Replace String CharArray with internal uint16_t array.

Summary of high level changes:
  - Adds compiler inliner support to identify string init methods
  - Adds compiler support (quick & optimizing) with new invoke code path
    that calls method off the thread pointer
  - Adds thread entrypoints for all string init methods
  - Adds map to verifier to log when receiver of string init has been
    copied to other registers. used by compiler and interpreter

Change-Id: I797b992a8feb566f9ad73060011ab6f51eb7ce01

9 files changed, 272 insertions, 132 deletions

diff --git a/runtime/mirror/class-inl.h b/runtime/mirror/class-inl.h
index 712286f..cc6f5c4 100644
--- a/runtime/mirror/class-inl.h
+++ b/runtime/mirror/class-inl.h
@@ -548,6 +548,10 @@ inline void Class::CheckObjectAlloc() {
       << PrettyClass(this)
       << "A class object shouldn't be allocated through this "
       << "as it requires a pre-fence visitor that sets the class size.";
+  DCHECK(!IsStringClass())
+      << PrettyClass(this)
+      << "A string shouldn't be allocated through this "
+      << "as it requires a pre-fence visitor that sets the class size.";
   DCHECK(IsInstantiable()) << PrettyClass(this);
   // TODO: decide whether we want this check. It currently fails during bootstrap.
   // DCHECK(!Runtime::Current()->IsStarted() || IsInitializing()) << PrettyClass(this);
diff --git a/runtime/mirror/class.cc b/runtime/mirror/class.cc
index 1739019..56c586a 100644
--- a/runtime/mirror/class.cc
+++ b/runtime/mirror/class.cc
@@ -330,10 +330,6 @@ bool Class::IsInSamePackage(Class* that) {
   return IsInSamePackage(klass1->GetDescriptor(&temp1), klass2->GetDescriptor(&temp2));
 }
 
-bool Class::IsStringClass() const {
-  return this == String::GetJavaLangString();
-}
-
 bool Class::IsThrowableClass() {
   return WellKnownClasses::ToClass(WellKnownClasses::java_lang_Throwable)->IsAssignableFrom(this);
 }
diff --git a/runtime/mirror/class.h b/runtime/mirror/class.h
index 18496fd..d3cfd01 100644
--- a/runtime/mirror/class.h
+++ b/runtime/mirror/class.h
@@ -235,6 +235,15 @@ class MANAGED Class FINAL : public Object {
     SetAccessFlags(flags | kAccClassIsFinalizable);
   }
 
+  ALWAYS_INLINE bool IsStringClass() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    return (GetField32(AccessFlagsOffset()) & kAccClassIsStringClass) != 0;
+  }
+
+  ALWAYS_INLINE void SetStringClass() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    uint32_t flags = GetField32(OFFSET_OF_OBJECT_MEMBER(Class, access_flags_));
+    SetAccessFlags(flags | kAccClassIsStringClass);
+  }
+
   // Returns true if the class is abstract.
   ALWAYS_INLINE bool IsAbstract() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     return (GetAccessFlags() & kAccAbstract) != 0;
@@ -416,8 +425,6 @@ class MANAGED Class FINAL : public Object {
            ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
   bool IsClassClass() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  bool IsStringClass() const SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
-
   bool IsThrowableClass() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   template<ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
@@ -484,10 +491,10 @@ class MANAGED Class FINAL : public Object {
   template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags,
            ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
   bool IsVariableSize() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    // Classes and arrays vary in size, and so the object_size_ field cannot
+    // Classes, arrays, and strings vary in size, and so the object_size_ field cannot
     // be used to Get their instance size
     return IsClassClass<kVerifyFlags, kReadBarrierOption>() ||
-        IsArrayClass<kVerifyFlags, kReadBarrierOption>();
+        IsArrayClass<kVerifyFlags, kReadBarrierOption>() || IsStringClass();
   }
 
   template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags,
diff --git a/runtime/mirror/object-inl.h b/runtime/mirror/object-inl.h
index 2581fad..7c73f9f 100644
--- a/runtime/mirror/object-inl.h
+++ b/runtime/mirror/object-inl.h
@@ -28,8 +28,9 @@
 #include "monitor.h"
 #include "object_array-inl.h"
 #include "read_barrier-inl.h"
-#include "runtime.h"
 #include "reference.h"
+#include "runtime.h"
+#include "string-inl.h"
 #include "throwable.h"
 
 namespace art {
@@ -337,9 +338,14 @@ inline DoubleArray* Object::AsDoubleArray() {
   return down_cast<DoubleArray*>(this);
 }
 
