OpenCL程序分為兩個部份，一部份是內核代碼，負責具體算法。另一部份是主程序負責初始化OpenCL和準備數據。主程序加載內核代碼，并按照即定方法進行運算。

內核代碼可以寫在主程序里面，也可以寫在另一個文本文件里，有點像DX中的HLSL和OPENGL里的GLSL。哈哈，明白意思就行了。我們用第一種方法，把代碼跟源程序分開寫。

調用OpenCL大至分7個步驟

1：初始化OpenCL

2：創建上下文設備

3：創建命令隊列

4：創建數據緩沖區

5：將數據上傳到緩沖區

6：加載編譯代碼，創建內核調用函數

7：設置參數，執行內核

8：讀回計算結果。

下面我們通過一個向量相加的程序來了解OpenCL 。有A，B兩個四維向量，相加后值存在C向量里。OpenCL會根據用戶提供的維數，將向量分解成多個任務分發給多個CPU計算。

源碼分兩部份

（一）vecadd.cl核心代碼。

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__kernel void vecAdd(__global int* A, ????????__global int* B, ????????__global int* C) { ????//獲取當前工作項所在位置（線程索引號） ????int idx = get_global_id(0); ????C[idx] = A[idx] + B[idx]; }

__kernel 指明這是一個OpenCL內核，__global 說明指針指向的是全局的設備內存空間，其它的就是C語言的函數的語法。kernel必須返回空類型。

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（二）main.cpp代碼

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#include <iostream> #include <stdio.h> #include <string.h> #include <string> #include <CL/cl.h>//包含CL的頭文件 using namespace std; //四維向量 #define elements 4 //從外部文件獲取cl內核代碼 bool GetFileData(const char* fname,string& str) { ????FILE* fp = fopen(fname,"r"); ????if(fp==NULL) ????{ ????????printf("no found file\n"); ????????return false; ????} ????int n=0; ????while(feof(fp)==0) ????{ ????????str += fgetc(fp); ????} ????return true; } int main() { ????//先讀外部CL核心代碼，如果失敗則退出。 ????//代碼存buf_code里面 ????string code_file; ????if(false == GetFileData("vecadd.cl",code_file)) ????????return 0; ????char* buf_code = new char[code_file.size()]; ????strcpy(buf_code,code_file.c_str()); ????buf_code[code_file.size()-1] = NULL; ????//聲明CL所需變量。 ????cl_device_id device; ????cl_platform_id platform_id = NULL; ????cl_context context; ????cl_command_queue cmdQueue; ????cl_mem bufferA,bufferB,bufferC; ????cl_program program; ????cl_kernel kernel = NULL; ????//我們使用的是一維向量 ????//設定向量大小（維數） ????size_t globalWorkSize[1]; ????globalWorkSize[0] = elements; ????cl_int err; ????/* ????????定義輸入變量和輸出變量，并設定初值 ????*/ ????int* buf_A = new int[elements]; ????int* buf_B = new int[elements]; ????int* buf_C = new int[elements]; ????size_t datasize = sizeof(int) * elements; ????buf_A[0] = 1; ????buf_A[1] = 2; ????buf_A[2] = 3; ????buf_A[3] = 4; ????buf_B[0] = 5; ????buf_B[1] = 6; ????buf_B[2] = 7; ????buf_B[3] = 8; ????//step 1:初始化OpenCL ????err = clGetPlatformIDs(1,&platform_id,NULL); ????if(err!=CL_SUCCESS) ????{ ????????cout<<"clGetPlatformIDs error"<<endl; ????????return 0; ????} ????//這次我們只用CPU來進行并行運算，當然你也可以該成GPU ????clGetDeviceIDs(platform_id,CL_DEVICE_TYPE_CPU,1,&device,NULL); ????//step 2:創建上下文 ????context = clCreateContext(NULL,1,&device,NULL,NULL,NULL); ????//step 3:創建命令隊列 ????cmdQueue = clCreateCommandQueue(context,device,0,NULL); ????//step 4:創建數據緩沖區 ????bufferA = clCreateBuffer(context, ?????????????????????????????CL_MEM_READ_ONLY, ?????????????????????????????datasize,NULL,NULL); ????bufferB = clCreateBuffer(context, ?????????????????????????????CL_MEM_READ_ONLY, ?????????????????????????????datasize,NULL,NULL); ????bufferC = clCreateBuffer(context, ?????????????????????????????CL_MEM_WRITE_ONLY, ?????????????????????????????datasize,NULL,NULL); ????//step 5:將數據上傳到緩沖區 ????clEnqueueWriteBuffer(cmdQueue, ?????????????????????????bufferA,CL_FALSE, ?????????????????????????0,datasize, ?????????????????????????buf_A,0, ?????????????????????????NULL,NULL); ????clEnqueueWriteBuffer(cmdQueue, ?????????????????????????bufferB,CL_FALSE, ?????????????????????????0,datasize, ?????????????????????????buf_B,0, ?????????????????????????NULL,NULL); ????//step 6:加載編譯代碼,創建內核調用函數 ????program = clCreateProgramWithSource(context,1, ????????????????????????????????????????(const char**)&buf_code, ????????????????????????????????????????NULL,NULL); ????clBuildProgram(program,1,&device,NULL,NULL,NULL); ????kernel = clCreateKernel(program,"vecAdd",NULL); ????//step 7:設置參數，執行內核 ????clSetKernelArg(kernel,0,sizeof(cl_mem),&bufferA); ????clSetKernelArg(kernel,1,sizeof(cl_mem),&bufferB); ????clSetKernelArg(kernel,2,sizeof(cl_mem),&bufferC); ????clEnqueueNDRangeKernel(cmdQueue,kernel, ???????????????????????????1,NULL, ???????????????????????????globalWorkSize, ???????????????????????????NULL,0,NULL,NULL); ????//step 8:取回計算結果 ????clEnqueueReadBuffer(cmdQueue,bufferC,CL_TRUE,0, ????????????????????????datasize,buf_C,0,NULL,NULL); ????//輸出計算結果 ????cout<<"["<<buf_A[0]<<","<<buf_A[1]<<","<<buf_A[2]<<","<<buf_A[3]<<"]+[" ????<<buf_B[0]<<","<<buf_B[1]<<","<<buf_B[2]<<","<<buf_B[3]<<"]=[" ????<<buf_C[0]<<","<<buf_C[1]<<","<<buf_C[2]<<","<<buf_C[3]<<"]"<<endl; ????//釋放所有調用和內存 ????clReleaseKernel(kernel); ????clReleaseProgram(program); ????clReleaseCommandQueue(cmdQueue); ????clReleaseMemObject(bufferA); ????clReleaseMemObject(bufferB); ????clReleaseMemObject(bufferC); ????clReleaseContext(context); ????delete buf_A; ????delete buf_B; ????delete buf_C; ????delete buf_code; ????return 0; }

運算結果：

?[1,2,3,4] + [5,6,7,8] = [6,8,10,12]

總結

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