Use hash for library placeholders.

libevmasm/LinkerObject.cpp

@@ -21,6 +21,7 @@
 
 #include <libevmasm/LinkerObject.h>
 #include <libdevcore/CommonData.h>
+#include <libdevcore/SHA3.h>
 
 using namespace dev;
 using namespace dev::eth;
@@ -50,14 +51,19 @@ string LinkerObject::toHex() const
 	for (auto const& ref: linkReferences)
 	{
 		size_t pos = ref.first * 2;
-		string const& name = ref.second;
+		string hash = libraryPlaceholder(ref.second);
 		hex[pos] = hex[pos + 1] = hex[pos + 38] = hex[pos + 39] = '_';
 		for (size_t i = 0; i < 36; ++i)
-			hex[pos + 2 + i] = i < name.size() ? name[i] : '_';
+			hex[pos + 2 + i] = hash.at(i);
 	}
 	return hex;
 }
 
+string LinkerObject::libraryPlaceholder(string const& _libraryName)
+{
+	return keccak256(_libraryName).hex().substr(0, 36);
+}
+
 h160 const*
 LinkerObject::matchLibrary(
 	string const& _linkRefName,

libevmasm/LinkerObject.h

@@ -50,6 +50,11 @@ struct LinkerObject
 	/// addresses by placeholders.
 	std::string toHex() const;
 
+	/// @returns a 36 character string that is used as a placeholder for the library
+	/// address (enclosed by `__` on both sides). The placeholder is the hex representation
+	/// of the first 18 bytes of the keccak-256 hash of @a _libraryName.
+	static std::string libraryPlaceholder(std::string const& _libraryName);
+
 private:
 	static h160 const* matchLibrary(
 		std::string const& _linkRefName,