Numerically stable sum so we can pass mxnet unit tests (#381)

* Numerically stable sum so we can pass mxnet unit tests

* Add a small initial residual

src/ngraph/runtime/cpu/cpu_kernel_emitters.cpp

@@ -303,12 +303,15 @@ void ngraph::runtime::cpu::kernel::emit_sum(codegen::CodeWriter& writer,
     if (out_shape.size() == 0)
     {
         writer << dest_nd_name << " = 0;\n";
+        writer << element_type << " residual = 0;\n";
     }
     else
     {
+        writer << element_type << " residual" << emit_bracketed_string(out_shape) << ";\n";
         auto output_vars = open_for_loops(writer, out_shape);
 
         writer << dest_nd_name << emit_bracketed_string(output_vars) << " = 0;\n";
+        writer << "residual" << emit_bracketed_string(output_vars) << " = 1e-8;\n";
 
         close_for_loops(writer, output_vars);
     }
@@ -357,10 +360,13 @@ void ngraph::runtime::cpu::kernel::emit_sum(codegen::CodeWriter& writer,
                 writer.indent++;
             }
         }
-
-        writer << dest_nd_name << emit_bracketed_string(out_indexes) << " += " << source_nd_name
-               << emit_bracketed_string(index_vars) << ";\n";
-
+        auto out_brackets = emit_bracketed_string(out_indexes);
+        auto dst = dest_nd_name + out_brackets;
+        auto src = source_nd_name + emit_bracketed_string(index_vars);
+        writer << element_type << " y = " << src << " - residual" << out_brackets << ";\n";
+        writer << element_type << " t = " << dst << " + y;\n";
+        writer << "residual" << out_brackets << " = (t - " << dst << ") - y;\n";
+        writer << dst << " = t;\n";
         close_for_loops(writer, index_vars);
     }
 }

test/backend_test.in.cpp

@@ -3257,6 +3257,50 @@ TEST(${BACKEND_NAME}, sum_3d_eliminate_zero_dim)
     EXPECT_EQ((vector<float>{0, 0, 0, 0, 0, 0}), result->get_vector<float>());
 }
 
+TEST(${BACKEND_NAME}, sum_to_scalar_stable)
+{
+    auto shape = Shape{2, 2};
+    auto A = make_shared<op::Parameter>(element::f32, shape);
+    auto f = make_shared<Function>(make_shared<op::Sum>(A, AxisSet{0, 1}), op::Parameters{A});
+
+    auto manager = runtime::Manager::get("${BACKEND_NAME}");
+    auto external = manager->compile(f);
+    auto backend = manager->allocate_backend();
+    auto cf = backend->make_call_frame(external);
+
+    // Create some tensors for input/output
+    auto a = backend->make_primary_tensor_view(element::f32, shape);
+    copy_data(a, vector<float>{1e-6, -1, 0, 1});
+    auto result = backend->make_primary_tensor_view(element::f32, Shape{});
+
+    cf->call({a}, {result});
+    EXPECT_TRUE(test::all_close(result->get_vector<float>(), vector<float>{1e-6}, 5e-2f));
+    // EXPECT_EQ(vector<float>{1e-6}, result->get_vector<float>());
+}
+
+TEST(${BACKEND_NAME}, sum_3d_to_vector_stable)
+{
+    auto shape_a = Shape{3, 3, 3};
+    auto A = make_shared<op::Parameter>(element::f32, shape_a);
+    auto shape_rt = Shape{3};
+    auto f = make_shared<Function>(make_shared<op::Sum>(A, AxisSet{0, 1}), op::Parameters{A});
+
+    auto manager = runtime::Manager::get("${BACKEND_NAME}");
+    auto external = manager->compile(f);
+    auto backend = manager->allocate_backend();
+    auto cf = backend->make_call_frame(external);
+
+    // Create some tensors for input/output
+    auto a = backend->make_primary_tensor_view(element::f32, shape_a);
+    copy_data(a, vector<float>{1, 1,  1,  1,  1,  1,  1e-4, 1e-5, 1e-6, 1,  1,  1,  1, 1,
+                               1, -1, -1, -1, -1, -1, -1,   -1,   -1,   -1, -1, -1, -1});
+    auto result = backend->make_primary_tensor_view(element::f32, shape_rt);
+
+    cf->call({a}, {result});
+    EXPECT_TRUE(
+        test::all_close(result->get_vector<float>(), vector<float>{1e-4, 1e-5, 1e-6}, 5e-2f));
+}
+
 TEST(${BACKEND_NAME}, sign)
 {
     auto shape = Shape{2, 3};