creating workflow for mirror::String compression

All-ASCII String characters are stored in 8-bit blocks
instead of 16-bit. The compression has not taken place, but all
workflow are in the code already (changing kUseStringCompression in
string.h file to TRUE will enable the feature)

Notes: Feature works on interpreter only without optimizing

Test art: m ART_TEST_INTERPRETER=true ART_TEST_OPTIMIZING=false
          test-art-host

Also tested with String tests from libcore/:
1. libcore.java.lang.StringTest
2. libcore.java.lang.StringBufferTest
3. libcore.java.lang.StringBuilderTest
4. libcore.java.lang.OldStringTest
5. libcore.java.lang.OldStringBufferTest

Memory improvement is 33% (from 6.03% to 4.03%, total String memory
from all apps per total memory of all apps) measured on Angler
with Hprof tools

Bug: 31040547
Change-Id: I9cc92c265ebf1305fc06b5fc33efd83797660cce

24 files changed, 519 insertions, 129 deletions

diff --git a/compiler/image_test.cc b/compiler/image_test.cc
index 91579e9daf..e1ee0d2966 100644
--- a/compiler/image_test.cc
+++ b/compiler/image_test.cc
@@ -188,6 +188,7 @@ void ImageTest::TestWriteRead(ImageHeader::StorageMode storage_mode) {
   }
 
   uint64_t image_file_size;
+  size_t image_size;
   {
     std::unique_ptr<File> file(OS::OpenFileForReading(image_file.GetFilename().c_str()));
     ASSERT_TRUE(file.get() != nullptr);
@@ -206,6 +207,7 @@ void ImageTest::TestWriteRead(ImageHeader::StorageMode storage_mode) {
     ASSERT_TRUE(space->IsMallocSpace());
 
     image_file_size = file->GetLength();
+    image_size = image_header.GetImageSize();
   }
 
   ASSERT_TRUE(compiler_driver_->GetImageClasses() != nullptr);
@@ -255,10 +257,10 @@ void ImageTest::TestWriteRead(ImageHeader::StorageMode storage_mode) {
   ASSERT_TRUE(image_space != nullptr);
   if (storage_mode == ImageHeader::kStorageModeUncompressed) {
     // Uncompressed, image should be smaller than file.
-    ASSERT_LE(image_space->Size(), image_file_size);
+    ASSERT_LE(image_size, image_file_size);
   } else {
     // Compressed, file should be smaller than image.
-    ASSERT_LE(image_file_size, image_space->Size());
+    ASSERT_LE(image_file_size, image_size);
   }
 
   image_space->VerifyImageAllocations();
diff --git a/runtime/arch/stub_test.cc b/runtime/arch/stub_test.cc
index 80bb51d9b6..10adb3ac05 100644
--- a/runtime/arch/stub_test.cc
+++ b/runtime/arch/stub_test.cc
@@ -1203,6 +1203,7 @@ TEST_F(StubTest, AllocObjectArray) {
 
 
 TEST_F(StubTest, StringCompareTo) {
+  TEST_DISABLED_FOR_STRING_COMPRESSION();
   // There is no StringCompareTo runtime entrypoint for __arm__ or __aarch64__.
 #if defined(__i386__) || defined(__mips__) || \
     (defined(__x86_64__) && !defined(__APPLE__))
diff --git a/runtime/common_runtime_test.h b/runtime/common_runtime_test.h
index f445e52d20..2d16a493c8 100644
--- a/runtime/common_runtime_test.h
+++ b/runtime/common_runtime_test.h
@@ -207,6 +207,12 @@ class CheckJniAbortCatcher {
     return; \
   }
 
+#define TEST_DISABLED_FOR_STRING_COMPRESSION() \
+  if (mirror::kUseStringCompression) { \
+    printf("WARNING: TEST DISABLED FOR STRING COMPRESSION\n"); \
+    return; \
+  }
+
 }  // namespace art
 
 namespace std {
diff --git a/runtime/debugger.cc b/runtime/debugger.cc
index 2a5198bf01..cbdf3dc636 100644
--- a/runtime/debugger.cc
+++ b/runtime/debugger.cc
@@ -1286,8 +1286,7 @@ JDWP::JdwpError Dbg::CreateObject(JDWP::RefTypeId class_id, JDWP::ObjectId* new_
   if (c->IsStringClass()) {
     // Special case for java.lang.String.
     gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-    mirror::SetStringCountVisitor visitor(0);
-    new_object = mirror::String::Alloc<true>(self, 0, allocator_type, visitor);
+    new_object = mirror::String::AllocEmptyString<true>(self, allocator_type);
   } else {
     new_object = c->AllocObject(self);
   }
@@ -4327,10 +4326,16 @@ void Dbg::DdmSendThreadNotification(Thread* t, uint32_t type) {
     Handle<mirror::String> name(hs.NewHandle(t->GetThreadName(soa)));
     size_t char_count = (name.Get() != nullptr) ? name->GetLength() : 0;
     const jchar* chars = (name.Get() != nullptr) ? name->GetValue() : nullptr;
+    bool is_compressed = (name.Get() != nullptr) ? name->IsCompressed() : false;
 
     std::vector<uint8_t> bytes;
     JDWP::Append4BE(bytes, t->GetThreadId());
-    JDWP::AppendUtf16BE(bytes, chars, char_count);
+    if (is_compressed) {
+      const uint8_t* chars_compressed = name->GetValueCompressed();
+      JDWP::AppendUtf16CompressedBE(bytes, chars_compressed, char_count);
+    } else {
+      JDWP::AppendUtf16BE(bytes, chars, char_count);
+    }
     CHECK_EQ(bytes.size(), char_count*2 + sizeof(uint32_t)*2);
     Dbg::DdmSendChunk(type, bytes);
   }
diff --git a/runtime/hprof/hprof.cc b/runtime/hprof/hprof.cc
index 9895395169..4005f054d5 100644
--- a/runtime/hprof/hprof.cc
+++ b/runtime/hprof/hprof.cc
@@ -223,6 +223,12 @@ class EndianOutput {
     HandleU1List(values, count);
     length_ += count;
   }
+  void AddU1AsU2List(const uint8_t* values, size_t count) {
+    HandleU1AsU2List(values, count);
+    // Array of char from compressed String (8-bit) is added as 16-bit blocks
+    int ceil_count_to_even = count + ((count & 1) ? 1 : 0);
+    length_ += ceil_count_to_even * sizeof(uint8_t);
+  }
   void AddU2List(const uint16_t* values, size_t count) {
     HandleU2List(values, count);
     length_ += count * sizeof(uint16_t);
@@ -268,6 +274,9 @@ class EndianOutput {
   virtual void HandleU1List(const uint8_t* values ATTRIBUTE_UNUSED,
                             size_t count ATTRIBUTE_UNUSED) {
   }
+  virtual void HandleU1AsU2List(const uint8_t* values ATTRIBUTE_UNUSED,
+                                size_t count ATTRIBUTE_UNUSED) {
+  }
   virtual void HandleU2List(const uint16_t* values ATTRIBUTE_UNUSED,
                             size_t count ATTRIBUTE_UNUSED) {
   }
@@ -308,6 +317,19 @@ class EndianOutputBuffered : public EndianOutput {
     buffer_.insert(buffer_.end(), values, values + count);
   }
 
