CGemv handles 0-size arguments properly.

theano/tensor/blas_c.py

@@ -332,7 +332,7 @@ def gemv_c_code(aa, xx, yy, zz, alpha, beta, destructive, fail):
         int Sz = %(zz)s->strides[0] / elemsize;
         int Sy = %(yy)s->strides[0] / elemsize;
 
-        if (1)
+        if (Nx0 * Nx1)
         {
             if (%(xx)s->strides[0] == elemsize)
             {
@@ -398,7 +398,18 @@ def gemv_c_code(aa, xx, yy, zz, alpha, beta, destructive, fail):
                 // pair of for loops.
                 assert (0);
             }
-        } // if(1)
+        }
+        else if (dbeta != 1.0)
+        {
+            // the matrix has at least one dim of length 0
+            // so we do this loop, which either iterates over 0 elements
+            // or else it does the right thing for length-0 x.
+            dtype_%(zz)s * zptr = (dtype_%(zz)s*)(%(zz)s->data);
+            for (int i = 0; i < Nx0; ++i)
+            {
+                zptr[i * Sz] *= dbeta;
+            }
+        }
     }
 
     """ % locals()