ci: cache compiled objects between runs

Every CI leg compiled the whole tree from scratch, 42 to 57 minutes of
each build job. Objects are now cached with ccache, one entry per leg
kept on the branch that built it: a push saves the cache and drops the
previous entry, a pull request restores main's and keeps nothing.

The precompiled header is turned off whenever the cache is on: Clang
stamps it with the build time, so every file including it missed. With
it off, a warm run hits 98.5 to 98.9 % of compiles and the compile steps
take 1 to 4 minutes; a cold run costs 25 to 60 % more than before, and a
change to a widely included header lands in between.

.github/workflows/build_orca.yml

@@ -76,6 +76,56 @@ jobs:
           if (-not (Test-Path "$cmakeBin\cmake.exe")) { throw "cmake.exe not found at $cmakeBin" }
           Add-Content -Path $env:GITHUB_PATH -Value $cmakeBin
 
+      # Compiler cache. Pushes save it, so main keeps it warm; pull requests
+      # restore it and discard what they compiled. Objects are keyed on the
+      # preprocessed source, the compiler and the flags, so a leg only ever
+      # hits its own entries. A failed install costs the caching, not the build.
+      - name: Name the compiler cache leg
+        if: ${{ !inputs.macos-combine-only }}
+        shell: bash
+        run: |
+          leg="${{ runner.os }}-${{ inputs.arch || 'amd64' }}${{ runner.os == 'Windows' && format('-{0}', inputs.compiler) || '' }}"
+          echo "CCACHE_LEG=$leg" >> "$GITHUB_ENV"
+          echo "CCACHE_ENTRY=ccache-$leg-${{ github.run_id }}-${{ github.run_attempt }}" >> "$GITHUB_ENV"
+
+      # The action only installs and configures ccache. Restore and save go
+      # through actions/cache with one path string, since the cache service
+      # only matches entries saved under the identical path and the action
+      # spells it differently on Windows.
+      - name: Compiler cache
+        id: ccache
+        if: ${{ !inputs.macos-combine-only }}
+        continue-on-error: true
+        uses: hendrikmuhs/ccache-action@v1.2
+        with:
+          key: ${{ env.CCACHE_LEG }}
+          max-size: 3G
+          restore: false
+          save: false
+
+      - name: Restore compiler cache
+        if: ${{ steps.ccache.outcome == 'success' }}
+        uses: actions/cache/restore@v6
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ${{ env.CCACHE_ENTRY }}
+          restore-keys: ccache-${{ env.CCACHE_LEG }}-
+
+      - name: Enable compiler cache
+        if: ${{ steps.ccache.outcome == 'success' }}
+        shell: bash
+        run: |
+          echo "CMAKE_C_COMPILER_LAUNCHER=ccache" >> "$GITHUB_ENV"
+          echo "CMAKE_CXX_COMPILER_LAUNCHER=ccache" >> "$GITHUB_ENV"
+          # Headers a fresh checkout has just written, and the few files that
+          # use __DATE__ or __TIME__.
+          echo "CCACHE_SLOPPINESS=time_macros,include_file_mtime,include_file_ctime" >> "$GITHUB_ENV"
+          # Clang rebuilds the precompiled header with a fresh timestamp on
+          # every run, so everything that includes it would miss.
+          echo "ORCA_EXTRA_BUILD_ARGS=-DSLIC3R_PCH=OFF" >> "$GITHUB_ENV"
+          # The restored directory carries the previous run's counters.
+          ccache -z
+
       - name: Get the version and date on Ubuntu and macOS
         if: runner.os != 'Windows'
         run: |
@@ -670,3 +720,32 @@ jobs:
           asset_name: orca_custom_preset_tests.zip
           asset_content_type: application/octet-stream
           max_releases: 1
+
+      - name: Compiler cache statistics
+        if: ${{ always() && steps.ccache.outcome == 'success' }}
+        shell: bash
+        run: ccache -s -v || ccache -s
+
+      # Entries are immutable, so the new one is saved first and the older
+      # ones for this leg on this ref are dropped afterwards: a failed save
+      # leaves the previous entry in place.
+      - name: Save compiler cache
+        id: ccache_save
+        if: ${{ steps.ccache.outcome == 'success' && github.event_name != 'pull_request' }}
+        uses: actions/cache/save@v6
+        with:
+          path: ${{ github.workspace }}/.ccache
+          key: ${{ env.CCACHE_ENTRY }}
+
+      - name: Drop older compiler cache entries
+        if: ${{ steps.ccache_save.outcome == 'success' }}
+        # A read-only token (fork PRs) cannot delete; that only costs storage.
+        continue-on-error: true
+        shell: bash
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          gh cache list --ref "$GITHUB_REF" --key "ccache-$CCACHE_LEG-" --limit 100 --json id,key \
+            | jq -r --arg keep "$CCACHE_ENTRY" '.[] | select(.key != $keep) | .id' \
+            | tr -d '\r' \
+            | while read -r id; do gh cache delete "$id"; done