performance_tests: measure ge_frombytes_vartime only

not an extra dynamic cast, which might be heavy

tests/performance_tests/ge_frombytes_vartime.h

@@ -49,22 +49,29 @@ public:
     if (!base_class::init())
       return false;
 
+    cryptonote::account_base m_alice;
+    cryptonote::transaction m_tx;
+
     m_alice.generate();
 
     std::vector<tx_destination_entry> destinations;
     destinations.push_back(tx_destination_entry(1, m_alice.get_keys().m_account_address, false));
 
-    return construct_tx(this->m_miners[this->real_source_idx].get_keys(), this->m_sources, destinations, boost::none, std::vector<uint8_t>(), m_tx, 0);
+    if (!construct_tx(this->m_miners[this->real_source_idx].get_keys(), this->m_sources, destinations, boost::none, std::vector<uint8_t>(), m_tx, 0))
+      return false;
+
+    const cryptonote::txin_to_key& txin = boost::get<cryptonote::txin_to_key>(m_tx.vin[0]);
+    m_key = rct::ki2rct(txin.k_image);
+
+    return true;
   }
 
   bool test()
   {
     ge_p3 unp;
-    const cryptonote::txin_to_key& txin = boost::get<cryptonote::txin_to_key>(m_tx.vin[0]);
-    return ge_frombytes_vartime(&unp, (const unsigned char*) &txin.k_image) == 0;
+    return ge_frombytes_vartime(&unp, (const unsigned char*) &m_key) == 0;
   }
 
 private:
-  cryptonote::account_base m_alice;
-  cryptonote::transaction m_tx;
+  rct::key m_key;
 };