+  void HandleU1AsU2List(const uint8_t* values, size_t count) OVERRIDE {
+    DCHECK_EQ(length_, buffer_.size());
+    // All 8-bits are grouped in 2 to make 16-bit block like Java Char
+    if (count & 1) {
+      buffer_.push_back(0);
+    }
+    for (size_t i = 0; i < count; ++i) {
+      uint8_t value = *values;
+      buffer_.push_back(value);
+      values++;
+    }
+  }
+
   void HandleU2List(const uint16_t* values, size_t count) OVERRIDE {
     DCHECK_EQ(length_, buffer_.size());
     for (size_t i = 0; i < count; ++i) {
@@ -1354,7 +1376,11 @@ void Hprof::DumpHeapInstanceObject(mirror::Object* obj, mirror::Class* klass) {
         string_value = reinterpret_cast<mirror::Object*>(
             reinterpret_cast<uintptr_t>(s) + kObjectAlignment);
       } else {
-        string_value = reinterpret_cast<mirror::Object*>(s->GetValue());
+        if (s->IsCompressed()) {
+          string_value = reinterpret_cast<mirror::Object*>(s->GetValueCompressed());
+        } else {
+          string_value = reinterpret_cast<mirror::Object*>(s->GetValue());
+        }
       }
       __ AddObjectId(string_value);
     }
@@ -1369,12 +1395,18 @@ void Hprof::DumpHeapInstanceObject(mirror::Object* obj, mirror::Class* klass) {
   CHECK_EQ(obj->IsString(), string_value != nullptr);
   if (string_value != nullptr) {
     mirror::String* s = obj->AsString();
+    // Compressed string's (8-bit) length is ceil(length/2) in 16-bit blocks
+    int length_in_16_bit = (s->IsCompressed()) ? ((s->GetLength() + 1) / 2) : s->GetLength();
     __ AddU1(HPROF_PRIMITIVE_ARRAY_DUMP);
     __ AddObjectId(string_value);
     __ AddStackTraceSerialNumber(LookupStackTraceSerialNumber(obj));
-    __ AddU4(s->GetLength());
+    __ AddU4(length_in_16_bit);
     __ AddU1(hprof_basic_char);
-    __ AddU2List(s->GetValue(), s->GetLength());
+    if (s->IsCompressed()) {
+      __ AddU1AsU2List(s->GetValueCompressed(), s->GetLength());
+    } else {
+      __ AddU2List(s->GetValue(), s->GetLength());
+    }
   }
 }
 
diff --git a/runtime/intern_table.cc b/runtime/intern_table.cc
index eceb593e08..1940d67316 100644
--- a/runtime/intern_table.cc
+++ b/runtime/intern_table.cc
@@ -386,8 +386,23 @@ bool InternTable::StringHashEquals::operator()(const GcRoot<mirror::String>& a,
   if (a_length != b.GetUtf16Length()) {
     return false;
   }
-  const uint16_t* a_value = a_string->GetValue();
-  return CompareModifiedUtf8ToUtf16AsCodePointValues(b.GetUtf8Data(), a_value, a_length) == 0;
+  if (a_string->IsCompressed()) {
+    size_t b_byte_count = strlen(b.GetUtf8Data());
+    size_t b_utf8_length = CountModifiedUtf8Chars(b.GetUtf8Data(), b_byte_count);
+    // Modified UTF-8 single byte character range is 0x01 .. 0x7f
+    // The string compression occurs on regular ASCII with same exact range,
+    // not on extended ASCII which up to 0xff
+    const bool is_b_regular_ascii = (b_byte_count == b_utf8_length);
+    if (is_b_regular_ascii) {
+      return memcmp(b.GetUtf8Data(),
+                    a_string->GetValueCompressed(), a_length * sizeof(uint8_t)) == 0;
+    } else {
+      return false;
+    }
+  } else {
+    const uint16_t* a_value = a_string->GetValue();
+    return CompareModifiedUtf8ToUtf16AsCodePointValues(b.GetUtf8Data(), a_value, a_length) == 0;
+  }
 }
 
 size_t InternTable::Table::AddTableFromMemory(const uint8_t* ptr) {
diff --git a/runtime/interpreter/interpreter_common.h b/runtime/interpreter/interpreter_common.h
index 90c8227443..acdc270b28 100644
--- a/runtime/interpreter/interpreter_common.h
+++ b/runtime/interpreter/interpreter_common.h
@@ -450,7 +450,7 @@ static inline void TraceExecution(const ShadowFrame& shadow_frame, const Instruc
       oss << StringPrintf(" vreg%u=0x%08X", i, raw_value);
       if (ref_value != nullptr) {
         if (ref_value->GetClass()->IsStringClass() &&
-            ref_value->AsString()->GetValue() != nullptr) {
+            !ref_value->AsString()->IsValueNull()) {
           oss << "/java.lang.String \"" << ref_value->AsString()->ToModifiedUtf8() << "\"";
         } else {
           oss << "/" << PrettyTypeOf(ref_value);
diff --git a/runtime/interpreter/interpreter_goto_table_impl.cc b/runtime/interpreter/interpreter_goto_table_impl.cc
index 43b27781e4..6aba898412 100644
--- a/runtime/interpreter/interpreter_goto_table_impl.cc
+++ b/runtime/interpreter/interpreter_goto_table_impl.cc
@@ -530,8 +530,7 @@ JValue ExecuteGotoImpl(Thread* self, const DexFile::CodeItem* code_item, ShadowF
     if (LIKELY(c != nullptr)) {
       if (UNLIKELY(c->IsStringClass())) {
         gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-        mirror::SetStringCountVisitor visitor(0);
-        obj = String::Alloc<true>(self, 0, allocator_type, visitor);
+        obj = mirror::String::AllocEmptyString<true>(self, allocator_type);
       } else {
         obj = AllocObjectFromCode<do_access_check, true>(
             inst->VRegB_21c(), shadow_frame.GetMethod(), self,
diff --git a/runtime/interpreter/interpreter_switch_impl.cc b/runtime/interpreter/interpreter_switch_impl.cc
index a6349fcf88..582fc9b438 100644
--- a/runtime/interpreter/interpreter_switch_impl.cc
+++ b/runtime/interpreter/interpreter_switch_impl.cc
@@ -477,8 +477,7 @@ JValue ExecuteSwitchImpl(Thread* self, const DexFile::CodeItem* code_item,
         if (LIKELY(c != nullptr)) {
           if (UNLIKELY(c->IsStringClass())) {
             gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-            mirror::SetStringCountVisitor visitor(0);
-            obj = String::Alloc<true>(self, 0, allocator_type, visitor);
+            obj = mirror::String::AllocEmptyString<true>(self, allocator_type);
           } else {
             obj = AllocObjectFromCode<do_access_check, true>(
               inst->VRegB_21c(), shadow_frame.GetMethod(), self,
diff --git a/runtime/interpreter/mterp/mterp.cc b/runtime/interpreter/mterp/mterp.cc
index c25cd78309..20a0753dd9 100644
--- a/runtime/interpreter/mterp/mterp.cc
+++ b/runtime/interpreter/mterp/mterp.cc
@@ -358,8 +358,7 @@ extern "C" size_t MterpNewInstance(ShadowFrame* shadow_frame, Thread* self, uint
   if (LIKELY(c != nullptr)) {
     if (UNLIKELY(c->IsStringClass())) {
       gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-      mirror::SetStringCountVisitor visitor(0);
-      obj = String::Alloc<true>(self, 0, allocator_type, visitor);
+      obj = mirror::String::AllocEmptyString<true>(self, allocator_type);
     } else {
       obj = AllocObjectFromCode<false, true>(
         inst->VRegB_21c(), shadow_frame->GetMethod(), self,
diff --git a/runtime/interpreter/unstarted_runtime_test.cc b/runtime/interpreter/unstarted_runtime_test.cc
index 7e1f7950eb..c3246008a1 100644
--- a/runtime/interpreter/unstarted_runtime_test.cc
+++ b/runtime/interpreter/unstarted_runtime_test.cc
@@ -401,8 +401,23 @@ TEST_F(UnstartedRuntimeTest, StringInit) {
   interpreter::DoCall<false, false>(method, self, *shadow_frame, inst, inst_data[0], &result);
   mirror::String* string_result = reinterpret_cast<mirror::String*>(result.GetL());
   EXPECT_EQ(string_arg->GetLength(), string_result->GetLength());
-  EXPECT_EQ(memcmp(string_arg->GetValue(), string_result->GetValue(),
-                   string_arg->GetLength() * sizeof(uint16_t)), 0);
+
+  if (string_arg->IsCompressed() && string_result->IsCompressed()) {
+    EXPECT_EQ(memcmp(string_arg->GetValueCompressed(), string_result->GetValueCompressed(),
+                     string_arg->GetLength() * sizeof(uint8_t)), 0);
+  } else if (!string_arg->IsCompressed() && !string_result->IsCompressed()) {
+    EXPECT_EQ(memcmp(string_arg->GetValue(), string_result->GetValue(),
+                     string_arg->GetLength() * sizeof(uint16_t)), 0);
+  } else {
+    bool equal = true;
+    for (int i = 0; i < string_arg->GetLength(); ++i) {
+      if (string_arg->CharAt(i) != string_result->CharAt(i)) {
+        equal = false;
+        break;
+      }
+    }
+    EXPECT_EQ(equal, true);
+  }
 
