代码是从OpengGL-Superbible 10并行计算前缀和。

所示着色器的本地工作组大小为1024，这意味着它将处理包含2048个元素的数组，因为每次调用都会计算输出数组的两个元素。共享变量SHARED_DATA用于存储正在传输的数据。执行开始时，着色器将两个相邻元素从输入数组加载到数组中。接下来，它执行BAILAR()函数。此步骤确保在内部循环开始之前，所有着色器调用都已将其数据加载到共享数组中。

# version 450 core

layout (local_size_x = 1024) in;
layout (binding = 0) coherent buffer block1
{
    float input_data[gl_WorkGroupSize.x];
};
layout (binding = 1) coherent buffer block2
{
    float output_data[gl_WorkGroupSize.x];
};
shared float shared_data[gl_WorkGroupSize.x * 2];
void main(void)
{
    uint id = gl_LocalInvocationID.x;
    uint rd_id;
    uint wr_id;
    uint mask;// The number of steps is the log base 2 of the
    // work group size, which should be a power of 2
    const uint steps = uint(log2(gl_WorkGroupSize.x)) + 1;
    uint step = 0;
    // Each invocation is responsible for the content of
    // two elements of the output array
    shared_data[id * 2] = input_data[id * 2];
    shared_data[id * 2 + 1] = input_data[id * 2 + 1];
    // Synchronize to make sure that everyone has initialized
    // their elements of shared_data[] with data loaded from
    // the input arrays
    barrier();
    memoryBarrierShared();
    // For each step...
    for (step = 0; step < steps; step++)
    {
        // Calculate the read and write index in the
        // shared array
        mask = (1 << step) - 1;
        rd_id = ((id >> step) << (step + 1)) + mask;
        wr_id = rd_id + 1 + (id & mask);
        // Accumulate the read data into our element
        shared_data[wr_id] += shared_data[rd_id];
        // Synchronize again to make sure that everyone
        // has caught up with us
        barrier();
        memoryBarrierShared();
    } // Finally write our data back to the output image
    output_data[id * 2] = shared_data[id * 2];
    output_data[id * 2 + 1] = shared_data[id * 2 + 1];
}

如何直观地理解rd_id和wr_id的移位操作？为什么它会起作用？