Generate dot() in the Metal backend (#7085)

* dot() support for Metal backend)

* Restrict dot() to floats

src/CodeGen_Metal_Dev.cpp

@@ -91,6 +91,7 @@ class CodeGen_Metal_Dev : public CodeGen_GPU_Dev {
         void visit(const Allocate *op) override;
         void visit(const Free *op) override;
         void visit(const Cast *op) override;
+        void visit(const VectorReduce *op) override;
         void visit(const Atomic *op) override;
     };
 
@@ -223,6 +224,20 @@ void CodeGen_Metal_Dev::CodeGen_Metal_C::visit(const Min *op) {
     print_expr(Call::make(op->type, "min", {op->a, op->b}, Call::Extern));
 }
 
+void CodeGen_Metal_Dev::CodeGen_Metal_C::visit(const VectorReduce *op) {
+    if (op->op == VectorReduce::Add && op->type.is_float() && (op->type.lanes() == 1)) {
+        if (const Mul *maybe_mul = op->value.as<Mul>()) {
+            string a = print_expr(maybe_mul->a);
+            string b = print_expr(maybe_mul->b);
+            ostringstream rhs;
+            rhs << "dot(" << a << ", " << b << ")";
+            print_assignment(op->type, rhs.str());
+            return;
+        }
+    }
+    CodeGen_GPU_C::visit(op);
+}
+
 void CodeGen_Metal_Dev::CodeGen_Metal_C::visit(const Div *op) {
     int bits;
     if (is_const_power_of_two_integer(op->b, &bits)) {