# Issue with cudaMemcpyToSymbol and Separable Compilation.

Hi,

I have discovered a problem with the cudaMemcpyToSymbol function and separable compilation when an executable is defined as a .cu file. I am using CMake to build my program.

Our application has a static initialized object which initializes the CUDA library and copies a constant to the constant memory cache as a cuComplex. This is occurring in the “Constants.cu” file listed below.

However, when the executable is defined as a .cu I am getting a CUDA error which states “invalid device symbol”. This issue does NOT occur if I make the executable a .cpp file. As every single file in our application goes through this startup process, I would like to allow .cu executables to be built, but at the moment I cannot allow this due this problem.

The static initialization code is defined in a static library which is linked into a shared library via separable compilation. The non functioning code has been placed inside either the static library or the shared library and works properly in neither of them.

Why is this not working when an executable is defined as a .cu file in CMake?

The code with non-relevant portions stripped out:

Constants.hpp

#include <cmath>
#include <complex>

#ifdef __CUDA_ARCH__
#include <cuComplex.h>
#else
#include <complex>
#endif

#include <DEWM/cuda/CudaInterface.hpp>

#ifndef DEWM_Constants_INC
#define DEWM_Constants_INC

namespace dewm
{
/*!< Average dielectric constant for polarization computation. Obtained from "Radar Reflectivity of Land and Sea".
Currently using the value stated for 'average land' of 10 for epsilon prime. Epsilon double prime isn't given, so a value was used inbetween moist and dry land in the table.*/

// CuComplex doesn't provide a constructor so this has to be stored in the constant memory cache.
// Also, this constant must be defined in a CPP file, to avoid multiple definition errors. This is due to how
// the CUDA file linking works.
extern __device__ __constant__ cuComplex CU_AVERAGE_LAND_DIELECTRIC_CONSTANT;

/*!
* \brief Structs that are defined as const in this header are not usable in CUDA kernels. CUDA can only use constants defind in the host
* if they are of floating or integral type. To use these structs in CUDA kernels, we must copy them to the constant memory cache on each
* cuda card.
*/
CUDA_HOST void writeCudaStructsToConstantMemory( );
}

#endif


Constants.cu ( in libDEWM.a or libDEWM.so, doesn’t work in either case )

#include <DEWM/Constants.hpp>
#include <iostream>

namespace dewm
{
#ifdef DEWM_CUDA_ENABLED

// Define it here so it is only defined once for the CUDA library.
__device__ __constant__ cuComplex  CU_AVERAGE_LAND_DIELECTRIC_CONSTANT;

CUDA_HOST void writeCudaStructsToConstantMemory( )
{
static const cuComplex dieletricConstant( make_cuFloatComplex( 4.0f, -0.006f ) );

// This fails when being called from another .cu file..
checkCudaError( cudaMemcpyToSymbol( CU_AVERAGE_LAND_DIELECTRIC_CONSTANT,
&dieletricConstant,
sizeof(cuComplex),
0,
cudaMemcpyHostToDevice ) );
}

#endif
}


CudaInterface.hpp

#ifndef DEWM_SignalCudaInterface_INC
#define DEWM_SignalCudaInterface_INC 1

#include <cuda.h>
#include <cuda_profiler_api.h>
//#define DEWM_CUDA_DEBUG_LITE 1
//  #define DEWM_CUDA_ERROR_OUTPUT 1
#define DEWM_CUDA_DEVICE_PROP_OUTPUT 1

// Define CUDA Host device macros for compatability with NON-CUDA builds.
#ifdef __CUDACC__
#define CUDA_HOST_DEVICE __host__ __device__
#define CUDA_KERNEL __global__
#define CUDA_DEVICE __device__
#define CUDA_HOST __host__
#define CUDA_CONSTANT_CACHE __constant__
#else
#define CUDA_HOST_DEVICE
#define CUDA_KERNEL
#define CUDA_CONSTANT_CACHE
#define CUDA_DEVICE
#define CUDA_HOST
#endif

