[Buildroot] ccache problem when compiling with different toolchains in different projects

[Mathias De Maré]

On Sat, Apr 20, 2013 at 6:08 PM, Danomi Manchego <
danomimanchego123 at gmail.com> wrote:

> On Fri, Apr 19, 2013 at 7:41 PM, Arnout Vandecappelle <arnout at mind.be>wrote:
>
>  Then, the effects of all the
>>> hidden variables are in some way accounted for without grepping for them
>>> in the .config.
>>>
>>
>>  Not entirely. Many toolchain options affect the C library, not the
>> compiler. A change in these options may not affect the gcc binary itself.
>> Even a gcc configuration option may not change the gcc binary, because most
>> of gcc is in cc1 and libgcc.a. But it's still an improvement over previous
>> proposals.
>>
>
> Arnout, I'm wavering on this.
>
> At work, we only use external toolchains (downloaded or pre-installed), so
> using the gcc -v output and the md5sum of the compiler/wrapper provided to
> ccache is perfect for our projects, because these discriminators capture
> everything.  But if toolchain configuration options in the general case
> (for everyone else) cannot be simply captured (without doing lots of
> variable renaming as you mentioned earlier), then it seems to me that
> buildroot's official position must remain that the user must know when to
> flush the cache, and do it manually.  What was not clear to me before is
> that I have to not only keep in mind my build configuration, but also the
> configuration of every other buildroot build on the machine, whether it's
> mine or somebody else's.  In light of that, would it not be better to limit
> the scope of self-policing to just the area you are working in, by
> including absolute paths (in the COMPILERCHECK, or HASHDIR) to disable
> sharing cached objects when you change output directories, or between
> several developers?  It's bad enough to have to remember to flush the cache
> based our our own activities, but it truly stinks to have to defend against
> getting screwed up by somebody else's build.
>
> I'm just trying to think this through from a community perspective ...
>

Very good point, Danomi.

One of the most important points about ccache is it being safe (no
incorrect behaviour) *. If this is not possible, it should at least be 'as
safe as possible', so be separate per buildroot build. It would be nice if
it were possible though: it could result in a big gain.

*Note: this is also why I, for a large project, use 'CCACHE_NODIRECT'. See
also: https://bugzilla.samba.org/show_bug.cgi?id=8424

Greetings,
Mathias
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