   ShadowFrame::DeleteDeoptimizedFrame(shadow_frame);
 }
diff --git a/runtime/jdwp/jdwp_bits.h b/runtime/jdwp/jdwp_bits.h
index f9cf9ca0d9..33b98f3efe 100644
--- a/runtime/jdwp/jdwp_bits.h
+++ b/runtime/jdwp/jdwp_bits.h
@@ -59,13 +59,22 @@ static inline void Append8BE(std::vector<uint8_t>& bytes, uint64_t value) {
   bytes.push_back(static_cast<uint8_t>(value));
 }
 
-static inline void AppendUtf16BE(std::vector<uint8_t>& bytes, const uint16_t* chars, size_t char_count) {
+static inline void AppendUtf16BE(std::vector<uint8_t>& bytes, const uint16_t* chars,
+                                 size_t char_count) {
   Append4BE(bytes, char_count);
   for (size_t i = 0; i < char_count; ++i) {
     Append2BE(bytes, chars[i]);
   }
 }
 
+static inline void AppendUtf16CompressedBE(std::vector<uint8_t>& bytes,
+                                           const uint8_t* chars, size_t char_count) {
+  Append4BE(bytes, char_count);
+  for (size_t i = 0; i < char_count; ++i) {
+    Append2BE(bytes, static_cast<uint16_t>(chars[i]));
+  }
+}
+
 // @deprecated
 static inline void Set1(uint8_t* buf, uint8_t val) {
   *buf = val;
diff --git a/runtime/jni_internal.cc b/runtime/jni_internal.cc
index c3224757d8..7bcadd8b78 100644
--- a/runtime/jni_internal.cc
+++ b/runtime/jni_internal.cc
@@ -592,9 +592,8 @@ class JNI {
     }
     if (c->IsStringClass()) {
       gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-      mirror::SetStringCountVisitor visitor(0);
-      return soa.AddLocalReference<jobject>(mirror::String::Alloc<true>(soa.Self(), 0,
-                                                                        allocator_type, visitor));
+      return soa.AddLocalReference<jobject>(mirror::String::AllocEmptyString<true>(soa.Self(),
+                                                                              allocator_type));
     }
     return soa.AddLocalReference<jobject>(c->AllocObject(soa.Self()));
   }
@@ -1673,8 +1672,14 @@ class JNI {
       ThrowSIOOBE(soa, start, length, s->GetLength());
     } else {
       CHECK_NON_NULL_MEMCPY_ARGUMENT(length, buf);
-      const jchar* chars = s->GetValue();
-      memcpy(buf, chars + start, length * sizeof(jchar));
+      if (s->IsCompressed()) {
+        for (int i = 0; i < length; ++i) {
+          buf[i] = static_cast<jchar>(s->CharAt(start+i));
+        }
+      } else {
+        const jchar* chars = static_cast<jchar*>(s->GetValue());
+        memcpy(buf, chars + start, length * sizeof(jchar));
+      }
     }
   }
 
@@ -1687,9 +1692,15 @@ class JNI {
       ThrowSIOOBE(soa, start, length, s->GetLength());
     } else {
       CHECK_NON_NULL_MEMCPY_ARGUMENT(length, buf);
-      const jchar* chars = s->GetValue();
-      size_t bytes = CountUtf8Bytes(chars + start, length);
-      ConvertUtf16ToModifiedUtf8(buf, bytes, chars + start, length);
+      if (s->IsCompressed()) {
+        for (int i = 0; i < length; ++i) {
+          buf[i] = s->CharAt(start+i);
+        }
+      } else {
+        const jchar* chars = s->GetValue();
+        size_t bytes = CountUtf8Bytes(chars + start, length);
+        ConvertUtf16ToModifiedUtf8(buf, bytes, chars + start, length);
+      }
     }
   }
 
@@ -1698,9 +1709,16 @@ class JNI {
     ScopedObjectAccess soa(env);
     mirror::String* s = soa.Decode<mirror::String*>(java_string);
     gc::Heap* heap = Runtime::Current()->GetHeap();
-    if (heap->IsMovableObject(s)) {
+    if (heap->IsMovableObject(s) || s->IsCompressed()) {
       jchar* chars = new jchar[s->GetLength()];
-      memcpy(chars, s->GetValue(), sizeof(jchar) * s->GetLength());
+      if (s->IsCompressed()) {
+        int32_t length = s->GetLength();
+        for (int i = 0; i < length; ++i) {
+          chars[i] = s->CharAt(i);
+        }
+      } else {
+        memcpy(chars, s->GetValue(), sizeof(jchar) * s->GetLength());
+      }
       if (is_copy != nullptr) {
         *is_copy = JNI_TRUE;
       }
@@ -1716,7 +1734,7 @@ class JNI {
     CHECK_NON_NULL_ARGUMENT_RETURN_VOID(java_string);
     ScopedObjectAccess soa(env);
     mirror::String* s = soa.Decode<mirror::String*>(java_string);
-    if (chars != s->GetValue()) {
+    if (s->IsCompressed() || (s->IsCompressed() == false && chars != s->GetValue())) {
       delete[] chars;
     }
   }
@@ -1737,15 +1755,27 @@ class JNI {
         heap->IncrementDisableThreadFlip(soa.Self());
       }
     }
-    if (is_copy != nullptr) {
-      *is_copy = JNI_FALSE;
+    if (s->IsCompressed()) {
+      if (is_copy != nullptr) {
+        *is_copy = JNI_TRUE;
+      }
+      int32_t length = s->GetLength();
+      jchar* chars = new jchar[length];
+      for (int i = 0; i < length; ++i) {
+        chars[i] = s->CharAt(i);
+      }
+      return chars;
+    } else {
+      if (is_copy != nullptr) {
+        *is_copy = JNI_FALSE;
+      }
+      return static_cast<jchar*>(s->GetValue());
     }
-    return static_cast<jchar*>(s->GetValue());
   }
 
   static void ReleaseStringCritical(JNIEnv* env,
                                     jstring java_string,
-                                    const jchar* chars ATTRIBUTE_UNUSED) {
+                                    const jchar* chars) {
     CHECK_NON_NULL_ARGUMENT_RETURN_VOID(java_string);
     ScopedObjectAccess soa(env);
     gc::Heap* heap = Runtime::Current()->GetHeap();
@@ -1757,6 +1787,9 @@ class JNI {
         heap->DecrementDisableThreadFlip(soa.Self());
       }
     }
+    if (s->IsCompressed() || (s->IsCompressed() == false && s->GetValue() != chars)) {
+      delete[] chars;
+    }
   }
 
