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Calling OpenCL from Octave / Matlab

From: andrew cooke <andrew@...>

Date: Wed, 9 Dec 2009 11:42:22 -0300

[I am optimising some Matlab code but, because I don't have a Matlab
licence on my local dev machine, am using Octave.  So what follows is
a bit odd, because I am using Octave, but via the Matlab-compatible
tools.  That doesn't affect the main points, though...]

Calling OpenCL from Octave / Matlab turns out to be (at least in a
simple case) very similar to calling C.  The main issues are (1) C/C++
interop and (2) libraries.


Currently the Octave code calls a C routine which, in turn, calls the
OpenCL C++ code.  The OpenCL C++ code is compiled into a library
(called "support" below).  I used this approach because I already knew
Octave (+ Matlab) can call C, and wasn't sure about C++.  So this
moves worries about C/C++ interop to the C level (where I know how to
deal with them) rather than the Octave/Matlab level (where live might
be more complicated).  In other words (to labour the point) I am using
C as the main interface to Octave / Matlab, not C++.

So the script to build the code looks like this:

g++ -fpic -Wall
-I/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/OpenCL/common/inc
-I/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/shared/inc -o support.o -c
support.cpp
gcc -shared -Wl,-soname,libsupport.so -olibsupport.so support.o

gcc -o main main.c -L. -lsupport -L/usr/lib64/
-L/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/OpenCL/common//lib
-L/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/shared/lib/
-L/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/shared/lib/linux -lOpenCL
-loclUtil -lshrutil -lstdc++

mkoctfile -v --mex singlec5.c -L. -lsupport -L/usr/lib64/
-L/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/OpenCL/common//lib
-L/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/shared/lib/
-L/home/andrew/pkg/NVIDIA_GPU_Computing_SDK/shared/lib/linux -lOpenCL
-loclUtil -lshrutil -lstdc++

(note the use of fpic - more on that below; also my toolkit is
installed in a non-standard location).  The "main" above is a separate
C program that calls the OpenCL routines.  Getting this working first
allowed me to fix basic errors without worrying about Octave.

The file singlec5.c is the "mex" file that handles the call from
Octave, unpacks/packs data structures and calls the OpenCl code in the
support library.


All the above is pretty normal, I think.  However, it won't work "out
of the box" because it requires that the support libraries (provided
by NVidia and used in their demos, which I based my code on) are
compiled only as ".a" (archive) libraries and not ".so" (shared
object) (this is on x86-64 Linux (OpenSuse 11.1)).

To build the libraries as shared objects, modify the following two
files so that "-fpic" is included in CXX and CC as well as LINK:

NVIDIA_GPU_Computing_SDK/OpenCL/common/common_opencl.mk
NVIDIA_GPU_Computing_SDK/shared/common.mk

Then rebuild the libraries:

cd NVIDIA_GPU_Computing_SDK/OpenCL/common/obj/release
gcc -shared -Wl,-soname,liboclUtil.so -oliboclUtil.so oclUtils.cpp.o
mv liboclUtil.so ../../../common/lib/

cd NVIDIA_GPU_Computing_SDK/shared/obj/release
gcc -shared -Wl,-soname,libshrUtil.so -olibshrUtil.so shrUtils.cpp.o
mv libshrUtil.so ../../lib/

Finally, the LD_LIBRARY_PATH should include

NVIDIA_GPU_Computing_SDK/shared/lib
NVIDIA_GPU_Computing_SDK/OpenCL/common/lib

(Paths will need to be changed as appropriate - I hope that's obvious....!)

Andrew

Extern C

From: andrew cooke <andrew@...>

Date: Wed, 9 Dec 2009 11:53:38 -0300

I should also have added, although I guess it's obvious, that the
header file for support.cpp, support.h, includes

  extern "C"

before each function declaration.  This lets C code link to the C++
routines (it removes the C++ name mangling, and may do other things I
do not understand).

Andrew

Or Simply Don't Use The Libs

From: andrew cooke <andrew@...>

Date: Wed, 16 Dec 2009 11:53:00 -0300

Hmmm.  On another machine, working with Matlab rather than Octave, and
with CentOS rather than OpenSuse, the above failed to fix things.

So I revised my code.  For now I have changed the logging to use
printf().  Apart from that, the only critical support code used was
oclLoadProgSource.  That just reads the source into memory.  Reading
files in C is tedious, but not impossible; here's a replacement:

    FILE *file;
    size_t blockAvailable = 512;
    size_t blockRead;
    char *kernelSource = NULL;
    size_t kernelLength = 0;
    size_t currentSize = 0;

    printf("Loading %s\n", kernelPath);
    file = fopen(kernelPath, "r");
    if (! file) {
        printf("Could not find kernel\n");
        err = 1;
        goto cleanup;
    }

    blockRead = blockAvailable;
    while (blockRead == blockAvailable) {
        blockAvailable *= 2;
        currentSize += blockAvailable;
        kernelSource = (char *)realloc(kernelSource, currentSize);
        blockRead = fread(kernelSource + kernelLength, sizeof(char),
                          blockAvailable, file);
        kernelLength += blockRead;
    }
    *(kernelSource + kernelLength + 1) = 0;
    printf(kernelSource);

You could add more checks (eg for realloc and fread) to handle more
errors, but the above works for now (this is very much proof of
concept code).

Andrew

More On OpenCL and Matlab Here

From: andrew cooke <andrew@...>

Date: Thu, 17 Dec 2009 19:47:47 -0300

http://www.acooke.org/cute/Experience0.html

Andrew

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