CI: Improve ccache handling in build and test jobs

Cache key isolation: Include the build flavour (ccwarn/asan/gcov) in the
build ccache key, so the coverage (gcov), asan, and RTLMeter
(ccwarn-off) configurations no longer share and thrash the single
'<runs-on>-<cc>' bucket, nor warm-start from each other's (near-useless)
caches.

Prune stale entries instead of a fixed size cap: After building/testing,
evict ccache entries this run did not touch (--evict-older-than, based
on the run's own duration).  This bounds the saved cache to the current
working set, so CCACHE_MAXSIZE and CCACHE_LIMIT_MULTIPLE are no longer
needed and are removed (ccache falls back to its 5G default ceiling, but
is realistically much smaller).

ccache configuration:
- Drop CCACHE_COMPRESS (already the default in ccache 4.x).
- Add CCACHE_COMPILERCHECK=content, so a changed compiler mtime on a rebuilt
  ephemeral runner image does not invalidate the whole restored cache.
- Hoist all ccache-related variables (CACHE_BASE_KEY, CCACHE_*, CXX) to the
  workflow-level env.

Remove dead ccache scaffolding:
- reusable-lint-py: the job never compiles or caches; drop its ccache env.
- reusable-rtlmeter-run: remove the disabled persistent-cache step and its
  storage variables; keep CCACHE_DISABLE=1, since caching must stay off to
  keep RTLMeter timings representative.

.github/workflows/reusable-build.yml

@@ -45,9 +45,9 @@ on:
         value: ${{ jobs.build.outputs.archive }}
 
 env:
-  CCACHE_COMPRESS: 1
+  CACHE_BASE_KEY: build-${{ inputs.runs-on }}-${{ inputs.cc }}${{ inputs.ccwarn && '-ccwarn' || '' }}${{ inputs.dev-asan && '-asan' || '' }}${{ inputs.dev-gcov && '-gcov' || '' }}
+  CCACHE_COMPILERCHECK: content
   CCACHE_DIR: ${{ github.workspace }}/.ccache
-  CCACHE_LIMIT_MULTIPLE: 0.95
 
 defaults:
   run:
@@ -61,9 +61,6 @@ jobs:
     runs-on: ${{ inputs.runs-on }}
     outputs:
       archive: ${{ steps.create-archive.outputs.archive }}
-    env:
-      CACHE_BASE_KEY: build-${{ inputs.runs-on }}-${{ inputs.cc }}
-      CCACHE_MAXSIZE: 1000M
     steps:
       - name: Checkout
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7
@@ -74,7 +71,13 @@ jobs:
           # behind 'verilator --version'
           fetch-depth: ${{ (inputs.install || inputs.dev-gcov) && '0' || '1' }}
 
+      # ccache is unreliable on the macOS runners; disable it there
+      - name: Disable ccache on macOS
+        if: ${{ startsWith(inputs.runs-on, 'macos') }}
+        run: echo "CCACHE_DISABLE=1" >> "$GITHUB_ENV"
+
       - name: Cache $CCACHE_DIR
+        if: ${{ env.CCACHE_DISABLE != '1' }}
         uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9  # v5
         env:
           CACHE_KEY: ${{ env.CACHE_BASE_KEY }}-ccache
@@ -82,7 +85,7 @@ jobs:
           path: ${{ env.CCACHE_DIR }}
           key: ${{ env.CACHE_KEY }}-${{ inputs.sha }}
           restore-keys: |
-            ${{ env.CACHE_KEY }}-
+            ${{ env.CACHE_KEY }}
 
       - name: Install packages for build
         run: ./ci/ci-install.bash build

.github/workflows/reusable-lint-py.yml

@@ -7,11 +7,6 @@ name: reusable-lint-py
 on:
   workflow_call:
 
-env:
-  CCACHE_COMPRESS: 1
-  CCACHE_DIR: ${{ github.workspace }}/.ccache
-  CCACHE_LIMIT_MULTIPLE: 0.95
-
 defaults:
   run:
     shell: bash

