[Mesa-dev] Proposal for a long-term shader compiler (and IR) architecture

[Jerome Glisse]

On Fri, Oct 15, 2010 at 7:44 PM, John Kessenich <johnk at lunarg.com> wrote:
> Hi,
> LunarG has decided to work on an open source, long-term, highly-functional,
> and modular shader and kernel compiler stack. Attached is our high-level
> proposal for this compiler architecture (LunarGLASS).  We would like to
> solicit feedback from the open source community on doing this.
> I have read several posts here where it seems the time has come for
> something like this, and in that spirit, I hope this is consistent with the
> desire and direction many contributors to this list have already alluded to.
> Perhaps the biggest point of the proposal is to standardize on LLVM as an
> intermediate representation.  This is actually done at two levels within the
> proposal; one at a high-level IR close to the source language and one at a
> low-level IR close to the target architecture.  The full picture is in the
> attached document.
> Based on feedback to this proposal, our next step is to more precisely
> define the two forms of LLVM IR.
> Please let me know if you have any trouble reading the attached, or any
> questions, or any feedback regarding the proposal.
> Thanks,
> JohnK


Just a quick reply (i won't have carefully read through this proposition before
couple weeks) last time i check LLVM didn't seemed to fit the bill for GPU,
newer GPU can be seen as close to scalar but not completely, there are
restriction on instruction packing and the amount of data computation
unit of gpu can access per cycle, also register allocation is different
from normal CPU, you don't wan to do register peeling on GPU. So from
my POV instruction scheduling & packing and register allocation are
interlace process (where you store variable impact instruction packing).
Also on newer gpu it makes sense to use a mixed scalar/vector representation
to preserve things like dot product. Last loop, jump, function have kind
of unsual restriction unlike any CPU (thought i haven't broad CPU knowledge)

Bottom line is i don't think LLVM is anywhere near what would help us.

Cheers,
Jerome