diff --git a/compiler/utils/assembler_test_base.h b/compiler/utils/assembler_test_base.h
index 8c71292..ac24ee9 100644
--- a/compiler/utils/assembler_test_base.h
+++ b/compiler/utils/assembler_test_base.h
@@ -106,7 +106,9 @@
   // Driver() assembles and compares the results. If the results are not equal and we have a
   // disassembler, disassemble both and check whether they have the same mnemonics (in which case
   // we just warn).
-  void Driver(const std::vector<uint8_t>& data, std::string assembly_text, std::string test_name) {
+  void Driver(const std::vector<uint8_t>& data,
+              const std::string& assembly_text,
+              const std::string& test_name) {
     EXPECT_NE(assembly_text.length(), 0U) << "Empty assembly";
 
     NativeAssemblerResult res;
@@ -229,7 +231,7 @@
     bool success = Exec(args, error_msg);
     if (!success) {
       LOG(ERROR) << "Assembler command line:";
-      for (std::string arg : args) {
+      for (const std::string& arg : args) {
         LOG(ERROR) << arg;
       }
     }
@@ -238,7 +240,7 @@
 
   // Runs objdump -h on the binary file and extracts the first line with .text.
   // Returns "" on failure.
-  std::string Objdump(std::string file) {
+  std::string Objdump(const std::string& file) {
     bool have_objdump = FileExists(FindTool(objdump_cmd_name_));
     EXPECT_TRUE(have_objdump) << "Cannot find objdump: " << GetObjdumpCommand();
     if (!have_objdump) {
@@ -287,8 +289,9 @@
   }
 
   // Disassemble both binaries and compare the text.
-  bool DisassembleBinaries(const std::vector<uint8_t>& data, const std::vector<uint8_t>& as,
-                           std::string test_name) {
+  bool DisassembleBinaries(const std::vector<uint8_t>& data,
+                           const std::vector<uint8_t>& as,
+                           const std::string& test_name) {
     std::string disassembler = GetDisassembleCommand();
     if (disassembler.length() == 0) {
       LOG(WARNING) << "No dissassembler command.";
@@ -324,7 +327,7 @@
     return result;
   }
 
-  bool DisassembleBinary(std::string file, std::string* error_msg) {
+  bool DisassembleBinary(const std::string& file, std::string* error_msg) {
     std::vector<std::string> args;
 
     // Encaspulate the whole command line in a single string passed to
@@ -345,7 +348,7 @@
     return Exec(args, error_msg);
   }
 
-  std::string WriteToFile(const std::vector<uint8_t>& buffer, std::string test_name) {
+  std::string WriteToFile(const std::vector<uint8_t>& buffer, const std::string& test_name) {
     std::string file_name = GetTmpnam() + std::string("---") + test_name;
     const char* data = reinterpret_cast<const char*>(buffer.data());
     std::ofstream s_out(file_name + ".o");
@@ -354,7 +357,7 @@
     return file_name + ".o";
   }
 
-  bool CompareFiles(std::string f1, std::string f2) {
+  bool CompareFiles(const std::string& f1, const std::string& f2) {
     std::ifstream f1_in(f1);
     std::ifstream f2_in(f2);
 
@@ -369,7 +372,9 @@
   }
 
   // Compile the given assembly code and extract the binary, if possible. Put result into res.
-  bool Compile(std::string assembly_code, NativeAssemblerResult* res, std::string test_name) {
+  bool Compile(const std::string& assembly_code,
+               NativeAssemblerResult* res,
+               const std::string& test_name) {
     res->ok = false;
     res->code.reset(nullptr);
 
@@ -438,7 +443,7 @@
   // Check whether file exists. Is used for commands, so strips off any parameters: anything after
   // the first space. We skip to the last slash for this, so it should work with directories with
   // spaces.
-  static bool FileExists(std::string file) {
+  static bool FileExists(const std::string& file) {
     if (file.length() == 0) {
       return false;
     }
@@ -478,7 +483,7 @@
     return getcwd(temp, 1024) ? std::string(temp) + "/" : std::string("");
   }
 
-  std::string FindTool(std::string tool_name) {
+  std::string FindTool(const std::string& tool_name) {
     // Find the current tool. Wild-card pattern is "arch-string*tool-name".
     std::string gcc_path = GetRootPath() + GetGCCRootPath();
     std::vector<std::string> args;
@@ -522,7 +527,8 @@
 
   // Helper for below. If name_predicate is empty, search for all files, otherwise use it for the
   // "-name" option.
-  static void FindToolDumpPrintout(std::string name_predicate, std::string tmp_file) {
+  static void FindToolDumpPrintout(const std::string& name_predicate,
+                                   const std::string& tmp_file) {
     std::string gcc_path = GetRootPath() + GetGCCRootPath();
     std::vector<std::string> args;
     args.push_back("find");
@@ -562,7 +568,7 @@
   }
 
   // For debug purposes.
-  void FindToolDump(std::string tool_name) {
+  void FindToolDump(const std::string& tool_name) {
     // Check with the tool name.
     FindToolDumpPrintout(architecture_string_ + "*" + tool_name, GetTmpnam());
     FindToolDumpPrintout("", GetTmpnam());