我有问题。我正在尝试学习OpenCl，所以我一直在尝试用OpenCl实现FFT算法。我正在尝试重新创建这个：
void FFT (cmplx* data, int dataSize){
if(dataSize == 1){
return;
}else{
cmplx* even = (cmplx*)malloc(dataSize/2*sizeof(cmplx));
cmplx* odd = (cmplx*)malloc(dataSize/2*sizeof(cmplx));
for (int i = 0;i<dataSize;i+=2){
even[i/2] = data[i];
odd[i/2] = data[i+1];
}
FFT(even,dataSize/2);
FFT(odd,dataSize/2);
for (int i = 0;i<dataSize;i++){
cmplx C = cmplx(-2*M_PI/dataSize*i);
data[i].real = even[i].real + C.real*odd[i].real - C.imag*odd[i].imag;
data[i].imag = even[i].imag + C.real*odd[i].imag + C.imag*odd[i].real;
}
}
}
cmplx只是一个包含复数的两个浮点实部和虚部的类，并且有一个构造函数，用Euler方程创建复数。其他一切都很直接
我可能不知道一些小的细微差别，在我的理解中，我可以在独立线程中进行循环计算，所以这样循环：
for (int i = 0;i<dataSize;i++){
cmplx C = cmplx(-2*M_PI/dataSize*i);
data[i].real = even[i].real + C.real*odd[i].real - C.imag*odd[i].imag;
data[i].imag = even[i].imag + C.real*odd[i].imag + C.imag*odd[i].real;
}
使用这样的OpenCl代码：
__kernel void FFTComplexSum(__global float *evenReal,__global float *evenImag,
__global float *oddReal,__global float *oddImag,
__global float *real,__global float *imag,
__global float *C){
int gid = get_global_id(0);
real[gid] = evenReal[gid] + cos(C[gid])*oddReal[gid] - sin(C[gid])*oddImag[gid];
imag[gid] = evenImag[gid] + cos(C[gid])*oddImag[gid] + sin(C[gid])*oddReal[gid];
}
但如果运行这个：
.... // instantiating stuff like platform, device_id, kernel, program...
size_t buffer_size;
cl_mem evenReal_mem, evenImag_mem, oddReal_mem, oddImag_mem, real_mem, imag_mem, c_mem;
float evenReal[dataSize];
float evenImag[dataSize];
float tReal[dataSize];
float tImag[dataSize];
float oddReal[dataSize];
float oddImag[dataSize];
float C[dataSize];
for (int i = 0;i<dataSize;i+=2){
evenReal[i/2] = real[i];
evenImag[i/2] = imag[i];
oddReal[i/2] = real[i+1];
oddImag[i/2] = imag[i+1];
C[i] = -2*M_PI/dataSize*i;
C[i+1] = -2*M_PI/dataSize*(i+1);
}
doubleArray(evenReal,dataSize); // doubleArray function just makes array to loop
doubleArray(evenImag,dataSize);
doubleArray(oddReal,dataSize);
doubleArray(oddImag,dataSize);
buffer_size = sizeof(float) * dataSize;
evenReal_mem = clCreateBuffer(context, CL_MEM_READ_ONLY, buffer_size, NULL, NULL);
err = clEnqueueWriteBuffer(cmd_queue, evenReal_mem, CL_TRUE, 0, buffer_size,(void*)evenReal, 0, NULL, NULL);
assert(err == CL_SUCCESS); // Fail check
evenImag_mem = clCreateBuffer(context, CL_MEM_READ_ONLY, buffer_size, NULL, NULL);
err = clEnqueueWriteBuffer(cmd_queue, evenImag_mem, CL_TRUE, 0, buffer_size,(void*)evenImag, 0, NULL, NULL);
assert(err == CL_SUCCESS); // Fail check
oddReal_mem = clCreateBuffer(context, CL_MEM_READ_ONLY, buffer_size, NULL, NULL);
err = clEnqueueWriteBuffer(cmd_queue, oddReal_mem, CL_TRUE, 0, buffer_size,(void*)oddReal, 0, NULL, NULL);
assert(err == CL_SUCCESS); // Fail check
oddImag_mem = clCreateBuffer(context, CL_MEM_READ_ONLY, buffer_size, NULL, NULL);