   static const char* GetStringUTFChars(JNIEnv* env, jstring java_string, jboolean* is_copy) {
@@ -1771,8 +1804,14 @@ class JNI {
     size_t byte_count = s->GetUtfLength();
     char* bytes = new char[byte_count + 1];
     CHECK(bytes != nullptr);  // bionic aborts anyway.
-    const uint16_t* chars = s->GetValue();
-    ConvertUtf16ToModifiedUtf8(bytes, byte_count, chars, s->GetLength());
+    if (s->IsCompressed()) {
+      for (size_t i = 0; i < byte_count; ++i) {
+        bytes[i] = s->CharAt(i);
+      }
+    } else {
+      const uint16_t* chars = s->GetValue();
+      ConvertUtf16ToModifiedUtf8(bytes, byte_count, chars, s->GetLength());
+    }
     bytes[byte_count] = '\0';
     return bytes;
   }
diff --git a/runtime/jni_internal_test.cc b/runtime/jni_internal_test.cc
index 04ba8dfc64..64954743d4 100644
--- a/runtime/jni_internal_test.cc
+++ b/runtime/jni_internal_test.cc
@@ -880,8 +880,15 @@ TEST_F(JniInternalTest, FromReflectedField_ToReflectedField) {
   ASSERT_NE(fid2, nullptr);
   // Make sure we can actually use it.
   jstring s = env_->NewStringUTF("poop");
-  ASSERT_EQ(4, env_->GetIntField(s, fid2));
-
+  if (mirror::kUseStringCompression) {
+    // Negative because s is compressed (first bit is 1)
+    ASSERT_EQ(-2147483644, env_->GetIntField(s, fid2));
+    // Create incompressible string
+    jstring s_16 = env_->NewStringUTF("\u0444\u0444");
+    ASSERT_EQ(2, env_->GetIntField(s_16, fid2));
+  } else {
+    ASSERT_EQ(4, env_->GetIntField(s, fid2));
+  }
   // Bad arguments.
   GetFromReflectedField_ToReflectedFieldBadArgumentTest(false);
   GetFromReflectedField_ToReflectedFieldBadArgumentTest(true);
@@ -1632,13 +1639,28 @@ TEST_F(JniInternalTest, GetStringCritical_ReleaseStringCritical) {
 
   jboolean is_copy = JNI_TRUE;
   chars = env_->GetStringCritical(s, &is_copy);
-  EXPECT_EQ(JNI_FALSE, is_copy);
+  if (mirror::kUseStringCompression) {
+    // is_copy has to be JNI_TRUE because "hello" is all-ASCII
+    EXPECT_EQ(JNI_TRUE, is_copy);
+  } else {
+    EXPECT_EQ(JNI_FALSE, is_copy);
+  }
   EXPECT_EQ(expected[0], chars[0]);
   EXPECT_EQ(expected[1], chars[1]);
   EXPECT_EQ(expected[2], chars[2]);
   EXPECT_EQ(expected[3], chars[3]);
   EXPECT_EQ(expected[4], chars[4]);
   env_->ReleaseStringCritical(s, chars);
+
+  if (mirror::kUseStringCompression) {
+    // is_copy has to be JNI_FALSE because "\xed\xa0\x81\xed\xb0\x80" is incompressible
+    jboolean is_copy_16 = JNI_TRUE;
+    jstring s_16 = env_->NewStringUTF("\xed\xa0\x81\xed\xb0\x80");
+    chars = env_->GetStringCritical(s_16, &is_copy_16);
+    EXPECT_EQ(2, env_->GetStringLength(s_16));
+    EXPECT_EQ(4, env_->GetStringUTFLength(s_16));
+    env_->ReleaseStringCritical(s_16, chars);
+  }
 }
 
 TEST_F(JniInternalTest, GetObjectArrayElement_SetObjectArrayElement) {
diff --git a/runtime/mirror/object_test.cc b/runtime/mirror/object_test.cc
index 00342202cb..b35a479914 100644
--- a/runtime/mirror/object_test.cc
+++ b/runtime/mirror/object_test.cc
@@ -62,7 +62,7 @@ class ObjectTest : public CommonRuntimeTest {
     Handle<String> string(
         hs.NewHandle(String::AllocFromModifiedUtf8(self, expected_utf16_length, utf8_in)));
     ASSERT_EQ(expected_utf16_length, string->GetLength());
-    ASSERT_TRUE(string->GetValue() != nullptr);
+    ASSERT_EQ(string->IsValueNull(), false);
     // strlen is necessary because the 1-character string "\x00\x00" is interpreted as ""
     ASSERT_TRUE(string->Equals(utf8_in) || (expected_utf16_length == 1 && strlen(utf8_in) == 0));
     ASSERT_TRUE(string->Equals(StringPiece(utf8_in)) ||
diff --git a/runtime/mirror/string-inl.h b/runtime/mirror/string-inl.h
index d3660e5615..bc39ea86f7 100644
--- a/runtime/mirror/string-inl.h
+++ b/runtime/mirror/string-inl.h
@@ -13,7 +13,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 #ifndef ART_RUNTIME_MIRROR_STRING_INL_H_
 #define ART_RUNTIME_MIRROR_STRING_INL_H_
 
@@ -49,6 +48,7 @@ class SetStringCountVisitor {
     // Avoid AsString as object is not yet in live bitmap or allocation stack.
     String* string = down_cast<String*>(obj);
     string->SetCount(count_);
+    DCHECK(!string->IsCompressed() || kUseStringCompression);
   }
 
  private:
@@ -68,10 +68,19 @@ class SetStringCountAndBytesVisitor {
     // Avoid AsString as object is not yet in live bitmap or allocation stack.
     String* string = down_cast<String*>(obj);
     string->SetCount(count_);
-    uint16_t* value = string->GetValue();
+    DCHECK(!string->IsCompressed() || kUseStringCompression);
+    int32_t length = String::GetLengthFromCount(count_);
     const uint8_t* const src = reinterpret_cast<uint8_t*>(src_array_->GetData()) + offset_;
-    for (int i = 0; i < count_; i++) {
-      value[i] = high_byte_ + (src[i] & 0xFF);
+    if (string->IsCompressed()) {
+      uint8_t* valueCompressed = string->GetValueCompressed();
+      for (int i = 0; i < length; i++) {
+        valueCompressed[i] = (src[i] & 0xFF);
+      }
+    } else {
+      uint16_t* value = string->GetValue();
+      for (int i = 0; i < length; i++) {
+        value[i] = high_byte_ + (src[i] & 0xFF);
+      }
     }
   }
 
@@ -96,7 +105,16 @@ class SetStringCountAndValueVisitorFromCharArray {
     String* string = down_cast<String*>(obj);
     string->SetCount(count_);
     const uint16_t* const src = src_array_->GetData() + offset_;
-    memcpy(string->GetValue(), src, count_ * sizeof(uint16_t));
+    const int32_t length = String::GetLengthFromCount(count_);
+    bool compressible = kUseStringCompression && String::GetCompressionFlagFromCount(count_);
+    DCHECK(!compressible || kUseStringCompression);
+    if (compressible) {
+      for (int i = 0; i < length; ++i) {
+        string->GetValueCompressed()[i] = static_cast<uint8_t>(src[i]);
+      }
+    } else {
+      memcpy(string->GetValue(), src, length * sizeof(uint16_t));
+    }
   }
 
  private:
@@ -118,8 +136,22 @@ class SetStringCountAndValueVisitorFromString {
     // Avoid AsString as object is not yet in live bitmap or allocation stack.
     String* string = down_cast<String*>(obj);
     string->SetCount(count_);
-    const uint16_t* const src = src_string_->GetValue() + offset_;
-    memcpy(string->GetValue(), src, count_ * sizeof(uint16_t));
+    const int32_t length = String::GetLengthFromCount(count_);
+    bool compressible = kUseStringCompression && String::GetCompressionFlagFromCount(count_);
+    DCHECK(!compressible || kUseStringCompression);
+    if (src_string_->IsCompressed()) {
+      const uint8_t* const src = src_string_->GetValueCompressed() + offset_;
+      memcpy(string->GetValueCompressed(), src, length * sizeof(uint8_t));
+    } else {
+      const uint16_t* const src = src_string_->GetValue() + offset_;
+      if (compressible) {
+        for (int i = 0; i < length; ++i) {
+          string->GetValueCompressed()[i] = static_cast<uint8_t>(src[i]);
+        }
+      } else {
+        memcpy(string->GetValue(), src, length * sizeof(uint16_t));
+      }
+    }
   }
 
