float VS floatN

Question

Is there any advantage when using floatN instead float in OpenCL?

for example

float3 position;

and

float posX, posY, posZ;

Thank you

Answer 1

It depends on the hardware.

NVidia GPUs have a scalar architecture, so vectors provide little advantage on them over writing purely scalar code. Quoting the NVidia OpenCL best practices guide (PDF link):

The CUDA architecture is a scalar architecture. Therefore, there is no performance benefit from using vector types and instructions. These should only be used for convenience. It is also in general better to have more work-items than fewer using large vectors.

With CPUs and ATI GPUs, you will gain more benefits from using vectors as these architectures have vector instructions (though I've heard this might be different on the latest Radeons - wish I had a link to the article where I read this).

Quoting the ATI Stream OpenCL programming guide (PDF link), for CPUs:

The SIMD floating point resources in a CPU (SSE) require the use of vectorized types (float4) to enable packed SSE code generation and extract good performance from the SIMD hardware.

This article provides a performance comparison on ATI GPUs of a kernel written with vectors vs pure scalar types.