[ $CMAKE_GENERATOR != "Visual Studio"* ]]; then
+  unset CMAKE_GENERATOR_PLATFORM
+fi
+
 # Setup
 ./servatrice/check_schema_version.sh
 if [[ ! $BUILDTYPE ]]; then
@@ -158,6 +174,16 @@ export CMAKE_POLICY_VERSION_MINIMUM=3.10
 
 # Add cmake flags
 flags=("-DCMAKE_BUILD_TYPE=$BUILDTYPE")
+if [[ $USE_CCACHE && $USE_SCCACHE ]]; then
+  echo "::error file=$0::ccache and sccache at the same time is not a supported combination"
+  exit 1
+fi
+if [[ $USE_SCCACHE && $CMAKE_GENERATOR != "Ninja"* ]]; then
+  # A compiler cache wraps the compiler, and only generators that run the compiler
+  # themselves are given a launcher to wrap: cmake does not pass one to MSBuild.
+  echo "::error file=$0::sccache needs a Ninja generator, not '$CMAKE_GENERATOR', which would ignore it"
+  exit 1
+fi
 if [[ $MAKE_SERVER ]]; then
   flags+=("-DWITH_SERVER=1")
 fi
@@ -188,6 +214,10 @@ if [[ $USE_CCACHE ]]; then
     ccache --max-size "$CCACHE_SIZE"
   fi
 fi
+if [[ $USE_SCCACHE ]]; then
+  flags+=("-DUSE_SCCACHE=1")
+  flags+=("-DCMAKE_C_COMPILER_LAUNCHER=sccache" "-DCMAKE_CXX_COMPILER_LAUNCHER=sccache")
+fi
 if [[ $PACKAGE_TYPE ]]; then
   flags+=("-DCPACK_GENERATOR=$PACKAGE_TYPE")
 fi
@@ -209,6 +239,22 @@ 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 +339,33 @@ if [[ $RUNNER_OS == macOS ]]; then
   fi
 
 elif [[ $RUNNER_OS == Windows ]]; then
-  # 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)
+  if [[ $CMAKE_GENERATOR == "Visual Studio"* ]]; then
+    # cmake only hands MSBuild a job count when it 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 "$BUILD_PARALLEL_LEVEL")
+
+    # Enable MTT, see https://devblogs.microsoft.com/cppblog/improved-parallelism-in-msbuild/
+    # and https://devblogs.microsoft.com/cppblog/cpp-build-throughput-investigation-and-tune-up/#multitooltask-mtt
+    # EnforceProcessCountAcrossBuilds makes the CL_MPCount ceiling apply across all
+    # the projects the nodes are building, instead of each node running its own
+    # cl.exe processes and /m and MTT multiplying into an oversubscribed machine.
+    buildflags+=(-- -p:UseMultiToolTask=true -p:EnableClServerMode=true -p:EnforceProcessCountAcrossBuilds=true -p:CL_MPCount="$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
+  else
+    # Ninja runs the compiler itself, so it neither needs the MSVC switches nor
+    # wants a job count: it sizes its own pool for the cores it finds.
+    echo "ninja $(ninja --version)"
+  fi
 fi
 
 if [[ $USE_CCACHE ]]; then
@@ -311,10 +381,19 @@ cmake .. "${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"
@@ -324,6 +403,10 @@ if [[ $USE_CCACHE ]]; then
   echo "::group::Show ccache stats again"
   ccachestatsverbose
   echo "::endgroup::"
+elif [[ $USE_SCCACHE ]]; then
+  echo "::group::Show sccache stats"
+  sccache --show-stats
+  echo "::endgroup::"
 elif [[ $CCACHE_EVICTION_AGE ]]; then
   echo "::error file=$0::ccache eviction is enabled while ccache is disabled!"
 fi

