What's the OpenCL idiom for elementwise array-lookup / gather operation with vectorized types?

Question

Consider the following OpenCL code in which each element in a vector-type variable gets its value via array lookup:

float* tbl = get_data();
int4 offsets = get_offsets();
float4 my_elements = { 
    tbl[offsets.x],
    tbl[offsets.y],
    tbl[offsets.z],
    tbl[offsets.w] 
};

... and never mind the specific types and vector-size I used, i.e. I used float and int and size-4 vectors, but it might have been, say, ushort values, long indices and a size-8 vector.

I don't like the repetitiveness of that code. What is a better OpenCL idiom for doing this kind of "elementwise lookup" without the excessive repetition?

Answer 1

You could use float4 my_elements = vload4(offsets.x, tbl);, assuming that offsets = {n,n+1,n+2,n+3}. If offsets is more random, perhaps this would work:

float* tbl = get_data();
int* offsets = get_offsets();
int s = sizeof(offsets)/4;// divide by 4 because sizeof returns number of bytes
float[s] my_elements; 
for (int i = 0; i<s; i++)
{
    my_elements[i]=tbl[offsets[i]];
};

or

float* tbl = get_data();
int4 offsets = get_offsets();
float4 my_elements; 
for (int i = 0; i<4; i++)
{
    my_elements[i]=tbl[offsets[i]];
};