  private:
@@ -133,17 +165,38 @@ inline String* String::Intern() {
 }
 
 inline uint16_t String::CharAt(int32_t index) {
-  int32_t count = GetField32(OFFSET_OF_OBJECT_MEMBER(String, count_));
+  int32_t count = GetLength();
   if (UNLIKELY((index < 0) || (index >= count))) {
     ThrowStringIndexOutOfBoundsException(index, count);
     return 0;
   }
-  return GetValue()[index];
+  if (IsCompressed()) {
+    return GetValueCompressed()[index];
+  } else {
+    return GetValue()[index];
+  }
+}
+
+template <typename MemoryType>
+int32_t String::FastIndexOf(MemoryType* chars, int32_t ch, int32_t start) {
+  const MemoryType* p = chars + start;
+  const MemoryType* end = chars + GetLength();
+  while (p < end) {
+    if (*p++ == ch) {
+      return (p - 1) - chars;
+    }
+  }
+  return -1;
 }
 
 template<VerifyObjectFlags kVerifyFlags>
 inline size_t String::SizeOf() {
-  size_t size = sizeof(String) + (sizeof(uint16_t) * GetLength<kVerifyFlags>());
+  size_t size = sizeof(String);
+  if (IsCompressed()) {
+    size += (sizeof(uint8_t) * GetLength<kVerifyFlags>());
+  } else {
+    size += (sizeof(uint16_t) * GetLength<kVerifyFlags>());
+  }
   // String.equals() intrinsics assume zero-padding up to kObjectAlignment,
   // so make sure the zero-padding is actually copied around if GC compaction
   // chooses to copy only SizeOf() bytes.
@@ -152,31 +205,35 @@ inline size_t String::SizeOf() {
 }
 
 template <bool kIsInstrumented, typename PreFenceVisitor>
-inline String* String::Alloc(Thread* self, int32_t utf16_length, gc::AllocatorType allocator_type,
+inline String* String::Alloc(Thread* self, int32_t utf16_length_with_flag,
+                             gc::AllocatorType allocator_type,
                              const PreFenceVisitor& pre_fence_visitor) {
   constexpr size_t header_size = sizeof(String);
-  static_assert(sizeof(utf16_length) <= sizeof(size_t),
+  const bool compressible = kUseStringCompression &&
+                            String::GetCompressionFlagFromCount(utf16_length_with_flag);
+  const size_t block_size = (compressible) ? sizeof(uint8_t) : sizeof(uint16_t);
+  size_t length = String::GetLengthFromCount(utf16_length_with_flag);
+  static_assert(sizeof(length) <= sizeof(size_t),
                 "static_cast<size_t>(utf16_length) must not lose bits.");
-  size_t length = static_cast<size_t>(utf16_length);
-  size_t data_size = sizeof(uint16_t) * length;
+  size_t data_size = block_size * length;
   size_t size = header_size + data_size;
   // String.equals() intrinsics assume zero-padding up to kObjectAlignment,
   // so make sure the allocator clears the padding as well.
   // http://b/23528461
   size_t alloc_size = RoundUp(size, kObjectAlignment);
-  Class* string_class = GetJavaLangString();
 
+  Class* string_class = GetJavaLangString();
   // Check for overflow and throw OutOfMemoryError if this was an unreasonable request.
   // Do this by comparing with the maximum length that will _not_ cause an overflow.
-  constexpr size_t overflow_length = (-header_size) / sizeof(uint16_t);  // Unsigned arithmetic.
-  constexpr size_t max_alloc_length = overflow_length - 1u;
+  const size_t overflow_length = (-header_size) / block_size;   // Unsigned arithmetic.
+  const size_t max_alloc_length = overflow_length - 1u;
   static_assert(IsAligned<sizeof(uint16_t)>(kObjectAlignment),
                 "kObjectAlignment must be at least as big as Java char alignment");
-  constexpr size_t max_length = RoundDown(max_alloc_length, kObjectAlignment / sizeof(uint16_t));
+  const size_t max_length = RoundDown(max_alloc_length, kObjectAlignment / block_size);
   if (UNLIKELY(length > max_length)) {
     self->ThrowOutOfMemoryError(StringPrintf("%s of length %d would overflow",
                                              PrettyDescriptor(string_class).c_str(),
-                                             utf16_length).c_str());
+                                             static_cast<int>(length)).c_str());
     return nullptr;
   }
 
@@ -187,11 +244,22 @@ inline String* String::Alloc(Thread* self, int32_t utf16_length, gc::AllocatorTy
 }
 
 template <bool kIsInstrumented>
+inline String* String::AllocEmptyString(Thread* self, gc::AllocatorType allocator_type) {
+  SetStringCountVisitor visitor(0);
+  return Alloc<kIsInstrumented>(self, 0, allocator_type, visitor);
+}
+
+template <bool kIsInstrumented>
 inline String* String::AllocFromByteArray(Thread* self, int32_t byte_length,
                                           Handle<ByteArray> array, int32_t offset,
                                           int32_t high_byte, gc::AllocatorType allocator_type) {
-  SetStringCountAndBytesVisitor visitor(byte_length, array, offset, high_byte << 8);
-  String* string = Alloc<kIsInstrumented>(self, byte_length, allocator_type, visitor);
+  const uint8_t* const src = reinterpret_cast<uint8_t*>(array->GetData()) + offset;
+  const bool compressible = kUseStringCompression && String::AllASCII<uint8_t>(src, byte_length)
+                                            && (high_byte == 0);
+  const int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(byte_length)
+                                                  : byte_length;
+  SetStringCountAndBytesVisitor visitor(length_with_flag, array, offset, high_byte << 8);
+  String* string = Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
   return string;
 }
 
@@ -201,16 +269,24 @@ inline String* String::AllocFromCharArray(Thread* self, int32_t count,
                                           gc::AllocatorType allocator_type) {
   // It is a caller error to have a count less than the actual array's size.
   DCHECK_GE(array->GetLength(), count);
-  SetStringCountAndValueVisitorFromCharArray visitor(count, array, offset);
-  String* new_string = Alloc<kIsInstrumented>(self, count, allocator_type, visitor);
+  const bool compressible = kUseStringCompression &&
+                            String::AllASCII<uint16_t>(array->GetData() + offset, count);
+  const int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(count) : count;
+  SetStringCountAndValueVisitorFromCharArray visitor(length_with_flag, array, offset);
+  String* new_string = Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
   return new_string;
 }
 
 template <bool kIsInstrumented>
 inline String* String::AllocFromString(Thread* self, int32_t string_length, Handle<String> string,
                                        int32_t offset, gc::AllocatorType allocator_type) {
-  SetStringCountAndValueVisitorFromString visitor(string_length, string, offset);
-  String* new_string = Alloc<kIsInstrumented>(self, string_length, allocator_type, visitor);
+  const bool compressible = kUseStringCompression &&
+      ((string->IsCompressed()) ? true : String::AllASCII<uint16_t>(string->GetValue() + offset,
+                                                                    string_length));
+  const int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(string_length)
+                                                  : string_length;
+  SetStringCountAndValueVisitorFromString visitor(length_with_flag, string, offset);
+  String* new_string = Alloc<kIsInstrumented>(self, length_with_flag, allocator_type, visitor);
   return new_string;
 }
 
@@ -219,11 +295,28 @@ inline int32_t String::GetHashCode() {
   if (UNLIKELY(result == 0)) {
     result = ComputeHashCode();
   }
-  DCHECK(result != 0 || ComputeUtf16Hash(GetValue(), GetLength()) == 0)
-      << ToModifiedUtf8() << " " << result;
+  if (kIsDebugBuild) {
+    if (IsCompressed()) {
+      DCHECK(result != 0 || ComputeUtf16Hash(GetValueCompressed(), GetLength()) == 0)
+          << ToModifiedUtf8() << " " << result;
+    } else {
+      DCHECK(result != 0 || ComputeUtf16Hash(GetValue(), GetLength()) == 0)
+          << ToModifiedUtf8() << " " << result;
+    }
+  }
   return result;
 }
 
