CUDA, "illegal memory access was encountered" in Memcpy - c++

Question

I have this cuda file:
#include "cuda.h"
#include "../../HandleError.h"
#include "Sphere.hpp"
#include <stdlib.h>
#include <CImg.h>
#define WIDTH 1280
#define HEIGHT 720
#define rnd(x) (x*rand()/RAND_MAX)
#define SPHERES_COUNT 5
using namespace cimg_library;
__global__
void kernel(unsigned char* bitmap, Sphere* s)
{
// Map threadIdx/blockIdx to pixel position
int x = threadIdx.x + blockIdx.x * blockDim.x;
int y = threadIdx.y + blockIdx.y * blockDim.y;
int offset = x + y * blockDim.x * gridDim.x;
float ox = x - blockDim.x * gridDim.x / 2;
float oy = y - blockDim.y * gridDim.y / 2;
float r = 0.2, g = 0.2, b = 0.5;
float maxz = -INF;
for (int i = 0; i < SPHERES_COUNT; i++) {
float n, t = s[i].hit(ox, oy, &n);
if (t > maxz) {
float fscale = n;
r = s[i].r * fscale;
g = s[i].g * fscale;
b = s[i].b * fscale;
maxz = t;
}
}
bitmap[offset*3] = (int)(r * 255);
bitmap[offset*3 + 1] = (int)(g * 255);
bitmap[offset*3 + 2] = (int)(b * 255);
}
__constant__ Sphere s[SPHERES_COUNT];
int main ()
{
//Capture start time
cudaEvent_t start, stop;
HANDLE_ERROR(cudaEventCreate(&start));
HANDLE_ERROR(cudaEventCreate(&stop));
HANDLE_ERROR(cudaEventRecord(start, 0));
//Create host bitmap
CImg<unsigned char> image(WIDTH, HEIGHT, 1, 3);
image.permute_axes("cxyz");
//Allocate device bitmap data
unsigned char* dev_bitmap;
HANDLE_ERROR(cudaMalloc((void**)&dev_bitmap, image.size()*sizeof(unsigned char)));
//Generate spheres and copy them on the GPU one by one
Sphere* temp_s = (Sphere*)malloc(SPHERES_COUNT*sizeof(Sphere));
for (int i=0; i <SPHERES_COUNT; i++) {
temp_s[i].r = rnd(1.0f);
temp_s[i].g = rnd(1.0f);
temp_s[i].b = rnd(1.0f);
temp_s[i].x = rnd(1000.0f) - 500;
temp_s[i].y = rnd(1000.0f) - 500;
temp_s[i].z = rnd(1000.0f) - 500;
temp_s[i].radius = rnd(100.0f) + 20;
}
HANDLE_ERROR(cudaMemcpyToSymbol(s, temp_s, sizeof(Sphere)*SPHERES_COUNT));
free(temp_s);
//Generate a bitmap from spere data
dim3 grids(WIDTH/16, HEIGHT/16);
dim3 threads(16, 16);
kernel<<<grids, threads>>>(dev_bitmap, s);
//Copy the bitmap back from the GPU for display
HANDLE_ERROR(cudaMemcpy(image.data(), dev_bitmap,
image.size()*sizeof(unsigned char),
cudaMemcpyDeviceToHost));
cudaFree(dev_bitmap);
image.permute_axes("yzcx");
image.save("render.bmp");
}
It compiles fine, but when executed I get this error:
an illegal memory access was encountered in main.cu at line 82
that is, here:
//Copy the bitmap back from the GPU for display
HANDLE_ERROR(cudaMemcpy(image.data(), dev_bitmap,
image.size()*sizeof(unsigned char),
cudaMemcpyDeviceToHost));
I cannot understand why...
I know that If remove this:
bitmap[offset*3] = (int)(r * 255);
bitmap[offset*3 + 1] = (int)(g * 255);
bitmap[offset*3 + 2] = (int)(b * 255);
The error is not reported, so I thought It may be an out of index error, reported later, but I have An identical version of this program that makes no use of constant memory, and it works fine with the very same version of the kernel function...

Answer 1

There are two things at issue here. The first is this:
__constant__ Sphere s[SPHERES_COUNT];
int main ()
{
......
kernel<<<grids, threads>>>(dev_bitmap, s);
......
In host code, s is a host memory variable which provides a handle for the CUDA runtime to hook up with the device constant memory symbol. It doesn't contain a valid device pointer and can't be passed to kernel calls. The result is a invalid memory access error.
You could do this:
__constant__ Sphere s[SPHERES_COUNT];
int main ()
{
......
Sphere *d_s;
cudaGetSymbolAddress((void **)&d_s, s);
kernel<<<grids, threads>>>(dev_bitmap, d_s);
......
which would cause a symbol lookup to get the device address of s, and it would be valid to pass that to the kernel. However, the GPU relies on the compiler emitting specific instructions to access memory through the constant cache. The device compiler will only emit these instructions when it can detect that a __constant__ variable is being accessed within a kernel, which is not possible when using a pointer. You can see more about how the compiler will generate code for constant variable access in this Stack Overflow question and answer.