[LAD] Project proposition: llvm based dsp engine

Stéphane Letz letz at grame.fr
Tue Dec 7 13:58:57 UTC 2010

> Message: 3
> Date: Mon, 06 Dec 2010 14:11:07 +0100
> From: Maurizio De Cecco <jmax at dececco.name>
> Subject: Re: [LAD] Project proposition: llvm based dsp engine
> To: Paul Davis <paul at linuxaudiosystems.com>
> Cc: The Linux Audio Developers' Mailing List
> 	<linux-audio-dev at lists.linuxaudio.org>, llvmdev at cs.uiuc.edu
> Message-ID: <1291641067-065c11f3c738513db43420e758ad7e5d at dececco.name>
> Content-Type: text/plain; charset="iso-8859-1"
> Hi, and thanks for the feedback.
>> My own gut feeling on this is that you'd be better off figuring out
>> how to do this in the context of Faust, which already does a important
>> subset of what you are describing, though notably without (I think)
>> the LLVM part.
> Surely a contribution to the discussion by the Faust people would
> be welcome; and even more than to the discussion :->. By the way, they
> do something with LLVM, but i do not know exactly what (Faust is included
> in the list of projects using LLVM).
> I think the interest, if any, would be in having a *low* level
> library generic enough to be used by multiple applications, so
> to factor the work. It may be that an existing project have internally
> a large part of the needed code, though.
>> Note also that hosts which run plugins at the level of LADSPA/LV2,
>> VST, AU, DSSI etc are unlikely to be easy candidates for any
>> cross-plugin optimization.
> I was actually talking about two different context, reusing existing
> binaries, and having cross plugin optimisations for native plugins.
> But, if the plugins are compiled with a different compilation chain
> (using the LLVM compiler producing LLVM byte code, so introducing a different
> binary format for them), than cross plugin optimisation is possible also for 
> existing plugins like a LADSPA plugin.
> Would that be worthwhile ? Difficult to know for sure, but it is an interesting
> subject to explore.
> Maurizio

I'm quite dubious of any real optimization you would achieve with this kind of approach, using LLVM. AFAIKS LLVM link time optimization allows some kind of inter module inlining or possibly constant propagation (see http://llvm.org/docs/LinkTimeOptimization.html) but I am not sure it would help a lot in our domain.

Again staying in the imperative world makes things much more complicated... go functional!


PS: BTW LLVM is a really nice tool and very interesting things can be done with it, like using LLVM bitcode format as an interchange format for different architectures and using the JIT for dynamic compilation. We can already do that with Faust LLVM backend where we can generate a bitcode binary, then load it and JIT with a generic "LLVM faust plug-in" loader. This would allow the same bitcode object to be produced by the Faust online compiler for instance and be executed on any target machine.

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