Hi,
I am trying to use FP16 (half-precision) with pycuda. However I have
encountered an issue when trying to use this for element-wise kernels.
If I try a very simple kernel:
==============================
import numpy as np
import pycuda.autoinit
from pycuda.elementwise import ElementwiseKernel as CU_ElK
import pycuda.gpuarray as cua
cu_options = ['-use_fast_math', '-D__CUDA_NO_HALF_OPERATORS__',
'-D__CUDA_NO_HALF2_OPERATORS__']
testk = CU_ElK(name='testk', operation="d[i] *= 2", preamble='#include
<cuda_fp16.h>',
options=cu_options, arguments="float *d")
cu_d = cua.empty(128, dtype=np.float32)
testk(cu_d)
==============================
(the kernel does not even use half-precision, only the fp16 header is necessary
to trigger the issue)
This works on MacOS (it only requires the D__CUDA_NO_HALF_OPERATORS__
to avoid multiple linkage), but on debian9 and Ubuntu20 it fails with a bunch
of errors like:
...
/usr/include/c++/8/bits/stl_pair.h(446): error: this declaration may not have
extern "C" linkage
…
which come from the cuda_fp16.h using STL headers (std::move etc..).
This is due to the kernel being compiled with an ‘extern “C”’
directive, which is necessary to avoid C++ name mangling and still be able to
access the element wise kernel function.
The workaround is to include the cuda_fp16.h header _before_ the
‘extern “C”’ - I’ve tested this and that runs without a hitch.
So my question is how to proceed - I’d like as much as possible to
directly use pycuda without having to write a derived version of SourceModule
and the element-wise code.
I see two options:
1) if there is a way to have an element-wise kernel with no_extern_c=True - but
I don’t know how to resolve the name mangling issue to access the kernel
function ?
2) add a ‘cpp_preamble’ option to SourceModule and ElementwiseKernel (and
others) to add a preamble before the ‘extern “C”’
I could propose a PR for 2) but I’d like to know if that’d be
acceptable in pycuda. Note that it also removes the need for
D__CUDA_NO_HALF_OPERATORS__
Thanks,
Vincent
—
Vincent Favre-Nicolin
Co-editor, J. Synchrotron Radiation http://journals.iucr.org/s/
<http://journals.iucr.org/s/>
Director, HERCULES school http://hercules-school.eu
<http://hercules-school.eu/>
ESRF-The European Synchrotron http://www.esrf.eu <http://www.esrf.eu/>
71, Avenue des Martyrs
Grenoble, France
X-Ray NanoProbe (XNP) group
Tel: +33 4 76 88 28 11
On leave from Univ. Grenoble Alpes
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