Avoid Extracting Vector Components

Consider the following kernel:

__constant float4 oneVec = (float4)(1.0f, 1.0f, 1.0f, 1.0f);
__kernel __attribute__((vec_type_hint(float4)))
void inverter2(__global float4* input, __global float4* output)
{
  int tid = get_global_id(0);
  output[tid] = oneVec – input[tid];
  output[tid].w = input[tid].w;
  output[tid] = sqrt(output[tid]);
}

For this example of the explicit vector code, the extraction of the w component is very costly. The reason is that the next vector operation forces reloading the same vector from memory. Consider loading a vector once and performing all changes by use of vector operations even for a single component.

In this specific case, two changes are required:

  1. Modify the oneVec, so that its w component is zero, causing only a sign flip in the w component of the input vector.
  2. Use float representation to manually flip the sign bit of the w component back.

As a result, the kernel appears as follows:

__constant float4 oneVec = (float4)(1.0f, 1.0f, 1.0f, 0.0f);
__constant int4 signChanger = (int4)(0, 0, 0, 0x80000000);
__kernel __attribute__((vec_type_hint(float4)))
void inverter3(__global float4* input, __global float4* output)
{
  int tid  = get_global_id(0);
  output[tid] = oneVec – input[tid];
  output[tid] = as_float4(as_int4(output[tid]) ^ signChanger);
  output[tid] = sqrt(output[tid]);
}

At the cost of another constant vector, this implementation performs all the required operations addressing only full vectors. All the computations might also be performed in float8.