[LLVMdev] PTX backend, BSD license

[Villmow, Micah]

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of David A. Greene
> Sent: Tuesday, August 10, 2010 12:05 PM
> To: Helge Rhodin
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] PTX backend, BSD license
> 
> Helge Rhodin <helge.rhodin at alice-dsl.net> writes:
> 
> >> But I didn't study their code thoroughly, so I might be wrong about
> this.
> >>
> > Yes, we don't use the target-independent code generator and the
> > backend is based on the CBackend.  We decided to not use the code
> > generator because PTX code is also an intermediate language. The
> > graphics driver contains a compiler which compiles PTX code to
> machine
> > code targeting a particular GPU architecture. It performs register
> > allocation, instruction scheduling, dead-code elimination, and other
> > late optimizations. Thus we don't need most of the target-independent
> > code generator features in the PTXBackend.
> 
> Some of these could still be useful to aid the NVIDIA compiler.  But I
> don't have any hard data to support that assertion.  :)
[Villmow, Micah] For the AMD backend that I work on, having these turned on are invaluable. If the NVIDIA compiler is anything like the ATI graphics compiler, it is written for speed and assumes smaller graphics kernels, but with more generic compute kernels, doing some preliminary optimizations/scheduling/allocation helps generate better code. 
> 
> > We already support most of the PTX instruction set. Texture lookup,
> > structs&arrays, function calls, vector types, different address
> spaces
> > and many intrinsics.
> 
> Do you generate masked operations?  If so, are you managing
> masks/predicates with your own target-specific representation _a_la_
> the
> current ARM backend?
> 
> >> If they are unrelated, can you do a comparison of the two?  Perhaps
> >> there are holes in each that can be filled by the other.  It would
> be
> >> a shame to have two completely different PTX backends.
> >>
> > I don't know much about the target-independent code generator but I
> > think we use distinct approaches which cannot
> > be merged in a reasonable way. Probably both approaches have their
> own
> > pros and cons.
> 
> Certainly.
> 
> >> Is there work to upstream this?  I've got a relatively unused NVIDIA
> >> card at home.   :)
> >>
> >>                                  -Dave
> >>
> > The PTXBackend probably needs more test cases. I'm currently covering
> a
> > lot of  LLVM and PTX features but the test suite is still not
> exhaustive.
> > I took the coding standards into account and the license is now
> > compatible to LLVM. I don't know what else needs to be done?
> 
> Checking it in.  :) Really, we probably should do some sort of code
> review, but Chris would have to indicate what he wants.
> 
>                             -Dave
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