[LLVMdev] PTX backend, BSD license

Helge Rhodin helge.rhodin at alice-dsl.net
Sun Aug 15 15:40:01 PDT 2010


Chris Lattner wrote:
> On Aug 10, 2010, at 12:05 PM, David A. Greene wrote:
>>> 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.
> It needs to be code reviewed.  Please split it into reasonable sized chunks and get them reviewed.
> -Chris
Are there any volunteers out there? :) Thanks!

David A. Greene wrote:
> Do you generate masked operations?  If so, are you managing
> masks/predicates with your own target-specific representation _a_la_ the
> current ARM backend?
No, currently not. I only insert perdicates for the conditional branch 
implementation. But I don't think they are that important. A divergent 
branch(inside one warp) is more or less the same. Still it would be nice 
to have them and investicate the integration into LLVM.

[Villmow, Micah] From looking at the llvmptxbackend, it does not fully support vector types.
This in my perspective is one of the greatest benefits of the backend code-generator, automatic support
for vector types in LLVM-IR that are not natively supported by the target machine via vector splitting.

You are right, my backend only supports vector types for load, store, texture fetches and extract element instructions(every vector instructions PTX supports). Nothing like vector splitting is done.

Villmow, Micah wrote:
>>> 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. 
I agree with you that the target-independent code generator approach has 
some benefits(vector splitting, may be late-optimizations..) in 
comparison to my implementation. But it I think my approach is the 
simpler one (at least in the short run) and it is further progressed. 
What do you think, does it make sense to put more work into my 
backend(code review..)? The backend already gives us the opportunity to 
develope and test new GPU related optimizations and improve 
LLVM(predicates) and Clang(address spaces). From my point of view the 
higher level optimizations, like choosing the "right" address space, are 
more intresting and profitable. Such optimizations are not dependent on 
the backend implementation and could seamlessly be used with the new ptx 
backend once it is finished.


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