+template<typename MemoryType>
+bool String::AllASCII(const MemoryType* const chars, const int length) {
+  for (int i = 0; i < length; ++i) {
+    if (chars[i] > 0x80) {
+      return false;
+    }
+  }
+  return true;
+}
+
 }  // namespace mirror
 }  // namespace art
 
diff --git a/runtime/mirror/string.cc b/runtime/mirror/string.cc
index 33aca0304c..46caa4d73f 100644
--- a/runtime/mirror/string.cc
+++ b/runtime/mirror/string.cc
@@ -41,15 +41,11 @@ int32_t String::FastIndexOf(int32_t ch, int32_t start) {
   } else if (start > count) {
     start = count;
   }
-  const uint16_t* chars = GetValue();
-  const uint16_t* p = chars + start;
-  const uint16_t* end = chars + count;
-  while (p < end) {
-    if (*p++ == ch) {
-      return (p - 1) - chars;
-    }
+  if (IsCompressed()) {
+    return FastIndexOf<uint8_t>(GetValueCompressed(), ch, start);
+  } else {
+    return FastIndexOf<uint16_t>(GetValue(), ch, start);
   }
-  return -1;
 }
 
 void String::SetClass(Class* java_lang_String) {
@@ -65,45 +61,91 @@ void String::ResetClass() {
 }
 
 int String::ComputeHashCode() {
-  const int32_t hash_code = ComputeUtf16Hash(GetValue(), GetLength());
+  int32_t hash_code = 0;
+  if (IsCompressed()) {
+    hash_code = ComputeUtf16Hash(GetValueCompressed(), GetLength());
+  } else {
+    hash_code = ComputeUtf16Hash(GetValue(), GetLength());
+  }
   SetHashCode(hash_code);
   return hash_code;
 }
 
 int32_t String::GetUtfLength() {
-  return CountUtf8Bytes(GetValue(), GetLength());
+  if (IsCompressed()) {
+    return GetLength();
+  } else {
+    return CountUtf8Bytes(GetValue(), GetLength());
+  }
 }
 
 void String::SetCharAt(int32_t index, uint16_t c) {
-  DCHECK((index >= 0) && (index < count_));
-  GetValue()[index] = c;
+  DCHECK((index >= 0) && (index < GetLength()));
+  if (IsCompressed()) {
+    // TODO: Handle the case where String is compressed and c is non-ASCII
+    GetValueCompressed()[index] = static_cast<uint8_t>(c);
+  } else {
+    GetValue()[index] = c;
+  }
 }
 
 String* String::AllocFromStrings(Thread* self, Handle<String> string, Handle<String> string2) {
   int32_t length = string->GetLength();
   int32_t length2 = string2->GetLength();
   gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-  SetStringCountVisitor visitor(length + length2);
-  String* new_string = Alloc<true>(self, length + length2, allocator_type, visitor);
+  const bool compressible = kUseStringCompression && (string->IsCompressed() && string2->IsCompressed());
+  const int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(length + length2)
+                                                  : (length + length2);
+
+  SetStringCountVisitor visitor(length_with_flag);
+  String* new_string = Alloc<true>(self, length_with_flag, allocator_type, visitor);
   if (UNLIKELY(new_string == nullptr)) {
     return nullptr;
   }
-  uint16_t* new_value = new_string->GetValue();
-  memcpy(new_value, string->GetValue(), length * sizeof(uint16_t));
-  memcpy(new_value + length, string2->GetValue(), length2 * sizeof(uint16_t));
+  if (compressible) {
+    uint8_t* new_value = new_string->GetValueCompressed();
+    memcpy(new_value, string->GetValueCompressed(), length * sizeof(uint8_t));
+    memcpy(new_value + length, string2->GetValueCompressed(), length2 * sizeof(uint8_t));
+  } else {
+    uint16_t* new_value = new_string->GetValue();
+    if (string->IsCompressed()) {
+      for (int i = 0; i < length; ++i) {
+        new_value[i] = string->CharAt(i);
+      }
+    } else {
+      memcpy(new_value, string->GetValue(), length * sizeof(uint16_t));
+    }
+    if (string2->IsCompressed()) {
+      for (int i = 0; i < length2; ++i) {
+        new_value[i+length] = string2->CharAt(i);
+      }
+    } else {
+      memcpy(new_value + length, string2->GetValue(), length2 * sizeof(uint16_t));
+    }
+  }
   return new_string;
 }
 
 String* String::AllocFromUtf16(Thread* self, int32_t utf16_length, const uint16_t* utf16_data_in) {
   CHECK(utf16_data_in != nullptr || utf16_length == 0);
   gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-  SetStringCountVisitor visitor(utf16_length);
-  String* string = Alloc<true>(self, utf16_length, allocator_type, visitor);
+  const bool compressible = kUseStringCompression &&
+                            String::AllASCII<uint16_t>(utf16_data_in, utf16_length);
+  int32_t length_with_flag = (compressible) ? String::GetFlaggedCount(utf16_length)
+                                            : utf16_length;
+  SetStringCountVisitor visitor(length_with_flag);
+  String* string = Alloc<true>(self, length_with_flag, allocator_type, visitor);
   if (UNLIKELY(string == nullptr)) {
     return nullptr;
   }
-  uint16_t* array = string->GetValue();
-  memcpy(array, utf16_data_in, utf16_length * sizeof(uint16_t));
+  if (compressible) {
+    for (int i = 0; i < utf16_length; ++i) {
+      string->GetValueCompressed()[i] = static_cast<uint8_t>(utf16_data_in[i]);
+    }
+  } else {
+    uint16_t* array = string->GetValue();
+    memcpy(array, utf16_data_in, utf16_length * sizeof(uint16_t));
+  }
   return string;
 }
 
@@ -121,13 +163,20 @@ String* String::AllocFromModifiedUtf8(Thread* self, int32_t utf16_length, const
 String* String::AllocFromModifiedUtf8(Thread* self, int32_t utf16_length,
                                       const char* utf8_data_in, int32_t utf8_length) {
   gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-  SetStringCountVisitor visitor(utf16_length);
-  String* string = Alloc<true>(self, utf16_length, allocator_type, visitor);
+  const bool compressible = kUseStringCompression && (utf16_length == utf8_length);
+  const int32_t utf16_length_with_flag = (compressible) ? String::GetFlaggedCount(utf16_length)
+                                                        : utf16_length;
+  SetStringCountVisitor visitor(utf16_length_with_flag);
+  String* string = Alloc<true>(self, utf16_length_with_flag, allocator_type, visitor);
   if (UNLIKELY(string == nullptr)) {
     return nullptr;
   }
-  uint16_t* utf16_data_out = string->GetValue();
-  ConvertModifiedUtf8ToUtf16(utf16_data_out, utf16_length, utf8_data_in, utf8_length);
+  if (compressible) {
+    memcpy(string->GetValueCompressed(), utf8_data_in, utf16_length * sizeof(uint8_t));
+  } else {
+    uint16_t* utf16_data_out = string->GetValue();
+    ConvertModifiedUtf8ToUtf16(utf16_data_out, utf16_length, utf8_data_in, utf8_length);
+  }
   return string;
 }
 
@@ -219,10 +268,16 @@ bool String::Equals(const StringPiece& modified_utf8) {
 
 // Create a modified UTF-8 encoded std::string from a java/lang/String object.
 std::string String::ToModifiedUtf8() {
-  const uint16_t* chars = GetValue();
   size_t byte_count = GetUtfLength();
   std::string result(byte_count, static_cast<char>(0));
-  ConvertUtf16ToModifiedUtf8(&result[0], byte_count, chars, GetLength());
+  if (IsCompressed()) {
+    for (size_t i = 0; i < byte_count; ++i) {
+      result[i] = static_cast<char>(CharAt(i));
+    }
+  } else {
+    const uint16_t* chars = GetValue();
+    ConvertUtf16ToModifiedUtf8(&result[0], byte_count, chars, GetLength());
+  }
   return result;
 }
 
