[Libclc-dev] Using clang + libclc to compile OpenCL kernels to ptx23

Tue Aug 25 09:59:53 PDT 2015

Hi Jeroen,

Thanks for answering my silly question.  I didn't realize that the backend
had changed.

Regards,
Alex

On Tue, Aug 25, 2015 at 9:51 AM, Jeroen Ketema <j.ketema at imperial.ac.uk>
wrote:

>
> On 25 Aug 2015, at 17:43, Alex Breslow <abreslow at cs.ucsd.edu> wrote:
>
> Hi Jeroen,
> Correct me if I am mistaken (there is a good chance of that :) ), but I
> think the issue here is that ptx32 and ptx23 appear to refer to completely
> different things.  ptx32 is the 32-bit version of the PTX assembly, and
> ptx23 appears to refer to the version of the PTX assembly from a feature
> point of view (see
> http://llvm.org/devmtg/2011-11/Holewinski_PTXBackend.pdf slide 9/37).
>
>
> No in the current backed ptx32 is an attribute like ptx23, that’s what the
> table I referred you to shows you. The architectures are called nvptx32 and
> nvptx64 nowadays (not ptx32 and ptx64).
>
> I'm a bit perplexed by all of this given that it appears that clang used
> to be able to compile OpenCL to ptx23.  For instance, I found some old
> slides by Justin Holewinski showing how one used to be able to compile to
> ptx23 circa 2011 (http://llvm.org/devmtg/2011-11/Holewinski_PTXBackend.pdf).
> Below is an example from slide 10/37:
>  *clang -ccc-host-triple ptx32 -Xclang -target-feature -Xclang +ptx23
> -Xclang -target-feature -Xclang +sm20 -I$LIBCLC/include/generic
> -I$LIBCLC/include/ptx  -include clc/clc.h
> -Dcl_clang_storage_class_specifiers -O3 SOURCE.cl -S*
>
>
> I tried generating the code this way, but it doesn't work for me.  Is this
> syntax out of date?  Are the slides referring to something else?  If not,
> what would be the equivalent for llvm 3.4?
>
>
> The slides are about the backend (ptx) that was there around the time the
> slides were written, and that backend has been completely replaced by a new
> one (nvptx). I don’t think the nvptx backend (which I was referring to)
> ever supported ptx23 as an attribute. I don’t know about the old ptx
> backend; I never used that.
>
> Jeroen
>
>
>
> -Alex
>
>
>
> On Mon, Aug 24, 2015 at 3:39 PM, Jeroen Ketema <j.ketema at imperial.ac.uk>
> wrote:
>
>>
>> Hi Alex,
>>
>> I think this question might be more appropriate for the cfe-dev list, but
>> to answer your question: looking at lib/Target/NVPTX/NVPTX.td and its
>> history, it seems that the llvm NVPTX backend at present (llvm trunk) only
>> supports ptx32 and up, and never supported anything below ptx30.
>>
>> Jeroen
>>
>> On 24 Aug 2015, at 23:24, Alex Breslow via Libclc-dev <
>> libclc-dev at lists.llvm.org> wrote:
>>
>> Hi there,
>>
>> I'm using clang to compile OpenCL kernels to PTX so that they can be run
>> on an architectural simulator.  The simulator is only compatible with ptx23
>> not ptx30 or ptx31.
>>
>> I'm compiling the kernels as follows:
>> *SRC=VectorAdd.cl*
>> *DEST=VectorAdd.ptx*
>>
>>
>> *clang -Dcl_clang_storage_class_specifiers -isystem
>> libclc/generic/include -include clc/clc.h -target nvptx-unknown-nvcl -xcl
>> $SRC -emit-llvm -S -o $DEST.ll*
>>
>> *opt -S -O3 -loop-unroll $DEST.ll -o $DEST.opt.ll*
>>
>> *llc -mcpu=sm_20 -mattr=+ptx30 $DEST.opt.ll -o $DEST"*
>>
>> The following code works to compile the kernel to ptx30, but when I
>> attempt to change -mattr=+ptx30 to -mattr=+ptx23, I get the following
>> warning:
>>
>> *'+ptx23' is not a recognized feature for this target (ignoring feature)*
>> *'+ptx23' is not a recognized feature for this target (ignoring feature)*
>>
>> Is there a way to around this warning so that ptx23 code is generated?
>>
>> I am using the Ubuntu 14.04 aptitude packages for libclc-dev, libclc-ptx,
>> clang, and llvm.
>> Ubuntu clang version 3.4-1ubuntu3 (tags/RELEASE_34/final) (based on LLVM
>> 3.4)
>>
>>
>> Thanks again for your time and let me know if you need further
>> information.
>>
>> Regards,
>> Alex
>>
>> --
>> Alex Breslow
>>
>> _______________________________________________
>> Libclc-dev mailing list
>> Libclc-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/libclc-dev
>>
>>
>>
>
>
> --
> Alex Breslow
> PhD student in computer science at UC San Diego
> Email: abreslow at cs.ucsd.edu
> Website: cseweb.ucsd.edu/~abreslow
>
>
>

-- 
Alex Breslow
PhD student in computer science at UC San Diego
Email: abreslow at cs.ucsd.edu
Website: cseweb.ucsd.edu/~abreslow
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