diff --git a/runtime/dex_file.cc b/runtime/dex_file.cc
index 0589cdd..20098e7 100644
--- a/runtime/dex_file.cc
+++ b/runtime/dex_file.cc
@@ -296,6 +296,12 @@
   return dex_file;
 }
 
+// Technically we do not have a limitation with respect to the number of dex files that can be in a
+// multidex APK. However, it's bad practice, as each dex file requires its own tables for symbols
+// (types, classes, methods, ...) and dex caches. So warn the user that we open a zip with what
+// seems an excessive number.
+static constexpr size_t kWarnOnManyDexFilesThreshold = 100;
+
 bool DexFile::OpenFromZip(const ZipArchive& zip_archive, const std::string& location,
                           std::string* error_msg,
                           std::vector<std::unique_ptr<const DexFile>>* dex_files) {
@@ -310,14 +316,13 @@
     dex_files->push_back(std::move(dex_file));
 
     // Now try some more.
-    size_t i = 2;
 
     // We could try to avoid std::string allocations by working on a char array directly. As we
     // do not expect a lot of iterations, this seems too involved and brittle.
 
-    while (i < 100) {
-      std::string name = StringPrintf("classes%zu.dex", i);
-      std::string fake_location = location + kMultiDexSeparator + name;
+    for (size_t i = 1; ; ++i) {
+      std::string name = GetMultiDexClassesDexName(i);
+      std::string fake_location = GetMultiDexLocation(i, location.c_str());
       std::unique_ptr<const DexFile> next_dex_file(Open(zip_archive, name.c_str(), fake_location,
                                                         error_msg, &error_code));
       if (next_dex_file.get() == nullptr) {
@@ -329,7 +334,16 @@
         dex_files->push_back(std::move(next_dex_file));
       }
 
-      i++;
+      if (i == kWarnOnManyDexFilesThreshold) {
+        LOG(WARNING) << location << " has in excess of " << kWarnOnManyDexFilesThreshold
+                     << " dex files. Please consider coalescing and shrinking the number to "
+                        " avoid runtime overhead.";
+      }
+
+      if (i == std::numeric_limits<size_t>::max()) {
+        LOG(ERROR) << "Overflow in number of dex files!";
+        break;
+      }
     }
 
     return true;
@@ -973,11 +987,19 @@
   return strrchr(location, kMultiDexSeparator) != nullptr;
 }
 
-std::string DexFile::GetMultiDexClassesDexName(size_t number, const char* dex_location) {
-  if (number == 0) {
+std::string DexFile::GetMultiDexClassesDexName(size_t index) {
+  if (index == 0) {
+    return "classes.dex";
+  } else {
+    return StringPrintf("classes%zu.dex", index + 1);
+  }
+}
+
+std::string DexFile::GetMultiDexLocation(size_t index, const char* dex_location) {
+  if (index == 0) {
     return dex_location;
   } else {
-    return StringPrintf("%s" kMultiDexSeparatorString "classes%zu.dex", dex_location, number + 1);
+    return StringPrintf("%s" kMultiDexSeparatorString "classes%zu.dex", dex_location, index + 1);
   }
 }
 
diff --git a/runtime/dex_file.h b/runtime/dex_file.h
index 0d07358..6b3f883 100644
--- a/runtime/dex_file.h
+++ b/runtime/dex_file.h
@@ -888,7 +888,13 @@
     return size_;
   }
 
-  static std::string GetMultiDexClassesDexName(size_t number, const char* dex_location);
+  // Return the name of the index-th classes.dex in a multidex zip file. This is classes.dex for
+  // index == 0, and classes{index + 1}.dex else.
+  static std::string GetMultiDexClassesDexName(size_t index);
+
+  // Return the (possibly synthetic) dex location for a multidex entry. This is dex_location for
+  // index == 0, and dex_location + multi-dex-separator + GetMultiDexClassesDexName(index) else.
+  static std::string GetMultiDexLocation(size_t index, const char* dex_location);
 
   // Returns the canonical form of the given dex location.
   //
diff --git a/runtime/dex_file_test.cc b/runtime/dex_file_test.cc
index 4d099e1..90b35a3 100644
--- a/runtime/dex_file_test.cc
+++ b/runtime/dex_file_test.cc
@@ -350,11 +350,20 @@
 }
 