@@ -242,11 +297,24 @@ int32_t String::CompareTo(String* rhs) {
   int32_t rhsCount = rhs->GetLength();
   int32_t countDiff = lhsCount - rhsCount;
   int32_t minCount = (countDiff < 0) ? lhsCount : rhsCount;
-  const uint16_t* lhsChars = lhs->GetValue();
-  const uint16_t* rhsChars = rhs->GetValue();
-  int32_t otherRes = MemCmp16(lhsChars, rhsChars, minCount);
-  if (otherRes != 0) {
-    return otherRes;
+  if (lhs->IsCompressed() && rhs->IsCompressed()) {
+    int32_t comparison = memcmp(lhs->GetValueCompressed(), rhs->GetValueCompressed(), minCount * sizeof(uint8_t));
+    if (comparison != 0) {
+      return comparison;
+    }
+  } else if (lhs->IsCompressed() || rhs->IsCompressed()) {
+    for (int32_t i = 0; i < minCount; ++i) {
+      if (lhs->CharAt(i) != rhs->CharAt(i)) {
+        return static_cast<int32_t>(lhs->CharAt(i)) - static_cast<int32_t>(rhs->CharAt(i));
+      }
+    }
+  } else {
+    const uint16_t* lhsChars = lhs->GetValue();
+    const uint16_t* rhsChars = rhs->GetValue();
+    int32_t otherRes = MemCmp16(lhsChars, rhsChars, minCount);
+    if (otherRes != 0) {
+      return otherRes;
+    }
   }
   return countDiff;
 }
@@ -260,7 +328,14 @@ CharArray* String::ToCharArray(Thread* self) {
   Handle<String> string(hs.NewHandle(this));
   CharArray* result = CharArray::Alloc(self, GetLength());
   if (result != nullptr) {
-    memcpy(result->GetData(), string->GetValue(), string->GetLength() * sizeof(uint16_t));
+    if (string->IsCompressed()) {
+      int32_t length = string->GetLength();
+      for (int i = 0; i < length; ++i) {
+        result->GetData()[i] = string->CharAt(i);
+      }
+    } else {
+      memcpy(result->GetData(), string->GetValue(), string->GetLength() * sizeof(uint16_t));
+    }
   } else {
     self->AssertPendingOOMException();
   }
@@ -269,8 +344,18 @@ CharArray* String::ToCharArray(Thread* self) {
 
 void String::GetChars(int32_t start, int32_t end, Handle<CharArray> array, int32_t index) {
   uint16_t* data = array->GetData() + index;
-  uint16_t* value = GetValue() + start;
-  memcpy(data, value, (end - start) * sizeof(uint16_t));
+  if (IsCompressed()) {
+    for (int i = start; i < end; ++i) {
+      data[i-start] = CharAt(i);
+    }
+  } else {
+    uint16_t* value = GetValue() + start;
+    memcpy(data, value, (end - start) * sizeof(uint16_t));
+  }
+}
+
+bool String::IsValueNull() {
+  return (IsCompressed()) ? (GetValueCompressed() == nullptr) : (GetValue() == nullptr);
 }
 
 }  // namespace mirror
diff --git a/runtime/mirror/string.h b/runtime/mirror/string.h
index d492ba3162..8695fe84f4 100644
--- a/runtime/mirror/string.h
+++ b/runtime/mirror/string.h
@@ -31,6 +31,9 @@ class StubTest_ReadBarrierForRoot_Test;
 
 namespace mirror {
 
+// String Compression
+static constexpr bool kUseStringCompression = false;
+
 // C++ mirror of java.lang.String
 class MANAGED String FINAL : public Object {
  public:
@@ -54,18 +57,28 @@ class MANAGED String FINAL : public Object {
     return &value_[0];
   }
 
+  uint8_t* GetValueCompressed() SHARED_REQUIRES(Locks::mutator_lock_) {
+    return &value_compressed_[0];
+  }
+
   template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
   size_t SizeOf() SHARED_REQUIRES(Locks::mutator_lock_);
 
+  // Taking out the first/uppermost bit because it is not part of actual length value
   template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
   int32_t GetLength() SHARED_REQUIRES(Locks::mutator_lock_) {
+    return GetLengthFromCount(GetCount<kVerifyFlags>());
+  }
+
+  template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
+  int32_t GetCount() SHARED_REQUIRES(Locks::mutator_lock_) {
     return GetField32<kVerifyFlags>(OFFSET_OF_OBJECT_MEMBER(String, count_));
   }
 
   void SetCount(int32_t new_count) SHARED_REQUIRES(Locks::mutator_lock_) {
     // Count is invariant so use non-transactional mode. Also disable check as we may run inside
     // a transaction.
-    DCHECK_LE(0, new_count);
+    DCHECK_LE(0, (new_count & INT32_MAX));
     SetField32<false, false>(OFFSET_OF_OBJECT_MEMBER(String, count_), new_count);
   }
 
@@ -82,12 +95,6 @@ class MANAGED String FINAL : public Object {
 
   String* Intern() SHARED_REQUIRES(Locks::mutator_lock_);
 
-  template <bool kIsInstrumented, typename PreFenceVisitor>
-  ALWAYS_INLINE static String* Alloc(Thread* self, int32_t utf16_length,
-                                     gc::AllocatorType allocator_type,
-                                     const PreFenceVisitor& pre_fence_visitor)
-      SHARED_REQUIRES(Locks::mutator_lock_) REQUIRES(!Roles::uninterruptible_);
-
   template <bool kIsInstrumented>
   ALWAYS_INLINE static String* AllocFromByteArray(Thread* self, int32_t byte_length,
                                                   Handle<ByteArray> array, int32_t offset,
@@ -107,6 +114,11 @@ class MANAGED String FINAL : public Object {
                                                gc::AllocatorType allocator_type)
       SHARED_REQUIRES(Locks::mutator_lock_) REQUIRES(!Roles::uninterruptible_);
 
+  template <bool kIsInstrumented>
+  ALWAYS_INLINE static String* AllocEmptyString(Thread* self,
+                                                gc::AllocatorType allocator_type)
+      SHARED_REQUIRES(Locks::mutator_lock_) REQUIRES(!Roles::uninterruptible_);
+
   static String* AllocFromStrings(Thread* self, Handle<String> string, Handle<String> string2)
       SHARED_REQUIRES(Locks::mutator_lock_) REQUIRES(!Roles::uninterruptible_);
 
@@ -149,6 +161,10 @@ class MANAGED String FINAL : public Object {
 
   int32_t FastIndexOf(int32_t ch, int32_t start) SHARED_REQUIRES(Locks::mutator_lock_);
 
+  template <typename MemoryType>
+  int32_t FastIndexOf(MemoryType* chars, int32_t ch, int32_t start)
+      SHARED_REQUIRES(Locks::mutator_lock_);
+
   int32_t CompareTo(String* other) SHARED_REQUIRES(Locks::mutator_lock_);
 
   CharArray* ToCharArray(Thread* self) SHARED_REQUIRES(Locks::mutator_lock_)
@@ -157,6 +173,28 @@ class MANAGED String FINAL : public Object {
   void GetChars(int32_t start, int32_t end, Handle<CharArray> array, int32_t index)
       SHARED_REQUIRES(Locks::mutator_lock_);
 
+  template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
+  bool IsCompressed() SHARED_REQUIRES(Locks::mutator_lock_) {
+    return kUseStringCompression && GetCompressionFlagFromCount(GetCount());
+  }
+
+  bool IsValueNull() SHARED_REQUIRES(Locks::mutator_lock_);
+
+  template<typename MemoryType>
+  static bool AllASCII(const MemoryType* const chars, const int length);
+
+  ALWAYS_INLINE static bool GetCompressionFlagFromCount(const int32_t count) {
+    return kUseStringCompression && ((count & (1u << 31)) != 0);
+  }
+
+  ALWAYS_INLINE static int32_t GetLengthFromCount(const int32_t count) {
+    return kUseStringCompression ? (count & INT32_MAX) : count;
+  }
+
+  ALWAYS_INLINE static int32_t GetFlaggedCount(const int32_t count) {
+    return kUseStringCompression ? (count | (1u << 31)) : count;
+  }
+
   static Class* GetJavaLangString() SHARED_REQUIRES(Locks::mutator_lock_) {
     DCHECK(!java_lang_String_.IsNull());
     return java_lang_String_.Read();
@@ -174,12 +212,24 @@ class MANAGED String FINAL : public Object {
     SetField32<false, false>(OFFSET_OF_OBJECT_MEMBER(String, hash_code_), new_hash_code);
   }
 
