CUDA In-place Transpose Error - cuda

Question

I'm implementing a CUDA program for transposing an image. I created 2 kernels. The first kernel does out of place transposition and works perfectly for any image size.
Then I created a kernel for in-place transposition of square images. However, the output is incorrect. The lower triangle of the image is transposed but the upper triangle remains the same. The resulting image has a stairs like pattern in the diagonal and the size of each step of the stairs is equal to the 2D block size which I used for my kernel.
Out-of-Place Kernel:
Works perfectly for any image size if src and dst are different.
template<typename T, int blockSize>
__global__ void kernel_transpose(T* src, T* dst, int width, int height, int srcPitch, int dstPitch)
{
__shared__ T block[blockSize][blockSize];
int col = blockIdx.x * blockSize + threadIdx.x;
int row = blockIdx.y * blockSize + threadIdx.y;
if((col < width) && (row < height))
{
int tid_in = row * srcPitch + col;
block[threadIdx.y][threadIdx.x] = src[tid_in];
}
__syncthreads();
col = blockIdx.y * blockSize + threadIdx.x;
row = blockIdx.x * blockSize + threadIdx.y;
if((col < height) && (row < width))
{
int tid_out = row * dstPitch + col;
dst[tid_out] = block[threadIdx.x][threadIdx.y];
}
}
In-Place Kernel:
template<typename T, int blockSize>
__global__ void kernel_transpose_inplace(T* srcDst, int width, int pitch)
{
__shared__ T block[blockSize][blockSize];
int col = blockIdx.x * blockDim.x + threadIdx.x;
int row = blockIdx.y * blockDim.y + threadIdx.y;
int tid_in = row * pitch + col;
int tid_out = col * pitch + row;
if((row < width) && (col < width))
block[threadIdx.x][threadIdx.y] = srcDst[tid_in];
__threadfence();
if((row < width) && (col < width))
srcDst[tid_out] = block[threadIdx.x][threadIdx.y];
}
Wrapper Function:
int transpose_8u_c1(unsigned char* pSrcDst, int width,int pitch)
{
//pSrcDst is allocated using cudaMallocPitch
dim3 block(16,16);
dim3 grid;
grid.x = (width + block.x - 1)/block.x;
grid.y = (width + block.y - 1)/block.y;
kernel_transpose_inplace<unsigned char,16><<<grid,block>>>(pSrcDst,width,pitch);
assert(cudaSuccess == cudaDeviceSynchronize());
return 1;
}
Sample Input & Wrong Output:
I know this problem has something to do with the logic of in-place transpose. This is because my out of place transpose kernel which is working perfectly for different source and destination, also gives the same wrong result if I pass it a single pointer for source and destination.
What am I doing wrong? Help me in correcting the In-place kernel.

Answer 1

Your in-place kernel is overwriting data in the image that will be subsequently picked up by another thread to use for its transpose operation. So for a square image, you should buffer the destination data before overwriting it, then place the destination data in it's proper transposed location. Since we're doing effectively 2 copies per thread using this method, there's only a need to use half as many threads. Something like this should work:
template<typename T, int blockSize>
__global__ void kernel_transpose_inplace(T* srcDst, int width, int pitch)
{
int col = blockIdx.x * blockDim.x + threadIdx.x;
int row = blockIdx.y * blockDim.y + threadIdx.y;
int tid_in = row * pitch + col;
int tid_out = col * pitch + row;
if((row < width) && (col < width) && (row<col)) {
T temp = srcDst[tid_out];
srcDst[tid_out] = srcDst[tid_in];
srcDst[tid_in] = temp;
}
}