diff --git a/runtime/mirror/string-inl.h b/runtime/mirror/string-inl.h
index d3660e5..bc39ea8 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 @@
     // 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 @@
     // 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 @@
     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 @@
     // 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 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 @@
 }
 
 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 @@
 }
 
 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 @@
                                           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 @@
   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