minor fixes (#631)

* minor fixes

* var with ddof >= nelements

mlx/backend/accelerate/matmul.cpp

@@ -46,6 +46,9 @@ inline void matmul_cblas_general(
   size_t N = b.shape(-1);
   size_t K = a.shape(-1);
 
+  if (M == 0 || N == 0) {
+    return;
+  }
   if (K == 0) {
     std::memset(static_cast<void*>(out.data<float>()), 0, out.nbytes());
     return;
@@ -94,6 +97,9 @@ inline void matmul_bnns_general(
   size_t N = b.shape(-1);
   size_t K = a.shape(-1);
 
+  if (M == 0 || N == 0) {
+    return;
+  }
   if (K == 0) {
     std::memset(static_cast<void*>(out.data<float>()), 0, out.nbytes());
     return;

mlx/backend/common/default_primitives.cpp

@@ -132,7 +132,9 @@ inline void matmul_common_general(
   size_t M = a.shape(-2);
   size_t N = b.shape(-1);
   size_t K = a.shape(-1);
-
+  if (M == 0 || N == 0) {
+    return;
+  }
   if (K == 0) {
     std::memset(static_cast<void*>(out.data<float>()), 0, out.nbytes());
     return;

mlx/ops.cpp

@@ -1323,8 +1323,8 @@ array mean(
   for (int axis : axes) {
     if (axis < -ndim || axis >= ndim) {
       std::ostringstream msg;
-      msg << "[mean] axis " << axis + " is out of bounds for array with "
-          << ndim + " dimensions.";
+      msg << "[mean] axis " << axis << " is out of bounds for array with "
+          << ndim << " dimensions.";
       throw std::invalid_argument(msg.str());
     }
   }
@@ -1364,7 +1364,7 @@ array var(
 
   if (ddof != 0) {
     auto nelements = compute_number_of_elements(a, axes);
-    float factor = nelements / (nelements - ddof);
+    auto factor = nelements / static_cast<float>(std::max(nelements - ddof, 0));
     v = multiply(v, array(factor, dtype), s);
   }
 

python/tests/test_blas.py

@@ -582,6 +582,25 @@ class TestBlas(mlx_tests.MLXTestCase):
                 self.assertEqual(r.shape, t.shape)
                 self.assertTrue(mx.allclose(r, t, atol=1e-4).item())
 
+    def test_empty_matmul(self):
+        a = mx.array([[], []]).T
+        b = mx.array([[1.0, 2.0], [2.0, 3.0]])
+        c = a @ b
+        mx.eval(c)
+        self.assertEqual(c.shape, (0, 2))
+
+        a = mx.array([[1.0, 2.0], [2.0, 3.0]])
+        b = mx.array([[], []])
+        c = a @ b
+        mx.eval(c)
+        self.assertEqual(c.shape, (2, 0))
+
+        a = mx.array([[], []]).T
+        b = mx.array([[], []])
+        c = a @ b
+        mx.eval(c)
+        self.assertEqual(c.shape, (0, 0))
+
 
 if __name__ == "__main__":
     unittest.main()

python/tests/test_ops.py

@@ -1,4 +1,4 @@
-# Copyright © 2023 Apple Inc.
+# Copyright © 2023-2024 Apple Inc.
 
 import math
 import unittest
@@ -690,6 +690,14 @@ class TestOps(mlx_tests.MLXTestCase):
         self.assertEqual(mx.var(x, axis=0).tolist(), [1.0, 1.0])
         self.assertEqual(mx.var(x, axis=1).tolist(), [0.25, 0.25])
 
+        x = mx.array([1.0, 2.0])
+        out = mx.var(x, ddof=2)
+        self.assertEqual(out.item(), float("inf"))
+
+        x = mx.array([1.0, 2.0])
+        out = mx.var(x, ddof=3)
+        self.assertEqual(out.item(), float("inf"))
+
     def test_abs(self):
         a = mx.array([-1.0, 1.0, -2.0, 3.0])
         result = mx.abs(a)