HLSL: Index to unaligned/packed floats

I have a vertex shader (2.0) doing some instancing - each vertex specifies an index into an array.

If I have an array like this:

float instanceData[1开发者_JAVA技巧00];

The compiler allocates it 100 constant registers. Each constant register is a float4, so it's allocating 4 times as much space as is needed.

I need a way to make it allocate just 25 constant registers and store four values in each of them.

Ideally I'd like a method where it still looks like a float[] on both the CPU and GPU (Right now I am calling EffectParamter.SetValue(Single[]), I'm using XNA). But manually packing and unpacking a float4[] is an option, too.

Also: what are the performance implications for doing this? Is it actually worth it? (For me, this will save about one batch in every four or five).

Does that helps?:

float4 packedInstanceData[25];
...
float data = packedInstanceData[index / 4][index % 4];