.github/workflows/desktop-build.yml

@@ -385,6 +385,12 @@ 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.
+            # Enabled here on purpose: this pull request measures MSBuild
+            # parallelism, so its run is the one worth profiling.
+            build_profile: 1
+
             cmake_generator: "Visual Studio 18 2026"
             cmake_generator_platform: x64
             make_package: 1
@@ -395,11 +401,37 @@ jobs:
             vcpkg_scope: 'vcpkg/windows10-x64-vs2026/'
             vcpkg_upload: 1    # only one leg per vcpkg_scope hands its archives over
 
-    name: ${{ matrix.os }} ${{ matrix.target }}${{ matrix.soc == 'Intel' && ' Intel' || '' }}${{ matrix.type == 'Debug' && ' Debug' || '' }}
+          # A/B run: is Ninja faster than the Visual Studio generator on this runner?
+          # It is allowed to fail and is never published, see the steps that build
+          # a release below. Delete this entry once the answer is known.
+          - os: Windows
+            target: 10
+            runner: windows-2025
+
+            cmake_generator: Ninja
+            experiment: yes
+            make_package: 1
+            package_suffix: "-Win10-ninja"
+            qt_version: 6.11.*
+            qt_modules: qtimageformats qtmultimedia qtshadertools qtwebsockets
+            type: Release
+            use_sccache: 1
+            variant: ' (Ninja)'
+            # The same scope as the Visual Studio leg, so both halves of the A/B
+            # start from the same vcpkg cache and the comparison is about the
+            # generator alone. Without it this leg's prefix is empty, which makes
+            # the cache action restore the whole vcpkg/ namespace and write entries
+            # that no scope claims. vcpkg_upload is deliberately left unset, so only
+            # the Visual Studio leg hands its archives over.
+            vcpkg_scope: 'vcpkg/windows10-x64-vs2026/'
+
+    name: ${{ matrix.os }} ${{ matrix.target }}${{ matrix.soc == 'Intel' && ' Intel' || '' }}${{ matrix.type == 'Debug' && ' Debug' || '' }}${{ matrix.variant }}
     needs: configure
     runs-on: ${{ matrix.runner }}
+    continue-on-error: ${{ matrix.experiment == 'yes' }}
     timeout-minutes: 100
     env:
+      BUILD_PROFILE: '${{ 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
 
@@ -482,6 +514,28 @@ jobs:
           modules: ${{ matrix.qt_modules }}
           version: ${{ matrix.qt_version }}
 
+      # Ninja invokes cl.exe directly and needs the MSVC environment on PATH; the
+      # Visual Studio generator locates the compiler on its own and skips this.
+      - name: "[Windows/Ninja] Set up MSVC environment"
+        if: matrix.os == 'Windows' && matrix.cmake_generator == 'Ninja'
+        uses: TheMrMilchmann/setup-msvc-dev@v4.1.0
+        with:
+          arch: x64
+
+      # sccache is only useful for the Ninja job: a compiler cache has to wrap the
+      # compiler, and MSBuild is driven by cmake instead of running it, so a
+      # launcher handed to MSBuild is ignored without a word.
+      - name: "[Windows/Ninja] Set up sccache"
+        if: matrix.os == 'Windows' && matrix.cmake_generator == 'Ninja'
+        uses: mozilla-actions/sccache-action@v0.0.11
+        env:
+          SCCACHE_GHA_ENABLED: "true"
+          # A fork pull request can read the cache, but the token it runs with
+          # cannot write to the base branch's scope.
+          SCCACHE_GHA_RW_MODE: ${{ github.event.pull_request.head.repo.fork && 'READ_ONLY' || 'READ_WRITE' }}
+          # A cache that cannot be reached must not fail the build.
+          SCCACHE_IGNORE_SERVER_IO_ERROR: "1"
+
       - name: "[Windows] Install NSIS"
         if: matrix.os == 'Windows'
         shell: bash
@@ -530,8 +584,9 @@ jobs:
           MACOS_CI_KEYCHAIN_PWD: ${{ secrets.PROD_MACOS_CI_KEYCHAIN_PWD }}
           MAKE_PACKAGE: '${{ matrix.make_package }}'
           PACKAGE_SUFFIX: '${{ matrix.package_suffix }}'
-          TARGET_MACOS_VERSION: ${{ matrix.override_target }}
+          TARGET_MACOS_VERSION: '${{ matrix.override_target }}'
           USE_CCACHE: ${{ matrix.use_ccache }}
+          USE_SCCACHE: ${{ matrix.use_sccache }}
           # Only pushes to the default branch may write the shared binary cache: the
           # archives a run compiles are only as trustworthy as the code they were
           # built from, and only merged code may populate the scope that master and
@@ -690,8 +745,11 @@ jobs:
           if-no-files-found: error
           path: ${{ steps.build.outputs.path }}
 
