feat(mgb/opr): allow empty ImmutableTensor

Fixes MGE-675.

GitOrigin-RevId: c6771740fc48226f1b7c79d519de61445e671290

src/gopt/impl/framework.cpp

@@ -761,7 +761,8 @@ ConstVarPropogateBase::AddOprResult ConstVarPropogateBase::add_opr(
 
     if (is_const_var(m_const_var_type, opr)) {
         auto sz = var_mem_size(opr->output(0));
-        mgb_assert(sz);
+        mgb_assert(sz || opr->output(0)->contain_flag(
+                                 VarNode::Flag::ALLOW_EMPTY_SHAPE));
         info.is_const = true;
         info.max_size = sz;
         return make_ret();

src/opr/impl/io.cpp

@@ -382,7 +382,7 @@ class ImmutableTensor::Value {
         void setup(CompNode cn, const HostTensorND &val);
 
         bool initialized() const {
-            return !m_dev.empty();
+            return m_dev.shape_valid();
         }
 
         //! value on comp node
@@ -400,8 +400,9 @@ class ImmutableTensor::Value {
 };
 
 void ImmutableTensor::Value::setup(CompNode cn, const HostTensorND &val) {
-    mgb_assert(m_dev.empty() && !val.empty());
+    mgb_assert(m_dev.empty() && !m_dev.shape_valid());
     m_dev.comp_node(cn).copy_from(val).sync();
+    mgb_assert(val.empty() == m_dev.empty());
 
     auto one_elem = [](const TensorShape& shape) {
         for (size_t i = 0; i < shape.ndim; ++i) {
@@ -446,6 +447,7 @@ class ImmutableTensor::DevValueCache final: public UserDataContainer::UserData {
         HostTensorND m_val_ref;
 
         const dt_byte* val_ptr() const {
+            mgb_assert(m_trait.size_bytes);
             return m_val.empty() ? m_val_ref.raw_ptr() : m_val.data();
         }
 
@@ -454,9 +456,8 @@ class ImmutableTensor::DevValueCache final: public UserDataContainer::UserData {
             TensorKey(const HostTensorND &v):
                 m_val_ref{v}
             {
-                mgb_assert(v.layout().is_contiguous());
+                mgb_assert(v.layout().is_contiguous() || v.layout().is_empty());
                 m_trait.size_bytes = v.layout().span().high_byte;
-                mgb_assert(m_trait.size_bytes);
 
                 auto &&layout = m_trait.layout;
                 // zero to enable byte-comparison
@@ -467,15 +468,19 @@ class ImmutableTensor::DevValueCache final: public UserDataContainer::UserData {
                     layout.shape[i] = v.layout().shape[i];
                     layout.stride[i] = v.layout().stride[i];
                 }
-                m_trait.hash = XXHash{}.
-                    update(v.raw_ptr(), m_trait.size_bytes).
-                    update(&m_trait.layout, sizeof(m_trait.layout)).
-                    digest();
+                XXHash hasher;
+                if (!v.empty()) {
+                    hasher.update(v.raw_ptr(), m_trait.size_bytes);
+                }
+                hasher.update(&m_trait.layout, sizeof(m_trait.layout));
+                m_trait.hash = hasher.digest();
             }
 
             bool operator == (const TensorKey &rhs) const {
                 return !memcmp(&m_trait, &rhs.m_trait, sizeof(Trait)) &&
-                    !memcmp(val_ptr(), rhs.val_ptr(), m_trait.size_bytes);
+                       ((m_trait.size_bytes == 0 &&
+                         rhs.m_trait.size_bytes == 0) ||
+                        !memcmp(val_ptr(), rhs.val_ptr(), m_trait.size_bytes));
             }
 
             size_t hash() const {
@@ -485,6 +490,7 @@ class ImmutableTensor::DevValueCache final: public UserDataContainer::UserData {
             //! copy from m_val_ref to m_val, to avoid refed value being
             //! modified
             void copy_val_permanent() {
+                if (m_trait.size_bytes == 0) return;
                 mgb_assert(m_val.empty());
                 m_val.resize(m_trait.size_bytes);
                 memcpy(m_val.data(), m_val_ref.raw_ptr(), m_trait.size_bytes);
@@ -544,7 +550,6 @@ class ImmutableTensor::DevValueCache final: public UserDataContainer::UserData {
         }
 
         const Value& get(const HostTensorND &tensor) {
-            mgb_assert(!tensor.empty());
             if (tensor.shape().is_scalar()) {
                 return get(DTypeScalar::make_from_raw(
                             tensor.dtype(), tensor.raw_ptr()));
@@ -595,6 +600,7 @@ ImmutableTensor::ImmutableTensor(ComputingGraph &graph,
 
     add_output(value.dev().dtype());
     add_equivalence_component<ScalarHash<const void*>>(&value);
+    output(0)->add_flag(VarNode::Flag::ALLOW_EMPTY_SHAPE);
 }
 
 ImmutableTensor::~ImmutableTensor() noexcept = default;

src/opr/test/io.cpp

@@ -177,6 +177,17 @@ TEST(TestOprIO, ImmutableTensorLarge) {
     }
 }
 
+TEST(TestOprIO, ImmutableTensorEmpty) {
+    HostTensorGenerator<> gen;
+    auto graph = ComputingGraph::make();
+    auto host_x = gen({1, 9, 1, 9, 8, 1, 0});
+    auto x = opr::ImmutableTensor::make(*graph, *host_x);
+    HostTensorND host_x2;
+    auto func = graph->compile({make_callback_copy(x, host_x2)});
+    func->execute();
+    ASSERT_TRUE(host_x2.shape().is_empty());
+}
+
 TEST(TestOprIO, SharedDeviceTensor) {
     HostTensorGenerator<> gen;
     auto hv = gen({123});