-template<VerifyObjectFlags kVerifyFlags>
+template<VerifyObjectFlags kVerifyFlags, ReadBarrierOption kReadBarrierOption>
+inline bool Object::IsString() {
+  return GetClass<kVerifyFlags, kReadBarrierOption>()->IsStringClass();
+}
+
+template<VerifyObjectFlags kVerifyFlags, ReadBarrierOption kReadBarrierOption>
 inline String* Object::AsString() {
-  DCHECK(GetClass<kVerifyFlags>()->IsStringClass());
+  DCHECK((IsString<kVerifyFlags, kReadBarrierOption>()));
   return down_cast<String*>(this);
 }
 
@@ -385,6 +391,9 @@ inline size_t Object::SizeOf() {
   } else if (IsClass<kNewFlags, kReadBarrierOption>()) {
     result = AsClass<kNewFlags, kReadBarrierOption>()->
         template SizeOf<kNewFlags, kReadBarrierOption>();
+  } else if (GetClass<kNewFlags, kReadBarrierOption>()->IsStringClass()) {
+    result = AsString<kNewFlags, kReadBarrierOption>()->
+        template SizeOf<kNewFlags>();
   } else {
     result = GetClass<kNewFlags, kReadBarrierOption>()->
         template GetObjectSize<kNewFlags, kReadBarrierOption>();
@@ -947,7 +956,7 @@ inline void Object::VisitReferences(const Visitor& visitor,
   mirror::Class* klass = GetClass<kVerifyFlags>();
   if (klass == Class::GetJavaLangClass()) {
     AsClass<kVerifyNone>()->VisitReferences<kVisitClass>(klass, visitor);
-  } else if (klass->IsArrayClass()) {
+  } else if (klass->IsArrayClass() || klass->IsStringClass()) {
     if (klass->IsObjectArrayClass<kVerifyNone>()) {
       AsObjectArray<mirror::Object, kVerifyNone>()->VisitReferences<kVisitClass>(visitor);
     } else if (kVisitClass) {
diff --git a/runtime/mirror/object.h b/runtime/mirror/object.h
index 343c9bc..3ab52eb 100644
--- a/runtime/mirror/object.h
+++ b/runtime/mirror/object.h
@@ -182,7 +182,12 @@ class MANAGED LOCKABLE Object {
   template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
   DoubleArray* AsDoubleArray() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
+  template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags,
+           ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
+  bool IsString() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags,
+           ReadBarrierOption kReadBarrierOption = kWithReadBarrier>
   String* AsString() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
diff --git a/runtime/mirror/object_test.cc b/runtime/mirror/object_test.cc
index 2262af5..8e50a7a 100644
--- a/runtime/mirror/object_test.cc
+++ b/runtime/mirror/object_test.cc
@@ -61,14 +61,13 @@ 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->GetCharArray() != nullptr);
-    ASSERT_TRUE(string->GetCharArray()->GetData() != nullptr);
+    ASSERT_TRUE(string->GetValue() != nullptr);
     // 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)) ||
                 (expected_utf16_length == 1 && strlen(utf8_in) == 0));
     for (int32_t i = 0; i < expected_utf16_length; i++) {
-      EXPECT_EQ(utf16_expected[i], string->UncheckedCharAt(i));
+      EXPECT_EQ(utf16_expected[i], string->CharAt(i));
     }
     EXPECT_EQ(expected_hash, string->GetHashCode());
   }
@@ -491,12 +490,6 @@ TEST_F(ObjectTest, StringLength) {
   Handle<String> string(hs.NewHandle(String::AllocFromModifiedUtf8(soa.Self(), "android")));
   EXPECT_EQ(string->GetLength(), 7);
   EXPECT_EQ(string->GetUtfLength(), 7);
-
-  string->SetOffset(2);
-  string->SetCount(5);
-  EXPECT_TRUE(string->Equals("droid"));
-  EXPECT_EQ(string->GetLength(), 5);
-  EXPECT_EQ(string->GetUtfLength(), 5);
 }
 
