Metal Shader language : float2 screws everything up.

I have hit a brick wall. I cannot explain the behavior I am seeing. What it boils down to is: SIMD2<Float> is incompatible with float2.

I am working through tutorials trying to get a handle on Metal. All the tutorials are woefully out of date, so no answers there. But I did find somewhere that I should be using SIMD4<Float> for my point data. Great. I was able to bridge that gap. but I added uv maps using a float2. and the result was a portion of my vertex data gets lost somewhere. The buffer is smaller, by about 25%.

and I cannot figure it out. help?

in anticipation of pertinent questions:

the vertex struct:

Code Block 
struct vertex {
    var position : SIMD4<Float> = SIMD4<Float>(x: 0.0,y: 0.0,z: 0.0,w: 0.0)
    var uv : SIMD2<Float> = SIMD2<Float>(x:0.0 ,y:0.0)
    init(x: Float ,y: Float, z: Float) {
        self.position.x = x
        self.position.y = y
        self.position.z = z
        self.position.w = 1.0
        uv = SIMD2<Float>(0.5 ,0.5)
    }
    init( x: Float, y: Float, z: Float, u: Float, v: Float ){
        position = SIMD4<Float>(x,y,z,1.0)
        uv = SIMD2<Float>(u ,v)
    }
}

if I change the SIMD2 to an SIMD4... the entire thing works.

how I measure the size of my buffer:

Code Block 
let matrixSize = MemoryLayout.size(ofValue:vertexField.vertices[0])
        let vertsSize = vertexField.vertices.count * matrixSize
        // now we make the vertex buffers.
        for i in 0..<maxFramesInFlight{
            if let aVertBuff = device.makeBuffer(bytes: vertexField.vertices, length: vertsSize, options: .storageModeManaged){
                aVertBuff.label = "verts buffer \(i)"
                vertexBuffs += [aVertBuff]
            }
        }

vertexField is a struct containing an array of vertices, and an array of offsets and lengths for the various meshes I make from those vertices. Eventually I'll figure out the elegant way of doing that.

the buffer winds up having 64 elements, out of 84. The fragment shader, assumes 84 entries, and winds up setting those values to 0,0,0,0 0,0.

I sent a single SIMD2<Float> variable to the Fragment shader, and examined it. It was correct.

any ideas are welcome. I am up the river without a paddle on this one.