#ifdef DEWM_CUDA_ENABLED
/**
* \def checkCudaError( cudaError )
* \brief Macro to call the cuda check function. Simply wrap this macro around any cuda runtime api library calls being made.
* \param cudaError result from a cuda runtime api function call.
*/
#define checkCudaError( cudaError ) __checkCudaError( cudaError, __FILE__, __LINE__ )

/**
* \brief Checks the return value of any cuda function for errors.
* If an error occurs, the line number and error type is displayed for debugging purposes. Additionally,
* the model will be terminated when an error is encountered. This method is preferred to the old method of
* checking cuda errors through looking at the last error as it allows the error to be isolated to a single line
* and file.
* NOTE: This function is not explicitly called. It can only be called properly via the use of the macro checkCudaError
* above.
*
* This can be used with kernal launches as well, by calling cudaPeekAtLastError and cudaDeviceSynchronize and
* wrapping both functions with the macro above.
* \param result_t result from a cuda runtime api library function
* \param file File that the cuda check was used in
* \param line Line number of the cuda check
*/
inline void __checkCudaError ( cudaError_t result_t, const char * file, const int line )
{
std::string error_string;

if ( cudaSuccess != result_t )
{
fprintf ( stderr, "\x1B[31m CUDA error encountered in file '%s', line %d\n Error %d: %s\n Terminating DEWM!\n \x1B[0m", file, line, result_t,
cudaGetErrorString ( result_t ) );
throw std::runtime_error ( "checkCUDAError : ERROR: CUDA Error" );
}
}

namespace dewm
{
namespace cuda
{
const std::vector<uint32_t> & CudaGetAvailableDevices();

class SignalCudaInterface
{
public:
SignalCudaInterface();

~SignalCudaInterface();

void initializeDeviceData();
};
}
}

#endif
#endif


CudaInterface.cpp (in libDEWM_CUDA.a )

#include <iostream>
#include <map>

#include <DEWM/cuda/CudaInterface.hpp>

#include <cuComplex.h>

void checkCUDAError(const std::string & message)
{
cudaError_t error = cudaGetLastError();
if (cudaSuccess != error)
{
std::cerr << "CUDA Error: " << message << ": " << cudaGetErrorString( error) << std::endl;
throw std::runtime_error( "checkCUDAError : ERROR: CUDA Error");
}
}
std::vector<uint32_t> availableDevicesId; /*!< Vector of available CUDA devices */
int currentDeviceIndex; /*!< Iterator to currently selected CUDA device (global) */

namespace dewm
{
namespace cuda
{

/**
* Performs CUDA library initialization functions, distinct from
* API locking.
*/
void CudaLibInitUnlocked()
{
if (availableDevices.empty())
{
checkCudaError( cudaProfilerStart());

int deviceCount;
checkCudaError( cudaGetDeviceCount( &deviceCount));

for (int device( 0); device < deviceCount; ++device)
{
checkCudaError( cudaGetDeviceProperties( &deviceProperties, device));

#ifdef DEWM_CUDA_DEVICE_PROP_OUTPUT
std::cout << "Device                               : " << device << " has compute capability " << deviceProperties.major << "." << deviceProperties.minor << std::endl;
std::cout << "Device Name                          : " << deviceProperties.name << std::endl;
std::cout << "Number of MultiProcessors            : " << deviceProperties.multiProcessorCount << std::endl;
std::cout << "Number of Threads Per MultiProcessor : " << deviceProperties.maxThreadsPerMultiProcessor << std::endl;
std::cout << "Number of Threads Per Block          : " << deviceProperties.maxThreadsPerBlock << std::endl;
std::cout << "Clock Rate in kHz                    : " << deviceProperties.clockRate << std::endl;
std::cout << "Memory Clock in kHz                  : " << deviceProperties.memoryClockRate << std::endl;
std::cout << "Concurrent Kernels                   : " << deviceProperties.concurrentKernels << std::endl;
std::cout << std::endl;
#endif

