Re: [PATCH v2 01/12] gitlab: enable ccache for many build jobs

[Philippe Mathieu-Daudé]

On 25/8/23 09:46, Michael Tokarev wrote:
> 24.08.2023 19:38, Alex Bennée wrote:
> > From: Daniel P. Berrangé <berrange@redhat.com>
> >
> > 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.
>
> I've been using ccache when building qemu in debian, for quite a while.
>
> The problem here, in the way qemu build system works, is that the cache
> is hugely dependent on the path to the source. You change just one char
> in there (/build/qemu/v8.1.0/ => /build/qemu/v8.1.1) and whole cache 
> becomes
> unusable, it all gets compiled anew.  This is because qemu build sys uses
> absolute file names when building, and this is detected by ccache, so
> the source dir gets mixed into the hash together with gcc version and
> other things.

__FILE__ is used by assert() family, some DEBUG_PRINTF(), but mainly
by "qapi/error.h", so all error_setg*() calls.

This has been bugging me since quite some time, since if you build
the same QEMU in different paths (usually on different machines) then
the output doesn't match.

GCC 8 & Clang 10 provides -ffile-prefix-map, but

1/ Our minimal GCC supported is v7.4,
2/ meson doesn't support it. Still there is a feature request:
   https://github.com/mesonbuild/meson/issues/10533


For more info, see also the reproducible build project:
https://reproducible-builds.org/docs/build-path/

Regards,

Phil.