gitlab: enable ccache for many build jobs

The `ccache` tool can be very effective at reducing compilation times
when re-running pipelines with only minor changes each time. For example
a fresh 'build-system-fedora' job will typically take 20 minutes on the
gitlab.com shared runners. With ccache this is reduced to as little as
6 minutes.

Normally meson would auto-detect existance of ccache in $PATH and use
it automatically, but the way we wrap meson from configure breaks this,
as we're passing in an config file with explicitly set compiler paths.
Thus we need to add $CCACHE_WRAPPERSPATH to the front of $PATH. For
unknown reasons if doing this in msys though, gcc becomes unable to
invoke 'cc1' when run from meson. For msys we thus set CC='ccache gcc'
before invoking 'configure' instead.

A second problem with msys is that cache misses are incredibly
expensive, so enabling ccache massively slows down the build when
the cache isn't well populated. This is suspected to be a result of
the cost of spawning processes under the msys architecture. To deal
with this we set CCACHE_DEPEND=1 which enables ccache's 'depend_only'
strategy. This avoids extra spawning of the pre-processor during
cache misses, with the downside that is it less likely ccache will
find a cache hit after semantically benign compiler flag changes.
This is the lesser of two evils, as otherwise we can't use ccache
at all under msys and remain inside the job time limit.

If people are finding ccache to hurt their pipelines, it can be
disabled by setting the 'CCACHE_DISABLE=1' env variable against
their gitlab fork CI settings.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230804111054.281802-2-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-2-alex.bennee@linaro.org>
diff --git a/.gitlab-ci.d/crossbuild-template.yml b/.gitlab-ci.d/crossbuild-template.yml
index d976110..3e5f4d9 100644
--- a/.gitlab-ci.d/crossbuild-template.yml
+++ b/.gitlab-ci.d/crossbuild-template.yml
@@ -2,10 +2,20 @@
   extends: .base_job_template
   stage: build
   image: $CI_REGISTRY_IMAGE/qemu/$IMAGE:$QEMU_CI_CONTAINER_TAG
+  cache:
+    paths:
+      - ccache
+    key: "$CI_JOB_NAME"
+    when: always
   timeout: 80m
   script:
+    - export CCACHE_BASEDIR="$(pwd)"
+    - export CCACHE_DIR="$CCACHE_BASEDIR/ccache"
+    - export CCACHE_MAXSIZE="500M"
+    - export PATH="$CCACHE_WRAPPERSDIR:$PATH"
     - mkdir build
     - cd build
+    - ccache --zero-stats
     - ../configure --enable-werror --disable-docs --enable-fdt=system
         --disable-user $QEMU_CONFIGURE_OPTS $EXTRA_CONFIGURE_OPTS
         --target-list-exclude="arm-softmmu cris-softmmu
@@ -18,6 +28,7 @@
       version="$(git describe --match v[0-9]* 2>/dev/null || git rev-parse --short HEAD)";
       mv -v qemu-setup*.exe qemu-setup-${version}.exe;
       fi
+    - ccache --show-stats
 
 # Job to cross-build specific accelerators.
 #
@@ -29,7 +40,15 @@
   stage: build
   image: $CI_REGISTRY_IMAGE/qemu/$IMAGE:$QEMU_CI_CONTAINER_TAG
   timeout: 30m
+  cache:
+    paths:
+      - ccache/
+    key: "$CI_JOB_NAME"
   script:
+    - export CCACHE_BASEDIR="$(pwd)"
+    - export CCACHE_DIR="$CCACHE_BASEDIR/ccache"
+    - export CCACHE_MAXSIZE="500M"
+    - export PATH="$CCACHE_WRAPPERSDIR:$PATH"
     - mkdir build
     - cd build
     - ../configure --enable-werror --disable-docs $QEMU_CONFIGURE_OPTS
@@ -40,7 +59,14 @@
   extends: .base_job_template
   stage: build
   image: $CI_REGISTRY_IMAGE/qemu/$IMAGE:$QEMU_CI_CONTAINER_TAG
+  cache:
+    paths:
+      - ccache/
+    key: "$CI_JOB_NAME"
   script:
+    - export CCACHE_BASEDIR="$(pwd)"
+    - export CCACHE_DIR="$CCACHE_BASEDIR/ccache"
+    - export CCACHE_MAXSIZE="500M"
     - mkdir build
     - cd build
     - ../configure --enable-werror --disable-docs $QEMU_CONFIGURE_OPTS