// If this is the only CUDA device then try to use it even if there are monitors attached to it
#ifndef _WIN32 // On Windows, deviceProperties.kernelExecTimeoutEnabled is equal to 1 without monitors attached

//if (deviceCount == 1 || deviceProperties.kernelExecTimeoutEnabled == 0)
#endif
//{
// Check the CUDA compute version. We need greater than 3.0
if (deviceProperties.major >= 3)
{
//#ifdef DEWM_CUDA_DEBUG_LITE
std::cout << "Added cuda device: " << device << std::endl;
//#endif
availableDevices.push_back( deviceProperties);
availableDevicesId.push_back( device);
}
//}
}

// Set the first device as our first device
currentDeviceIndex = 0;
currentDeviceProperties = availableDevices.begin();
}
}

void CudaLibInit()
{
CudaLibInitUnlocked();
}

void CudaLibRelease()
{
// NOTE: CUDA Library releases at DEWM destruction.

std::vector < cudaDeviceProp >::reverse_iterator propIter = availableDevices.rbegin();
std::vector < uint32_t >::reverse_iterator idIter = availableDevicesId.rbegin();

// Clear available ids and devices.
availableDevicesId.clear();
availableDevices.clear();

// Reset allocations on the current device.

checkCudaError( cudaProfilerStop() );
}

SignalCudaInterface::SignalCudaInterface()
{
dewm::cuda::CudaLibInit();

initializeDeviceData();
}
SignalCudaInterface::~SignalCudaInterface()
{
dewm::cuda::CudaLibRelease();
}

void SignalCudaInterface::initializeDeviceData()
{
// NOTE: CUDA Library releases at DEWM destruction.

uint32_t index(0);

for ( uint32_t currentDevice : CudaGetAvailableDevices())
{
std::cout << "Writing struct to device: " << currentDevice << std::endl;

checkCudaError( cudaSetDevice(currentDevice));

// Write any necessary structures to the constant memory cache...
writeCudaStructsToConstantMemory();

std::cout << "Finishing writing initial device data" << std::endl;
}
}

static SignalCudaInterface m_interface; /*!< CUDA Library object created at runtime in DEWM. Initializes and releases the CUDA libraries. >*/
}
}


Main: compiled executable

/*
* testCompilation.cu
*
*  Created on: Feb 22, 2019
*      Author: tstrutz
*/
#include <DEWM/cuda/CudaInterface.hpp>

// This does nothing but initialize the static object inside CudaInterface.cpp
int main( int argc, char ** argv )
{

}


making CUDA runtime API calls before the beginning of main or after the end of main is a no-no.

https://stackoverflow.com/questions/24869167/trouble-launching-cuda-kernels-from-static-initialization-code

The use of .cpp vs. .cu for the main executable is apparently affecting the initialization sequence in some way, but the fact that it works in the .cpp case is incidental. For safe CUDA coding, you should not do this.

So placing the SignalCudaInterface object inside Main should fix the problem? Or if I made the SignalCudaInterface a singleton that gets initialized on first use of the CUDA library?

I just don’t want to add too much complexity to things like test applications that would then have to include this initialization function or object in each of their main functions.

Thanks,

There are many ways to refactor the code. For example if you simply removed CUDA calls from constructor and destructor, that should be enough. If you put the object at main scope rather than file scope, yes, that should fix this issue. I wouldn’t be able to tell you what is best for your needs.

Robert,

I have refactored my code so that my Cuda interface initializes the CUDA library when any of the functions in the interface are first called. ( this will be after main ). However, I am still experiencing the original issue with my device symbol being invalid that I am writing to.

I have noticed that with cuObjDump that after I do a device link to my libDEWM.so the type associated with my device symbols changes from STO_CONSTANT U to STV_DEFAULT. Could this be why I am having issues with writing to them?