+      # Ninja writes its PDBs next to the binaries instead of into a Release
+      # subdirectory, and the experimental job is not a source of release debug
+      # symbols, so it keeps out of both.
       - name: "[Windows] Upload PDBs (Program Databases)"
-        if: matrix.os == 'Windows' && github.ref_type != 'tag'
+        if: matrix.os == 'Windows' && matrix.experiment != 'yes' && github.ref_type != 'tag'
         uses: actions/upload-artifact@v7
         with:
           if-no-files-found: error
@@ -701,8 +759,19 @@ 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'
+        if: needs.configure.outputs.tag != null && matrix.make_package == '1' && matrix.experiment != 'yes'
         id: upload_release
         shell: bash
         env:

CMakeLists.txt

@@ -8,8 +8,19 @@
 # 3.16 required for Qt6 and target_precompile_headers()
 cmake_minimum_required(VERSION 3.16)
 
+# Let CMake pick the MSVC debug information format through
+# CMAKE_MSVC_DEBUG_INFORMATION_FORMAT instead of writing it into the
+# CMAKE_<LANG>_FLAGS_<CONFIG> defaults. The policy has to be settled before the
+# first project() enables a language, and it does not exist before CMake 3.25,
+# where the old defaults are the only thing available.
+if(POLICY CMP0141)
+  cmake_policy(SET CMP0141 NEW)
+endif()
+
 # Use compiler cache (ccache)
 option(USE_CCACHE "Cache the build results with ccache" ON)
+# Use compiler cache (sccache), for generators that invoke the compiler themselves
+option(USE_SCCACHE "Cache the build results with sccache" OFF)
 # Treat warnings as errors (Debug builds only)
 option(WARNING_AS_ERROR "Treat warnings as errors in debug builds" ON)
 # Check for translation updates
@@ -156,8 +167,17 @@ endif()
 
 # Define proper compilation flags
 if(MSVC)
-  # Disable Warning C4251, C++20 compatibility, Multi-threaded Builds, Warn Detection, Unwind Semantics, Debug Symbols
-  set(CMAKE_CXX_FLAGS "/wd4251 /Zc:__cplusplus /std:c++20 /permissive- /W4 /MP /EHsc /Zi")
+  # Debug information: a cache can only store and restore objects that carry it
+  # inside, so a cached build asks for /Z7 (Embedded); otherwise /Zi (ProgramDatabase).
+  # Either way the linker is told to write the PDB, in every configuration.
+  if(USE_SCCACHE)
+    set(CMAKE_MSVC_DEBUG_INFORMATION_FORMAT Embedded)
+  else()
+    set(CMAKE_MSVC_DEBUG_INFORMATION_FORMAT ProgramDatabase)
+  endif()
+
+  # Disable Warning C4251, C++20 compatibility, Multi-threaded Builds, Warn Detection, Unwind Semantics
+  set(CMAKE_CXX_FLAGS "/wd4251 /Zc:__cplusplus /std:c++20 /permissive- /W4 /MP /EHsc")
   # Visual Studio: Maximum Optimization, Multi-threaded DLL
   set(CMAKE_CXX_FLAGS_RELEASE "/Ox /MD")
   # Visual Studio: No Optimization, Multi-threaded Debug DLL