Optimization to OpenCL FFT

platforms/opencl/src/OpenCLFFT3D.cpp

@@ -158,6 +158,8 @@ int OpenCLFFT3D::findLegalDimension(int minimum) {
 
 cl::Kernel OpenCLFFT3D::createKernel(int xsize, int ysize, int zsize, int& threads, int axis, bool forward, bool inputIsReal) {
     int maxThreads = std::min(256, (int) context.getDevice().getInfo<CL_DEVICE_MAX_WORK_GROUP_SIZE>());
+    while (maxThreads > 128 && maxThreads-64 >= zsize)
+        maxThreads -= 64;
     bool isCPU = context.getDevice().getInfo<CL_DEVICE_TYPE>() == CL_DEVICE_TYPE_CPU;
     while (true) {
         bool loopRequired = (zsize > maxThreads || isCPU);