+  template <bool kIsInstrumented, typename PreFenceVisitor>
+  ALWAYS_INLINE static String* Alloc(Thread* self, int32_t utf16_length_with_flag,
+                                     gc::AllocatorType allocator_type,
+                                     const PreFenceVisitor& pre_fence_visitor)
+      SHARED_REQUIRES(Locks::mutator_lock_) REQUIRES(!Roles::uninterruptible_);
+
   // Field order required by test "ValidateFieldOrderOfJavaCppUnionClasses".
+  // First bit (uppermost/leftmost) is taken out for Compressed/Uncompressed flag
+  // [0] Uncompressed: string uses 16-bit memory | [1] Compressed: 8-bit memory
   int32_t count_;
 
   uint32_t hash_code_;
 
-  uint16_t value_[0];
+  // Compression of all-ASCII into 8-bit memory leads to usage one of these fields
+  union {
+    uint16_t value_[0];
+    uint8_t value_compressed_[0];
+  };
 
   static GcRoot<Class> java_lang_String_;
 
diff --git a/runtime/native/java_lang_Class.cc b/runtime/native/java_lang_Class.cc
index 6d5e7c7705..d4e54cfa34 100644
--- a/runtime/native/java_lang_Class.cc
+++ b/runtime/native/java_lang_Class.cc
@@ -198,12 +198,25 @@ ALWAYS_INLINE static inline ArtField* FindFieldByName(
   }
   size_t low = 0;
   size_t high = fields->size();
-  const uint16_t* const data = name->GetValue();
+  const bool is_name_compressed = name->IsCompressed();
+  const uint16_t* const data = (is_name_compressed) ? nullptr : name->GetValue();
+  const uint8_t* const data_compressed = (is_name_compressed) ? name->GetValueCompressed()
+                                                              : nullptr;
   const size_t length = name->GetLength();
   while (low < high) {
     auto mid = (low + high) / 2;
     ArtField& field = fields->At(mid);
-    int result = CompareModifiedUtf8ToUtf16AsCodePointValues(field.GetName(), data, length);
+    int result = 0;
+    if (is_name_compressed) {
+      size_t field_length = strlen(field.GetName());
+      size_t min_size = (length < field_length) ? length : field_length;
+      result = memcmp(field.GetName(), data_compressed, min_size);
+      if (result == 0) {
+        result = field_length - length;
+      }
+    } else {
+      result = CompareModifiedUtf8ToUtf16AsCodePointValues(field.GetName(), data, length);
+    }
     // Alternate approach, only a few % faster at the cost of more allocations.
     // int result = field->GetStringName(self, true)->CompareTo(name);
     if (result < 0) {
@@ -636,8 +649,7 @@ static jobject Class_newInstance(JNIEnv* env, jobject javaThis) {
   // Invoke the string allocator to return an empty string for the string class.
   if (klass->IsStringClass()) {
     gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
-    mirror::SetStringCountVisitor visitor(0);
-    mirror::Object* obj = mirror::String::Alloc<true>(soa.Self(), 0, allocator_type, visitor);
+    mirror::Object* obj = mirror::String::AllocEmptyString<true>(soa.Self(), allocator_type);
     if (UNLIKELY(soa.Self()->IsExceptionPending())) {
       return nullptr;
     } else {
diff --git a/runtime/native/libcore_util_CharsetUtils.cc b/runtime/native/libcore_util_CharsetUtils.cc
index 1216824b5a..64d56f6b26 100644
--- a/runtime/native/libcore_util_CharsetUtils.cc
+++ b/runtime/native/libcore_util_CharsetUtils.cc
@@ -165,10 +165,9 @@ static jbyteArray charsToBytes(JNIEnv* env, jstring java_string, jint offset, ji
     return nullptr;
   }
 
-  const jchar* src = &(string->GetValue()[offset]);
   jbyte* dst = &bytes[0];
-  for (int i = length - 1; i >= 0; --i) {
-    jchar ch = *src++;
+  for (int i = 0; i < length; ++i) {
+    jchar ch = string->CharAt(offset + i);
     if (ch > maxValidChar) {
       ch = '?';
     }
diff --git a/runtime/utf.cc b/runtime/utf.cc
index 5e9fdf7fc8..7e06482635 100644
--- a/runtime/utf.cc
+++ b/runtime/utf.cc
@@ -170,14 +170,6 @@ void ConvertUtf16ToModifiedUtf8(char* utf8_out, size_t byte_count,
   }
 }
 
-int32_t ComputeUtf16Hash(const uint16_t* chars, size_t char_count) {
-  uint32_t hash = 0;
-  while (char_count--) {
-    hash = hash * 31 + *chars++;
-  }
-  return static_cast<int32_t>(hash);
-}
-
 int32_t ComputeUtf16HashFromModifiedUtf8(const char* utf8, size_t utf16_length) {
   uint32_t hash = 0;
   while (utf16_length != 0u) {
diff --git a/runtime/utf.h b/runtime/utf.h
index 27d2fd5f56..7c9c333126 100644
--- a/runtime/utf.h
+++ b/runtime/utf.h
@@ -82,7 +82,16 @@ void ConvertUtf16ToModifiedUtf8(char* utf8_out, size_t byte_count,
  */
 int32_t ComputeUtf16Hash(mirror::CharArray* chars, int32_t offset, size_t char_count)
     SHARED_REQUIRES(Locks::mutator_lock_);
-int32_t ComputeUtf16Hash(const uint16_t* chars, size_t char_count);
+
+template<typename MemoryType>
+int32_t ComputeUtf16Hash(const MemoryType* chars, size_t char_count) {
+  uint32_t hash = 0;
+  while (char_count--) {
+    hash = hash * 31 + *chars++;
+  }
+  return static_cast<int32_t>(hash);
+}
+
 int32_t ComputeUtf16HashFromModifiedUtf8(const char* utf8, size_t utf16_length);
 
 // Compute a hash code of a modified UTF-8 string. Not the standard java hash since it returns a
diff --git a/test/020-string/expected.txt b/test/020-string/expected.txt
index 76b8929bd7..83a0835718 100644
--- a/test/020-string/expected.txt
+++ b/test/020-string/expected.txt
@@ -1,6 +1,6 @@
 testStr is 'This is a very nice string'
 This is a very nice string
-Compare result is 32
+Compare result is greater than zero
 Compare unicode: -65302
 Got expected exception
 subStr is 'uick brown fox jumps over the lazy '
diff --git a/test/020-string/src/Main.java b/test/020-string/src/Main.java
index 710808255c..ccf94aabcc 100644
--- a/test/020-string/src/Main.java
+++ b/test/020-string/src/Main.java
@@ -45,7 +45,14 @@ public class Main {
         if (testStr.length() != testStr2.length())
             System.out.println("WARNING: stringTest length mismatch");
 
-        System.out.println("Compare result is " + testStr.compareTo(testStr2));
+        int compareResult = testStr.compareTo(testStr2);
+        if (compareResult > 0) {
+          System.out.println("Compare result is greater than zero");
+        } else if (compareResult == 0) {
+          System.out.println("Compare result is equal to zero");
+        } else {
+          System.out.println("Compare result is less than zero");
+        }
 
         // expected: -65302
         String s1 = "\u0c6d\u0cb6\u0d00\u0000\u0080\u0080\u0080\u0000\u0002\u0002\u0002\u0000\u00e9\u00e9\u00e9";