 TEST_F(DexFileTest, GetMultiDexClassesDexName) {
+  ASSERT_EQ("classes.dex", DexFile::GetMultiDexClassesDexName(0));
+  ASSERT_EQ("classes2.dex", DexFile::GetMultiDexClassesDexName(1));
+  ASSERT_EQ("classes3.dex", DexFile::GetMultiDexClassesDexName(2));
+  ASSERT_EQ("classes100.dex", DexFile::GetMultiDexClassesDexName(99));
+}
+
+TEST_F(DexFileTest, GetMultiDexLocation) {
   std::string dex_location_str = "/system/app/framework.jar";
   const char* dex_location = dex_location_str.c_str();
-  ASSERT_EQ("/system/app/framework.jar", DexFile::GetMultiDexClassesDexName(0, dex_location));
-  ASSERT_EQ("/system/app/framework.jar:classes2.dex", DexFile::GetMultiDexClassesDexName(1, dex_location));
-  ASSERT_EQ("/system/app/framework.jar:classes101.dex", DexFile::GetMultiDexClassesDexName(100, dex_location));
+  ASSERT_EQ("/system/app/framework.jar", DexFile::GetMultiDexLocation(0, dex_location));
+  ASSERT_EQ("/system/app/framework.jar:classes2.dex",
+            DexFile::GetMultiDexLocation(1, dex_location));
+  ASSERT_EQ("/system/app/framework.jar:classes101.dex",
+            DexFile::GetMultiDexLocation(100, dex_location));
 }
 
 TEST_F(DexFileTest, GetDexCanonicalLocation) {
@@ -363,7 +372,7 @@
   std::string dex_location(dex_location_real.get());
 
   ASSERT_EQ(dex_location, DexFile::GetDexCanonicalLocation(dex_location.c_str()));
-  std::string multidex_location = DexFile::GetMultiDexClassesDexName(1, dex_location.c_str());
+  std::string multidex_location = DexFile::GetMultiDexLocation(1, dex_location.c_str());
   ASSERT_EQ(multidex_location, DexFile::GetDexCanonicalLocation(multidex_location.c_str()));
 
   std::string dex_location_sym = dex_location + "symlink";
@@ -371,7 +380,7 @@
 
   ASSERT_EQ(dex_location, DexFile::GetDexCanonicalLocation(dex_location_sym.c_str()));
 
-  std::string multidex_location_sym = DexFile::GetMultiDexClassesDexName(1, dex_location_sym.c_str());
+  std::string multidex_location_sym = DexFile::GetMultiDexLocation(1, dex_location_sym.c_str());
   ASSERT_EQ(multidex_location, DexFile::GetDexCanonicalLocation(multidex_location_sym.c_str()));
 
   ASSERT_EQ(0, unlink(dex_location_sym.c_str()));
diff --git a/runtime/oat_file_assistant.cc b/runtime/oat_file_assistant.cc
index 37e85ab..2f67263 100644
--- a/runtime/oat_file_assistant.cc
+++ b/runtime/oat_file_assistant.cc
@@ -230,8 +230,8 @@
   dex_files.push_back(std::move(dex_file));
 
   // Load secondary multidex files
-  for (int i = 1; ; i++) {
-    std::string secondary_dex_location = DexFile::GetMultiDexClassesDexName(i, dex_location);
+  for (size_t i = 1; ; i++) {
+    std::string secondary_dex_location = DexFile::GetMultiDexLocation(i, dex_location);
     oat_dex_file = oat_file.GetOatDexFile(secondary_dex_location.c_str(), nullptr, false);
     if (oat_dex_file == nullptr) {
       // There are no more secondary dex files to load.
@@ -403,9 +403,9 @@
   }
 
   // Verify the dex checksums for any secondary multidex files
-  for (int i = 1; ; i++) {
+  for (size_t i = 1; ; i++) {
     std::string secondary_dex_location
-      = DexFile::GetMultiDexClassesDexName(i, dex_location_);
+      = DexFile::GetMultiDexLocation(i, dex_location_);
     const OatFile::OatDexFile* secondary_oat_dex_file
       = file.GetOatDexFile(secondary_dex_location.c_str(), nullptr, false);
     if (secondary_oat_dex_file == nullptr) {