For example the symbol in question changes if I run “cuobjdump -symbols libDEWM_CUDA.a” and “cuobjdump -symbols libDEWM.so”

Also: libDEWM.so has libDEWM_CUDA.a linked into it and is built as a shared library.

libDEWM_CUDA.a

STT_CUDA_OBJECT STB_GLOBAL STO_CONSTANT U _ZN4dewm35CU_AVERAGE_LAND_DIELECTRIC_CONSTANTE

libDEWM.so

STT_OBJECT STB_GLOBAL STV_DEFAULT _ZN4dewm35CU_AVERAGE_LAND_DIELECTRIC_CONSTANTE

I am building with separable compilation enabled and with position-independent-code enabled which is required for my CUDA library to build properly. I also followed the CMake blog a while back for using CMake with CUDA as a standard language. Is the issue that I am using position independent code?

And if so, is there a way I can get Position independent code to work correctly?

I wouldn’t be able to comment without seeing the updated code. And I would need to see the issue reproduced with ordinary build commands, not CMAKE.

I could get you the ordinary build commands that CMake generates if that helps.

I actually made a simple CMake build with the updated code using a initialization on first use approach to the CUDA library similar to the code above and the code ran correctly. However, unfortunately the above code isn’t useful without all the kernels and other source we traditionally compile with.

I’ve been posting an extremely stripped down version of our codebase because it’d be too much to show here. Is there anyone who regularly posts here who is more familiar with CMake? The best I can do right now is compare the build process between the two and try to find the differences.

I could also get you the generated build commands that CMake runs. Trying to recreate this manually with our project would take an eternity as there are several hundred source files in it.

I’ve done a bit of testing and apparently the problem occurs because my CMake script is device linking multiple times. I believe this is a no-no in CUDA as well, and I think this may actually just be a CMake issue, not a CUDA one. The test script I made did not do this and would device link on each executable only. ( no good for other libraries that need to link with libDEWM.so ) I want my code to simply be able to have the device code in libDEWM.so to be linked to each executable.

CMake wants to device link multiple times regardless of this and will device link libDEWM_CUDA.a again. I also have a dummy.cu file in the src directory so that the first device link will actually occur in DEWM.so ( otherwise it won’t). This is so that the device code is portable with the shared library.

So, this may be a CMake problem and not one that you can solve.

You’re not allowed to have device linking across a shared library interface. The device linker supports static libraries only. This is documented in the nvcc manual. Multiple device link operations are allowed as long as they are non-overlapping.

So are you saying our project cannot use a shared library at all if we use relocatable device code? Everything needs to be compiled as a static library?

CMake seemed to say otherwise in their documentation that you could compile all your CUDA code into a static library and then link that library to a shared library and use it to build executables and other projects. Is this incorrect?

According to this blog on NVIDIA’s website this is possible: https://devblogs.nvidia.com/building-cuda-applications-cmake/

I am using this approach:
“When working on large projects it is common to generate one or more shared libraries. Each object file that is part of a shared library usually needs to be compiled with position-independent code enabled, which is done by setting the fPIC compiler flag. Unfortunately fPIC isn’t consistently supported across all compilers, so CMake abstracts away the issue by automatically enabling position-independent code when building shared libraries. In the case of static libraries that will be linked into shared libraries, position-independent code needs to be explicitly enabled by setting the POSITION_INDEPENDENT_CODE target property as follows.”

I actually got this approach to work correctly by building a Object library ( Just a set of object files that get linked in to the shared library ) instead of a Static library. Each of these object files is also build as position independent code. Then when I linked my libDEWM.so to each executable the code worked correctly.

As you’re unfamiliar with CMake I will note that when CMake does a device link it links device code into an intermediate “cmake_device_link.o” which is linked into each executable or library.

You can’t use device linking across a shared library interface.

You can use relocatable device code in a shared library as long as the linking is entirely within the shared library.

https://docs.nvidia.com/cuda/cuda-compiler-driver-nvcc/index.html#libraries