ART: Make some variables references

Avoid copying values.

Bug: 32619234
Test: m
Change-Id: I86bc206d6f6fccdf730c62c60317a7a4ae26c11b
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index 1b87725..8503934 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -1522,7 +1522,7 @@
 
   if (!use_dex_cache) {
     bool method_in_image = false;
-    const std::vector<gc::space::ImageSpace*> image_spaces = heap->GetBootImageSpaces();
+    const std::vector<gc::space::ImageSpace*>& image_spaces = heap->GetBootImageSpaces();
     for (gc::space::ImageSpace* image_space : image_spaces) {
       const auto& method_section = image_space->GetImageHeader().GetMethodsSection();
       if (method_section.Contains(reinterpret_cast<uint8_t*>(method) - image_space->Begin())) {
diff --git a/runtime/gc/space/image_space.cc b/runtime/gc/space/image_space.cc
index 118daac..0b602e9 100644
--- a/runtime/gc/space/image_space.cc
+++ b/runtime/gc/space/image_space.cc
@@ -1606,7 +1606,7 @@
 
   std::ostringstream oss;
   bool first = true;
-  for (auto msg : error_msgs) {
+  for (const auto& msg : error_msgs) {
     if (!first) {
       oss << "\n    ";
     }
diff --git a/runtime/oat_file_manager.cc b/runtime/oat_file_manager.cc
index fde0a2c..d71ef54 100644
--- a/runtime/oat_file_manager.cc
+++ b/runtime/oat_file_manager.cc
@@ -398,7 +398,7 @@
     while (!temp.empty() && index < shared_libraries_split.size() - 1) {
       DexFileAndClassPair pair(temp.top());
       const DexFile* dex_file = pair.GetDexFile();
-      std::string dex_filename(dex_file->GetLocation());
+      const std::string& dex_filename = dex_file->GetLocation();
       uint32_t dex_checksum = dex_file->GetLocationChecksum();
       if (dex_filename != shared_libraries_split[index] ||
           dex_checksum != std::stoul(shared_libraries_split[index + 1])) {