path: root/docs/manual/ccache-support.txt
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authorGravatar Arnout Vandecappelle <arnout@mind.be>2015-10-04 16:25:00 +0100
committerGravatar Peter Korsgaard <peter@korsgaard.com>2015-10-04 18:22:21 +0200
commitf4682cf933a3e0e1017aa8827a8c57fdd79e30f3 (patch)
tree716b3bfe3d541fc5ccbe80b9811d15c581582478 /docs/manual/ccache-support.txt
parent792f1278e3fbc165086aebb8c07cfd18e10f374b (diff)
ccache: use mtime for external toolchain, CONF_OPTS for internal toolchain
Our current ccache disables hashing of the compiler executable itself, because using the default 'mtime' doesn't work in buildroot: we always rebuild the compiler, so the mtime is always different, so the cache always misses. However, in the current situation, if a user changes the compiler configuration (which would result in the compiler generating different object files than before) and does 'make clean all', ccache may in fact reuse object files from the previous run. This rarely gives problems, because (1) the cache expires quite quickly (it's only 1GB by default), (2) radically changing compiler options will cause cache misses because different header files are used, (3) many compiler changes (e.g. changing -mtune) have little practical effect because the resulting code is usually still compatible, (4) we currently don't use CCACHE_BASEDIR, and almost all object files will contain an absolute path (e.g. in debug info), so when building in a different directory, most of it will miss, (5) we do mostly build test, and many of the potential problems only appear at runtime. Still, when ccache _does_ use the wrong cached object files, the effects are really weird and hard to debug. Also, we want reproducible builds and obviously the above makes builds non-reproducible. So we have a FAQ entry that warns against using ccache and tells the user to clear the cache in case of problems. Now that ccache is called from the toolchain wrapper, it is in fact possible to at least use the 'mtime' compiler hash for the external toolchain and for the host-gcc. Indeed, in this case, the compiler executable comes from a tarball so the mtime will be a good reference for its state. Therefore, the patch (sed script) that changes the default from 'mtime' to 'none' is removed. For the internal toolchain, we can do better by providing a hash of the relevant toolchain options. We are only interested in things that affect the compiler itself, because ccache also processes the header files and it doesn't look at libraries because it doesn't cache the link step, just compilation. Everything that affects the compiler itself can nicely be summarised in $(HOST_GCC_FINAL_CONF_OPTS). Of course, also the compiler source itself is relevant, so the source tarball and all the patches are included in the hash. For this purpose, a new HOST_GCC_XTENSA_OVERLAY_TAR is introduced. The following procedure tests the ccache behaviour: Use this defconfig: BR2_arm=y BR2_CCACHE=y make readelf -A output/build/uclibc-1.0.6/libc/signal/signal.os -> Tag_CPU_name: "ARM926EJ-S" Now make menuconfig, change variant into BR2_cortex_a9 make clean; make readelf -A output/build/uclibc-1.0.6/libc/signal/signal.os -> Tag_CPU_name: "ARM926EJ-S" should be "Cortex-A9" After this commit, it is "Cortex-A9". Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be> Cc: Danomi Manchego <danomimanchego123@gmail.com> Cc: Károly Kasza <kaszak@gmail.com> Cc: Samuel Martin <s.martin49@gmail.com> Cc: Romain Naour <romain.naour@openwide.fr> Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
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