introduce a version of matmul which does not depend on first arg

Former-commit-id: 46c9a58

Author: yoavg

dynet/dynet.h

@@ -636,8 +636,10 @@ struct Node {
 
   Device* device; /**< pointer to the node, or null to inherit device from first input, or default when there is no input */
 
+  unsigned matmul_count; // how many matmul nodes am I an arg of?
+
 protected:
-  Node() : args(), device(default_device) {}
+  Node() : args(), device(default_device), matmul_count(0) {}
   explicit Node(const std::initializer_list<VariableIndex>& a) : args(a), device(default_device) {}
   template <typename T>
   explicit Node(const T&c) : args(c.begin(), c.end()), device(default_device) {}

dynet/expr.cc

@@ -52,7 +52,7 @@ Expression operator+(const Expression& x, real y) { return y + x; }
 Expression operator-(const Expression& x, const Expression& y) { return x + (-y); }
 Expression operator-(real x, const Expression& y) { return Expression(y.pg, y.pg->add_function<ConstantMinusX>({y.i}, x)); }
 Expression operator-(const Expression& x, real y) { return -(y - x); }
-Expression operator*(const Expression& x, const Expression& y) { return Expression(x.pg, x.pg->add_function<MatrixMultiply>({x.i, y.i})); }
+Expression operator*(const Expression& x, const Expression& y) { x.pg->nodes[x.i]->matmul_count++; return Expression(x.pg, x.pg->add_function<MatrixMultiply>({x.i, y.i})); }
 Expression operator*(const Expression& x, float y) { return Expression(x.pg, x.pg->add_function<ConstScalarMultiply>({x.i}, y)); }
 Expression cmult(const Expression& x, const Expression& y) { 
     if (x.dim().batch_size() == 1) 

dynet/nodes-common.cc

@@ -879,16 +879,25 @@ int MatrixMultiply::autobatch_sig(const ComputationGraph & cg, SigMap &sm) const
   // TODO do we want to treat different dimensions of first/second arg differently?
   if(dim.bd == 1) {
     Sig s(nt::matmul);
-    s.add_node(args[0]);
+    // if arg0 is likely to be shared, include it in the sig.
+    // otherwise, include both args dims in the sig.
+    if (cg.nodes[args[0]]->matmul_count > 2) {  //TODO why 2? can we set a better number?
+      s.add_node(args[0]); s.add_dim(cg.nodes[args[1]]->dim);
+    } else { 
+      s.add_dim(cg.nodes[args[0]]->dim); s.add_dim(cg.nodes[args[1]]->dim);
+    }
     return sm.get_idx(s);
   } else {
     return 0; // TODO handle the batched case as well? should it differ at all?
   }
 }
 
 std::vector<int> MatrixMultiply::autobatch_concat(const ComputationGraph & cg) const {
-  vector<int> ret(args.size(), 0);
-  if (dim.bd == 1) { ret[1] = 1; }
+  vector<int> ret(2, 0);
+  if (dim.bd == 1) { 
+    ret[1] = 1; 
+    if (cg.nodes[args[0]]->matmul_count <= 2) { ret[0] = 1; }
+  }
   return ret;
 }