Re: [Qemu-devel] [PATCH v3 4/4] configure: Add workaround for ccache and clang

[John Snow <jsnow@redhat.com>]

On 03/23/2015 09:11 AM, Peter Maydell wrote:
> On 19 March 2015 at 19:01, John Snow <jsnow@redhat.com> wrote:
>> Test if ccache is interfering with our life, and
>> disable its habit of trying to compile already pre-processed
>> versions of code if so.
>>
>> In particular, clang has different semantic warnings based on
>> if the warning arose from a macro or not. By trying to build
>> preprocessed versions of code, we get more errors than we should.
>>
>> ccache allows us to disable this feature, opting instead to
>> compile the original version instead of its preprocessed version.
>>
>> This makes ccache much slower for cache misses, but at least it
>> becomes usable with QEMU/clang.
>>
>> Thanks to Peter Eisentraut for his writeup on the issue:
>> http://peter.eisentraut.org/blog/2014/12/01/ccache-and-clang-part-3/
>
> This is really working around a bug in either ccache or
> in the way Fedora has configured ccache, so I kind of
> feel it ought to be dealt with there. However I don't
> object too much to our including the workaround in our
> configure...
>
> -- PMM
>

I feel like it might be an inescapable consequence of using both ccache 
and clang together on any system, not just Fedora.

This at least ONLY turns on the workaround if it is observed to 
interfering, so it should improve the "ease of build" for clang + ccache 
in general, and not just on Fedora.

Of course, even if it is a workaround that only benefits out of the box 
configurations on Fedora, It's still useful to be able to build from 
source on Fedora, instead of waiting for a package maintainer to update ...

So unless there's a strong reason against this workaround in particular, 
I think it is harmless enough for inclusion. It certainly won't make the 
build worse for anyone.

For cases where you might be using ccache and gcc, for instance, this 
workaround won't even trigger. Further good news: We don't suppress any 
extra warnings we don't have to. Instead, we just tell ccache to 
re-process a file instead of trying to compile the preprocessed version.

--js