[llvm-dev] cuda __shfl_sync problem

[Johannes Doerfert via llvm-dev]

Not that I am an expert but it looks like it defaults to the minimal PTX 
version that supports the compute capability. You might be able to 
choose PTX 6.0 though.

~ Johannes


On 9/24/20 1:02 PM, George K via llvm-dev wrote:
> Hi,
>
> First of all, i'm not sure if i should be posting this here or in 
> cfe-dev, but here it goes.
>
> In order to instrument CUDA kernels i first generate device IR with:
>
> clang++ -x cuda --cuda-device-only -emit-llvm --cuda-gpu-arch=sm_52 -o 
> device.bc
>
> I also have a library that contains the instrumentation stubs for 
> which i generate IR similarly and i link it with the device IR 
> programmatically with Linker::linkModules(..)
>
> Then after some analysis i use llc to get ptx:
>
> llc device.bc --march=nvptx64 --mcpu=sm_52 --filetype=asm -o device.ptx
>
> This works fine but the problem is that the instrumentation code uses 
> __shfl_sync() and ptxas gives me the following error:
>
> ptxas device.ptx, line 1033; error   : Feature 'shfl.sync' requires 
> PTX ISA .version 6.0 or later
>
> Now according to 
> https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#warp-shuffle-functions,
>
> __shfl_sync is supported by compute capability >= 3 and according to 
> https://developer.nvidia.com/cuda-gpus#compute my GTX950 has Compute 
> Capability 5.2.
>
> Also according to 
> https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#release-notes 
> PTX ISA 6.0 does support sm_52.
>
> However llc generates:
>
> .version 4.1
> .target sm_52, debug
> .address_size 64
>
> Any ideas why this is happening? Or am i doing something wrong?
>
> PS. I'm using CUDA 10, driver 440
>
> ~George
>
>
>
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