How make_int2 is working? - cuda

Question

in the CUDA C Programming Guide Version there is a small paragraph about Built-in Vector Types. It says that this structure has 4 components and they accessible with a specific way, e.i .x .y .z .w. What are the 4 components? Can someone give an example?
Moreover it says that with this line int2 make_int2(int x, int y); it constructs a vector with value x, y. Each of these variables has 4 components?
The reason I am trying to understand these things is because I am studying the following code:
/*1*/ int ny = num_ofElements_y_ofmyMatrix;
/*2*/ int nx = num_ofElements_x_ofmyMatrix;
/*3*/ int2 matrix_index_2d = make_int2( ( blockIdx.x * blockDim.x ) + threadIdx.x, ( blockIdx.y * blockDim.y ) + threadIdx.y );
/*4*/ int matrix_index_1d = ( nx * matrix_index_2d.y ) + matrix_index_2d.x;
/*5*/ if ( matrix_index_2d.x < nx && matrix_index_2d.y < ny )
/*6*/ {
/*7*/ float r = myMatrix[ matrix_index_1d ];
/*8*/ }
How the indexing in lines 3 and 4 is working? Subsequently, who does the access of the matrix myMatrix is working?
UPDATE:
As far as the code snippet is concerned usually when I am accessing an array I am using the following:
col = blockDim.x*blockIdx.x + threahIdx.x;
row = blockDim.x*blockIdx.x + threahIdx.x;
if (col < NUMCOLS && row < NUMROWS){...}
in order to have a row-major access of an array in c++, e.g myMatrix[row*NUMCOLS + col].
What is the connection with the type of indexing used in line 3 and 4?

Answer 1

Not all the CUDA built-in vector types have 4 components. For example, double2 has 2 double components. double2 is indeed defined as
struct __device_builtin__ __builtin_align__(16) double2
{
double x, y;
};
and can be used to deal with complex, double precision numbers. According to the definition above, you can use a declaration like
double2 foo;
and then initialize the two int2 make_int2(int x, int y); and y components as
foo.x = 1.;
foo.y = 3.4;
As another example, float4 has 4 float components and can be used to deal with four-vector in a Minkowski space.
In your example, int2 has 2 integer components and the instruction
int2 foo_int = make_int2(3,1);
constructs a foo_int struct of type int2 and initializes the x and y components to 3 and 1 respectively.