How do you count registers in HLSL?

Question

With shader model 2.0, you can have 256 constant registers. I have been looking at various shaders, and trying to figure out what constitutes a single register?

For example, in my instancing shader, I have the following variables declared at the top, outside of functions:

float4x4 InstanceTransforms[40];
float4 InstanceDiffuses[40];

float4x4 View;
float4x4 Projection;

float3 LightDirection = normalize(float3(-1, -1, -1));
float3 DiffuseLight = 1;
float3 AmbientLight = 0.66; 

float Alpha;

texture Texture;

How many registers have I consumed? How do I count them?

Answer 1

Each constant register is a float4.

float3, float2 and float will each allocate a whole register. float4x4 will use 4 registers. Arrays will simply multiply the number of registers allocated by the number of elements. And the compiler will probably allocate a few registers itself to use as constants in various calculations.

The only way to really tell what the shader is using is to disassemble it. To that end you may be interested in this question that I asked a while ago: HLSL: Enforce Constant Register Limit at Compile Time

You might also find this one worth a look: HLSL: Index to unaligned/packed floats. It explains why an array of 40 floats will use 40 registers, and how you can make it use 10 instead.

Your texture will use a texture sampler (you have 16 of these), not a constant register.

For reference, here are the list of ps_2_0 registers and vs_2_0 registers.