[Mesa-dev] [RFC PATCH 00/16] A new IR for Mesa

[Greg Fischer]

The GlassyMesa effort is ongoing despite the lack of recent activity. We
continue to embrace LLVM as a basis for shader compilation in Mesa and
elsewhere.

We agree that translating from LLVM back "up" to GLSL IR is problematic and
that an architecture that supports LLVM backends would be desirable.

The current published code is a work-in-progress. We are currently working
on tuning of compilation time and code generation quality, and some of that
work will likely be appearing soon. There have also been recent
improvements in glslang, the parser for this stack.

Packaging and ABI stability have not been an issue for us.

Best regards,
Greg




On Tue, Aug 26, 2014 at 2:34 PM, Thomas Helland <thomashelland90 at gmail.com>
wrote:

> While I haven't heard about those projects, there's also GlassyMesa.
> Greg from LunarG (CC'd) posted about this to the mailing list. [1]
> However it looks like the github activity has stopped,
> and there's no new info on the projects website since its announcement.
>
> While it's not exactly the same as we're describing here,
> maybe he can share some on their experiences?
> Status of the project? Performance numbers?
> Packaging issues, ABI stability, etc?
>
> FWIW, I would prefer a world without LLVM,
> as long as there's not a big performance per man-hour
> benefit compared to rolling our own solution.
> LLVM brings extra complexity to debugging and bisecting,
> and if we want help from the users to bisect bugs it
> should be as easy as absolutely possible.
>
> [1] http://lists.freedesktop.org/archives/mesa-dev/2014-June/060951.html
>
>
> 2014-08-26 19:59 GMT+02:00 Matt Turner <mattst88 at gmail.com>:
> > On Tue, Aug 26, 2014 at 9:00 AM, Jose Fonseca <jfonseca at vmware.com>
> wrote:
> >> If LLVM was a useless piece of junk, we wouldn't have any trouble
> adding it
> >> as a dependency, as we would be the sole user.  But precisely because
> LLVM
> >> is successful in so many use cases, hence several packages depend on
> it, we
> >> shouldn't depend, so we can avoid the dealing with the oh-so-hard
> dependency
> >> issue!?  I find it ridiculous: it's precisely because LLVM is
> potentially
> >> that good that it makes sense for us/distros/everybody to put up with
> the
> >> dependencies issues it may bring, and worth considering.
> >
> > It sounds like there are enough people in the Mesa community that are
> > familiar with LLVM and interested in using it in the GLSL compiler
> > that there would be someone willing to start working on it. Hopefully
> > that's the case.
> >
> > I tried going through the LLVM language frontend tutorial on LLVM.org
> > and only had to get to chapter 4 (the first two chapters don't use
> > LLVM) before the code didn't compile (and I couldn't figure out why)
> > with LLVM 3.4 on my system. I found this [1] tiny example (not updated
> > for 2 years) using the C API and thought it'd probably not work
> > either, but was happily surprised that it compiled and worked fine. I
> > see that the C API is used in radeonsi and gallivm in Mesa.
> >
> > Here's what I think would be compelling: Using the stable C API,
> > translate from GLSL IR into LLVM IR. Call LLVM's optimizations. Give
> > backends the option to consume LLVM IR. From here we can evaluate just
> > how significant the improvement from LLVM's optimizer is. At least two
> > people have written GLSL IR -> LLVM translators in the past -- Luca
> > Barbieri (what ever happened to him?!) and Vincent Lejeune (Cc'd).
> > Their code is [2] and [3]. I think this plan would also fit nicely
> > with existing LLVM backends, potentially avoiding a trip through TGSI.
> >
> > I think this is strictly superior to other ideas like throwing out the
> > GLSL frontend and translating LLVM IR back "up" to the higher level
> > GLSL IR.
> >
> > So, maybe some people experienced with LLVM from VMware and AMD are
> > interested in taking the lead?
> >
> > [1] https://github.com/wickedchicken/llvm-c-example
> > [2] http://cgit.freedesktop.org/~anholt/mesa/log/?h=llvm-4 (Eric's
> > branch based on Luca's code)
> > [3] http://cgit.freedesktop.org/~vlj/mesa/ (one of the glsl-to-llvm*
> branches)
>
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