Fix qmm_t for unaligned cases (#923)

Author: angeloskath

mlx/backend/metal/kernels/quantized.metal

@@ -520,6 +520,7 @@ template <typename T, const int BM, const int BK, const int BN, const int group_
   const int K_g = K / group_size;
   const int y_row = tid.y * BM;
   const int y_col = tid.x * BN;
+
   x += y_row * K;
   w += y_col * K_w;
   scales += y_col * K_g;
@@ -572,7 +573,10 @@ template <typename T, const int BM, const int BK, const int BN, const int group_
           const device uint32_t * w_local = w + offset_row * K_w + offset_col;
           threadgroup T * Ws_local = Ws + offset_row * BK + offset_col * el_per_int;
 
-          if (y_row + offset_row < N) {
+          // y_col corresponds to the row of the weight matrix and added to
+          // offset_row it should be less than the total number of rows
+          // otherwise skip.
+          if (y_col + offset_row < N) {
             uint32_t wi = *w_local;
             T scale = scales_block[offset_row * groups_per_block + offset_col / (group_size / el_per_int)];
             T bias = biases_block[offset_row * groups_per_block + offset_col / (group_size / el_per_int)];

python/tests/test_quantized.py

@@ -229,6 +229,16 @@ def test_non_multiples(self):
         self.assertEqual(y_q.shape, y_hat.shape)
         self.assertLess((y_q - y_hat).abs().max(), 1e-3)
 
+        # Test with larger than 128 unaligned sizes
+        w = mx.random.normal(shape=(99, 256))
+        w_q, scales, biases = mx.quantize(w)
+        w_hat = mx.dequantize(w_q, scales, biases)
+        x = mx.random.normal(shape=(129, 256))
+        y_q = mx.quantized_matmul(x, w_q, scales, biases, transpose=True)
+        y_hat = x @ w_hat.T
+        self.assertEqual(y_q.shape, y_hat.shape)
+        self.assertLess((y_q - y_hat).abs().max(), 1e-3)
+
 
 if __name__ == "__main__":
     unittest.main()