err = clEnqueueWriteBuffer(cmd_queue, oddImag_mem, CL_TRUE, 0, buffer_size,(void*)oddImag, 0, NULL, NULL);
assert(err == CL_SUCCESS); // Fail check
real_mem = clCreateBuffer(context, CL_MEM_WRITE_ONLY, buffer_size, NULL, NULL);
err = clEnqueueWriteBuffer(cmd_queue, real_mem, CL_TRUE, 0, buffer_size,(void*)real, 0, NULL, NULL);
assert(err == CL_SUCCESS); // Fail check
imag_mem = clCreateBuffer(context, CL_MEM_WRITE_ONLY, buffer_size, NULL, NULL);
err = clEnqueueWriteBuffer(cmd_queue, imag_mem, CL_TRUE, 0, buffer_size,(void*)imag, 0, NULL, NULL);
assert(err == CL_SUCCESS); // Fail check
c_mem = clCreateBuffer(context, CL_MEM_READ_ONLY, sizeof(float), NULL, NULL);
err = clEnqueueWriteBuffer(cmd_queue, c_mem, CL_TRUE, 0, sizeof(float),(void*)C, 0, NULL, NULL);
assert(err == CL_SUCCESS); // Fail check
clFinish(cmd_queue);
err = clSetKernelArg(kernel[0], 0, sizeof(cl_mem), &evenReal_mem);
err = clSetKernelArg(kernel[0], 1, sizeof(cl_mem), &evenImag_mem);
err = clSetKernelArg(kernel[0], 2, sizeof(cl_mem), &oddReal_mem);
err = clSetKernelArg(kernel[0], 3, sizeof(cl_mem), &oddImag_mem);
err = clSetKernelArg(kernel[0], 4, sizeof(cl_mem), &real_mem);
err = clSetKernelArg(kernel[0], 5, sizeof(cl_mem), &imag_mem);
err = clSetKernelArg(kernel[0], 6, sizeof(cl_mem), &c_mem);
assert(err == CL_SUCCESS); // Fail check
size_t global_work_size = dataSize;
err = clEnqueueNDRangeKernel(cmd_queue, kernel[0], 1, NULL, &global_work_size, NULL, 0, NULL, NULL);
assert(err == CL_SUCCESS);
clFinish(cmd_queue);
printf("test data:\n");
for (int i = 0;i<dataSize;i++){
float r,I;
r = evenReal[i] + cos(C[i])*oddReal[i] - sin(C[i])*oddImag[i];
I = evenImag[i] + cos(C[i])*oddImag[i] + sin(C[i])*oddReal[i];
printf("%f + %f\n",r,I);
}
err = clEnqueueReadBuffer(cmd_queue, real_mem, CL_TRUE, 0, buffer_size, tReal, 0, NULL, NULL);
assert(err == CL_SUCCESS);
clFinish(cmd_queue);
err = clEnqueueReadBuffer(cmd_queue, imag_mem, CL_TRUE, 0, buffer_size, tImag, 0, NULL, NULL);
assert(err == CL_SUCCESS);
clFinish(cmd_queue);
clReleaseMemObject(evenReal_mem);
clReleaseMemObject(evenImag_mem);
clReleaseMemObject(oddReal_mem);
clReleaseMemObject(oddImag_mem);
clReleaseMemObject(real_mem);
clReleaseMemObject(imag_mem);
clReleaseMemObject(c_mem);
prinf("data:");
for (int i = 0;i<dataSize;i++){
printf("%f + %f\n",tReal[i],tImag[i]);
}
它返回：
test data:
1.000000 + 0.000000
0.000000 + 1.000000
-1.000000 + 0.000000
-0.000000 + -1.000000
data:
1.000000 + 0.000000
-1.000000 + 0.000000
1.000000 + 0.000000
-1.000000 + 0.000000
我真的很困惑，为什么我会得到错误的答案。我错过了一些非常明显的东西？
很抱歉很长的问题。
答案 0 :(得分：0)
c_mem有问题。
您可以访问内核中的C C[gid]，但只创建了大小为sizeof(float)的{{1}}。因此主数据不能适合那个（4字节）存储空间，并且你只写4个字节。使用data_size将其创建大小和写入大小相乘就足够了。
这就是为什么当虚数变为零时（sin（0）），实数值变为1和-1。如果运气好的话，溢出的C会产生垃圾并给出实部和虚部的垃圾结果，这会立即显示错误的来源。