Update dce to no remove ops with no output

src/dead_code_elimination.cpp

@@ -15,8 +15,12 @@ void dead_code_elimination::apply(program& p) const
         // instruction
         if(ins == p.begin())
             continue;
+        const auto i = std::prev(ins);
+        // Skip instruction with empty shape as output
+        if(i->result.elements() == 0)
+            continue;
         // Skip the last instruction
-        if(std::prev(ins) == last)
+        if(i == last)
             break;
         fix([&](auto self, auto leaf) {
             assert(p.has_instruction(leaf));
@@ -28,7 +32,7 @@ void dead_code_elimination::apply(program& p) const
                 for(auto arg : args)
                     self(arg);
             }
-        })(std::prev(ins));
+        })(i);
     }
     p.remove_instructions(std::next(last), p.end());
 }

src/shape.cpp

@@ -43,6 +43,8 @@ const std::vector<std::size_t>& shape::strides() const { return this->m_strides;
 std::size_t shape::elements() const
 {
     assert(this->lens().size() == this->strides().size());
+    if(this->lens().empty())
+        return 0;
     return std::accumulate(
         this->lens().begin(), this->lens().end(), std::size_t{1}, std::multiplies<std::size_t>());
 }
@@ -101,6 +103,8 @@ bool shape::standard() const { return this->m_standard; }
 std::size_t shape::element_space() const
 {
     assert(this->lens().size() == this->strides().size());
+    if(this->lens().empty())
+        return 0;
     return std::inner_product(this->lens().begin(),
                               this->lens().end(),
                               this->strides().begin(),

src/simplify_reshapes.cpp

@@ -10,10 +10,14 @@ namespace migraph {
 
 bool is_reshaper(const std::string& name)
 {
-    static const std::unordered_set<std::string> names = {"reshape",
-                                                          "transpose",
-                                                          // "broadcast",
-                                                          "contiguous"};
+    // clang-format off
+    static const std::unordered_set<std::string> names = {
+        "reshape",
+        "transpose",
+        // "broadcast",
+        "contiguous"
+    };
+    // clang-format on
     return contains(names, name);
 }
 

src/targets/gpu/target.cpp

@@ -19,8 +19,8 @@ std::vector<pass> target::get_passes(migraph::context&) const
         simplify_reshapes{},
         lowering{},
         write_literals{},
-        dead_code_elimination{},
-        check_context<context>{}
+        check_context<context>{},
+        dead_code_elimination{}
     };
     // clang-format on
 }

test/dead_code_elimination_test.cpp

@@ -27,6 +27,22 @@ void simple_test()
     EXPECT(result != migraph::literal{4});
 }
 
+void simple_test_nop()
+{
+    migraph::program p;
+
+    auto one = p.add_literal(1);
+    auto two = p.add_literal(2);
+    p.add_instruction(nop{});
+    p.add_instruction(sum_op{}, one, two);
+    auto count = std::distance(p.begin(), p.end());
+    p.compile(dce_target{});
+    EXPECT(std::distance(p.begin(), p.end()) == count);
+    auto result = p.eval({});
+    EXPECT(result == migraph::literal{3});
+    EXPECT(result != migraph::literal{4});
+}
+
 void duplicate_test1()
 {
     migraph::program p;
@@ -82,6 +98,7 @@ void depth_test()
 int main()
 {
     simple_test();
+    simple_test_nop();
     duplicate_test1();
     duplicate_test2();
     depth_test();

test/include/basic_ops.hpp

@@ -80,3 +80,18 @@ struct pass_op
         return inputs.front();
     }
 };
+
+struct nop
+{
+    std::string name() const { return "nop"; }
+    migraph::argument
+    compute(migraph::context&, migraph::shape, std::vector<migraph::argument>) const
+    {
+        return {};
+    }
+
+    migraph::shape compute_shape(std::vector<migraph::shape>) const
+    {
+        return {};
+    }
+};

test/shape_test.cpp

@@ -5,6 +5,13 @@
 #include <numeric>
 #include "test.hpp"
 
+void test_shape_default()
+{
+    migraph::shape s{};
+    EXPECT(s.elements() == 0);
+    EXPECT(s.bytes() == 0);
+}
+
 void test_shape_assign()
 {
     migraph::shape s1{migraph::shape::float_type, {100, 32, 8, 8}};
@@ -49,7 +56,7 @@ void test_shape_broadcasted()
     EXPECT(s.broadcasted());
 }
 
-void test_shape_default()
+void test_shape_default_copy()
 {
     migraph::shape s1{};
     migraph::shape s2{};
@@ -136,12 +143,13 @@ void test_shape4_nonpacked()
 
 int main()
 {
+    test_shape_default();
     test_shape_assign();
     test_shape_packed_default();
     test_shape_packed();
     test_shape_transposed();
     test_shape_broadcasted();
-    test_shape_default();
+    test_shape_default_copy();
     test_shape4();
     test_shape4_nonpacked();
 }