.github/workflows/reusable-rtlmeter-run.yml

@@ -53,8 +53,8 @@ defaults:
     shell: bash
 
 env:
-  CCACHE_DIR: ${{ github.workspace }}/ccache
-  CCACHE_MAXSIZE: 512M
+  # RTLMeter measures compile/execute times, so caching must stay off to keep
+  # timings representative; ccache is still on PATH but acts as a pass-through
   CCACHE_DISABLE: 1
 
 jobs:
@@ -82,14 +82,6 @@ jobs:
         working-directory: rtlmeter
         run: make venv
 
-      - name: Use saved ccache
-        if: ${{ env.CCACHE_DISABLE == 0 }}
-        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9  # v5
-        with:
-          path: ${{ env.CCACHE_DIR }}
-          key: rtlmeter-run-ccache-${{ inputs.runs-on }}-${{ inputs.cc }}-${{ inputs.cases }}-${{ inputs.compileArgs }}-${{ github.run_id }}-${{ github.run_attempt }}
-          restore-keys: rtlmeter-run-ccache-${{ inputs.runs-on }}-${{ inputs.cc }}-${{ inputs.cases }}-${{ inputs.compileArgs }}
-
       ########################################################################
       # Run with new Verilator
       ########################################################################

.github/workflows/reusable-test.yml

@@ -35,9 +35,10 @@ on:
         type: string
 
 env:
-  CCACHE_COMPRESS: 1
+  CACHE_BASE_KEY: test-${{ inputs.runs-on }}-${{ inputs.cc }}-${{ inputs.suite }}
+  CCACHE_COMPILERCHECK: content
   CCACHE_DIR: ${{ github.workspace }}/.ccache
-  CCACHE_LIMIT_MULTIPLE: 0.95
+  CXX: ${{ inputs.cc == 'clang' && 'clang++' || 'g++' }}
 
 defaults:
   run:
@@ -49,10 +50,6 @@ jobs:
   test:
     runs-on: ${{ inputs.runs-on }}
     name: Test
-    env:
-      CXX: ${{ inputs.cc == 'clang' && 'clang++' || 'g++' }}
-      CACHE_BASE_KEY: test-${{ inputs.runs-on }}-${{ inputs.cc }}-${{ inputs.reloc }}-${{ inputs.suite }}
-      CCACHE_MAXSIZE: 100M  # Per build per suite (* 5 * 5 = 2500M in total)
     steps:
 
       - name: Download repository archive
@@ -70,12 +67,12 @@ jobs:
       - name: Cache $CCACHE_DIR
         uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9  # v5
         env:
-          CACHE_KEY: ${{ env.CACHE_BASE_KEY }}-ccache2
+          CACHE_KEY: ${{ env.CACHE_BASE_KEY }}-ccache
         with:
           path: ${{ env.CCACHE_DIR }}
           key: ${{ env.CACHE_KEY }}-${{ github.sha }}
           restore-keys: |
-            ${{ env.CACHE_KEY }}-
+            ${{ env.CACHE_KEY }}
 
       - name: Install test dependencies
         run: |

ci/ci-build.bash

@@ -78,5 +78,8 @@ autoconf
 # Build
 
 ccache -z
+BUILD_START=$SECONDS
 "$MAKE" -j "$NPROC" -k
 ccache -svv
+ccache --evict-older-than "$((SECONDS - BUILD_START + 60))s"
+ccache -svv

ci/ci-common.bash

@@ -26,8 +26,6 @@ case "$(uname -s)" in
     HOST_OS=macOS
     MAKE=make
     NPROC=$(sysctl -n hw.logicalcpu)
-    # Disable ccache, doesn't always work in GitHub Actions
-    export OBJCACHE=
     ;;
   *)
     fatal "Unknown host OS: '$(uname -s)'"

ci/ci-test.bash

@@ -73,6 +73,7 @@ fi
 
 # Run the specified suite
 ccache -z
+TEST_START=$SECONDS
 case $OPT_SUITE in
   dist-vlt-0)
     "$MAKE" -C "$TEST_REGRESS" SCENARIOS="--dist --vlt --driver-clean" DRIVER_HASHSET=--hashset=0/4
@@ -163,4 +164,6 @@ case $OPT_SUITE in
     ;;
 esac
 ccache -svv
+ccache --evict-older-than "$((SECONDS - TEST_START + 60))s"
+ccache -svv
 uptime # To see load average