q8_matvec_padstage

// Q8_0 matvec: a decode-step projection that reads the weights quantized.
//
// Q8_0 stores signed bytes with one scale per block of 32 values along the
// contraction axis. Reading the bytes directly moves a quarter of the data
// an f32 copy would.
//
// The scale factors out of a block's dot product, so it is applied once per
// block and column, not once per weight.
@autotune(TILE_N in [64], BLK in [32])
@launch(256)
@padstage
kernel q8_matvec(A: tensor<f32>[1, K],
                 W: tensor<i8>[K, N],
                 S: tensor<f32>[KB, N],
                 C: tensor<f32>[1, N]) {
  let pn = ctx.block.x
  var acc: tile<f32>[1, TILE_N] = 0.0

  for k in range(0, K, BLK) {
    let b = k / BLK
    let a = A[0 :+ 1, k :+ BLK]
    let w = W[k :+ BLK, pn * TILE_N :+ TILE_N]
    let s = S[b :+ 1, pn * TILE_N :+ TILE_N]

    var wf: tile<f32>[BLK, TILE_N] = f32(w)
    var part: tile<f32>[1, TILE_N] = dot(a, wf)
    acc += part * s
  }

  C[0 :+ 1, pn * TILE_N :+ TILE_N] = acc
}