Address execution hot spots (#154)

* Address execution hot spots

* extend the reserve logic a bit

* FIx more hot spots

* Performance: use resize(0) instead of clear() in order to avoid a free/malloc

nnvm/src/c_api/c_api_symbolic.cc

@@ -15,7 +15,8 @@ int NNListAllOpNames(nn_uint *out_size,
   API_BEGIN();
   NNAPIThreadLocalEntry *ret = NNAPIThreadLocalStore::Get();
   ret->ret_vec_str = dmlc::Registry<Op>::ListAllNames();
-  ret->ret_vec_charp.clear();
+  ret->ret_vec_charp.resize(0);
+  ret->ret_vec_charp.reserve(ret->ret_vec_str.size());
   for (size_t i = 0; i < ret->ret_vec_str.size(); ++i) {
     ret->ret_vec_charp.push_back(ret->ret_vec_str[i].c_str());
   }
@@ -64,7 +65,8 @@ int NNGetOpInfo(OpHandle handle,
   *description = op->description.c_str();
   *num_doc_args = static_cast<nn_uint>(op->arguments.size());
   if (return_type) *return_type = nullptr;
-  ret->ret_vec_charp.clear();
+  ret->ret_vec_charp.resize(0);
+  ret->ret_vec_charp.reserve(op->arguments.size() * 3);
   for (size_t i = 0; i < op->arguments.size(); ++i) {
     ret->ret_vec_charp.push_back(op->arguments[i].name.c_str());
   }
@@ -207,13 +209,15 @@ int NNSymbolListAttrs(SymbolHandle symbol,
       s->ListAttrs(static_cast<Symbol::ListAttrOption>(option));  // NOLINT(*)
 
   std::vector<std::string>& attr_list = ret->ret_vec_str;
-  attr_list.clear();
+  attr_list.resize(0);
+  attr_list.reserve(attr.size());
   for (const auto& kv : attr) {
     attr_list.push_back(kv.first);
     attr_list.push_back(kv.second);
   }
   *out_size = attr.size();
   ret->ret_vec_charp.clear();
+  ret->ret_vec_charp.reserve(ret->ret_vec_str.size());
   for (size_t i = 0; i < ret->ret_vec_str.size(); ++i) {
     ret->ret_vec_charp.push_back(ret->ret_vec_str[i].c_str());
   }
@@ -229,7 +233,8 @@ int NNSymbolListInputVariables(SymbolHandle symbol,
   NNAPIThreadLocalEntry *ret = NNAPIThreadLocalStore::Get();
   API_BEGIN();
   std::vector<NodePtr> vs = s->ListInputs(Symbol::ListInputOption(option));
-  ret->ret_handles.clear();
+  ret->ret_handles.resize(0);
+  ret->ret_handles.reserve(vs.size());
   for (size_t i = 0; i < vs.size(); ++i) {
     nnvm::Symbol* rs = new nnvm::Symbol();
     rs->outputs.push_back(NodeEntry{vs[i], 0, 0});
@@ -249,7 +254,8 @@ int NNSymbolListInputNames(SymbolHandle symbol,
   API_BEGIN();
   ret->ret_vec_str =
       s->ListInputNames(Symbol::ListInputOption(option));
-  ret->ret_vec_charp.clear();
+  ret->ret_vec_charp.resize(0);
+  ret->ret_vec_charp.reserve(ret->ret_vec_str.size());
   for (size_t i = 0; i < ret->ret_vec_str.size(); ++i) {
     ret->ret_vec_charp.push_back(ret->ret_vec_str[i].c_str());
   }
@@ -265,7 +271,8 @@ int NNSymbolListOutputNames(SymbolHandle symbol,
   NNAPIThreadLocalEntry *ret = NNAPIThreadLocalStore::Get();
   API_BEGIN();
   ret->ret_vec_str = s->ListOutputNames();
-  ret->ret_vec_charp.clear();
+  ret->ret_vec_charp.resize(0);
+  ret->ret_vec_charp.reserve(ret->ret_vec_str.size());
   for (size_t i = 0; i < ret->ret_vec_str.size(); ++i) {
     ret->ret_vec_charp.push_back(ret->ret_vec_str[i].c_str());
   }

nnvm/src/core/symbolic.cc

@@ -189,6 +189,7 @@ Symbol Symbol::operator[] (size_t index) const {
 std::vector<NodePtr> Symbol::ListInputs(ListInputOption option) const {
   std::vector<NodePtr> ret;
   if (option == kAll) {
+    ret.reserve(this->outputs.size());
     DFSVisit(this->outputs, [&ret](const NodePtr &node) {
         if (node->is_variable()) {
           ret.push_back(node);
@@ -197,6 +198,7 @@ std::vector<NodePtr> Symbol::ListInputs(ListInputOption option) const {
   } else {
     std::unordered_set<Node*> mutable_set;
     std::vector<NodePtr> vlist;
+    vlist.reserve(this->outputs.size());
     static auto& fmutate_inputs = Op::GetAttr<FMutateInputs>("FMutateInputs");
     DFSVisit(this->outputs, [&ret, &mutable_set, &vlist](const NodePtr &node) {
         if (node->is_variable()) {
@@ -207,6 +209,7 @@ std::vector<NodePtr> Symbol::ListInputs(ListInputOption option) const {
           }
         }
       });
+    ret.reserve(vlist.size());
     for (const NodePtr& node : vlist) {
       if ((option == kReadOnlyArgs && mutable_set.count(node.get()) == 0) ||
           (option == kAuxiliaryStates && mutable_set.count(node.get()) != 0)) {
@@ -230,6 +233,7 @@ std::vector<std::string> Symbol::ListOutputNames() const {
   static auto& flist_ouputs = Op::GetAttr<FListOutputNames>("FListOutputNames");
 
   std::vector<std::string> ret;
+  ret.reserve(outputs.size());
   for (auto &head : outputs) {
     if (head.node->is_variable()) {
       ret.push_back(head.node->attrs.name);