Re: [Qemu-devel] converting build system to Meson?

From: Paolo Bonzini <pbonzini@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: qemu-devel <qemu-devel@nongnu.org>,
	"Peter Maydell" <peter.maydell@linaro.org>,
	"Thomas Huth" <thuth@redhat.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Richard Henderson" <rth@twiddle.net>
Subject: Re: [Qemu-devel] converting build system to Meson?
Date: Thu, 18 Apr 2019 10:35:36 +0200	[thread overview]
Message-ID: <1ae50a8a-b669-db47-f1e6-995097c556f3@redhat.com> (raw)
In-Reply-To: <87v9zbpvtk.fsf@dusky.pond.sub.org>

On 18/04/19 10:21, Markus Armbruster wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> Hi all,
>>
>> lately I have been thinking of converting the QEMU build system to
>> Meson.  Meson is a relatively new build system that can replace
>> Autotools or hand-written Makefiles such as QEMU; as a die-hard
>> Autotools fan, I must say that Meson is by far better than anything else
>> that has ever tried to replace Autotools, and actually has the potential
>> to do so.
>>
>> Advantages of Meson that directly matter for QEMU include:
>>
>> - build definitions in a very readable and user friendly DSL, which
>> supports looping and conditions.
>>
>> - ability to introspect the build definitions so that you can find out
>> what is built without building it (the DSL is not Turing complete and
>> most objects in it are immutable, so it cannot be abused that much :))
>>
>> - support for a non-recursive build from per-subdirectory input (similar
>> to Makefile.objs)
> 
> Could Meson build the $(TARGET_DIRS) non-recursively?

Yes.  It doesn't support recursive builds at all, in fact.

If you can stand the C++, you can find a "testcase slash example with
obvious QEMU inspiration" at
https://github.com/mesonbuild/meson/pull/5028/commits/9495ea241aaca5c2242dac1654ef3168bee1657a.
 In fact, that pull request is basically the last obvious missing piece
before one could seriously try Meson for QEMU.  Then of course there are
the non-obvious missing pieces, but those are more bugfixes than
anything else.

Paolo

> 
> I'm asking because I find the make recursion there annoying.  As usual
> with recursion, we have to dumb down dependencies: if anything made by
> the recursion needs X, then everything does.  If making X fails, we
> don't recurse.  Defeats -k.
> 
>> - ease of distributing a full copy of Meson to support distros that ship
>> an older version (no dependencies apart from Python 3.5).  At 40000
>> lines of Python, Meson is relatively small.
> 
> [...]
> 