the noise primitive only appears to provide 1d noise when used in fx_4_0.  All other dimensions appear to get mapped down to 1d, which is wrong.

noise(float arg); // works

noise(float2 arg); // does not return 2d noise

noise(float3 arg); // does not return 3d noise

noise(float4 arg); // does not return 4d noise

I am confused by your response.  There is a noise instruction in HLSL.

noise (DirectX HLSL)

Generates a random value using the Perlin-noise algorithm.

When compiling under fx_4_0 it only generates 1d noise, regardless of the number of dimensions in the vector passed to it.

It may be helpful if you can tell us what you exactly mean with “1d noise“. Do you talk about that noise always return only a scalar value no matter how many elements the input vector has? Or does it only respect the value from the first element of the vector to calculate the noise?

What, exactly, is a "2d noise"? The noise intrinsic always returns a single scalar value, so if you're expecting it to return a vector, then you're mistaken.

If you are saying that the scalar value being returned is only using a single value from the input vector, then please post a small repro example with your test input and results.