 TEST_F(ObjectTest, DescriptorCompare) {
diff --git a/runtime/mirror/string-inl.h b/runtime/mirror/string-inl.h
index b367cff..8f5a7d4 100644
--- a/runtime/mirror/string-inl.h
+++ b/runtime/mirror/string-inl.h
@@ -19,6 +19,7 @@
 
 #include "array.h"
 #include "class.h"
+#include "gc/heap-inl.h"
 #include "intern_table.h"
 #include "runtime.h"
 #include "string.h"
@@ -29,33 +30,139 @@ namespace art {
 namespace mirror {
 
 inline uint32_t String::ClassSize() {
-  uint32_t vtable_entries = Object::kVTableLength + 51;
+  uint32_t vtable_entries = Object::kVTableLength + 52;
   return Class::ComputeClassSize(true, vtable_entries, 0, 1, 0, 1, 2);
 }
 
-inline uint16_t String::UncheckedCharAt(int32_t index) {
-  return GetCharArray()->Get(index + GetOffset());
+// Sets string count in the allocation code path to ensure it is guarded by a CAS.
+class SetStringCountVisitor {
+ public:
+  explicit SetStringCountVisitor(int32_t count) : count_(count) {
+  }
+
+  void operator()(Object* obj, size_t usable_size ATTRIBUTE_UNUSED) const
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    // Avoid AsString as object is not yet in live bitmap or allocation stack.
+    String* string = down_cast<String*>(obj);
+    string->SetCount(count_);
+  }
+
+ private:
+  const int32_t count_;
+};
+
+// Sets string count and value in the allocation code path to ensure it is guarded by a CAS.
+class SetStringCountAndBytesVisitor {
+ public:
+  SetStringCountAndBytesVisitor(int32_t count, uint8_t* src, int32_t high_byte)
+      : count_(count), src_(src), high_byte_(high_byte) {
+  }
+
+  void operator()(Object* obj, size_t usable_size ATTRIBUTE_UNUSED) const
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    // 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();
+    for (int i = 0; i < count_; i++) {
+      value[i] = high_byte_ + (src_[i] & 0xFF);
+    }
+  }
+
+ private:
+  const int32_t count_;
+  const uint8_t* const src_;
+  const int32_t high_byte_;
+};
+
+// Sets string count and value in the allocation code path to ensure it is guarded by a CAS.
+class SetStringCountAndValueVisitor {
+ public:
+  SetStringCountAndValueVisitor(int32_t count, uint16_t* src) : count_(count), src_(src) {
+  }
+
+  void operator()(Object* obj, size_t usable_size) const
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    UNUSED(usable_size);
+    // Avoid AsString as object is not yet in live bitmap or allocation stack.
+    String* string = down_cast<String*>(obj);
+    string->SetCount(count_);
+    memcpy(string->GetValue(), src_, count_ * sizeof(uint16_t));
+  }
+
+ private:
+  const int32_t count_;
+  const uint16_t* const src_;
+};
+
+inline String* String::Intern() {
+  return Runtime::Current()->GetInternTable()->InternWeak(this);
 }
 
-inline CharArray* String::GetCharArray() {
-  return GetFieldObject<CharArray>(ValueOffset());
+inline uint16_t String::CharAt(int32_t index) {
+  int32_t count = GetField32(OFFSET_OF_OBJECT_MEMBER(String, count_));
+  if (UNLIKELY((index < 0) || (index >= count))) {
+    Thread* self = Thread::Current();
+    self->ThrowNewExceptionF("Ljava/lang/StringIndexOutOfBoundsException;",
+                             "length=%i; index=%i", count, index);
+    return 0;
+  }
+  return GetValue()[index];
 }
 
-inline int32_t String::GetLength() {
-  int32_t result = GetField32(OFFSET_OF_OBJECT_MEMBER(String, count_));
-  DCHECK(result >= 0 && result <= GetCharArray()->GetLength());
-  return result;
+template<VerifyObjectFlags kVerifyFlags>
+inline size_t String::SizeOf() {
+  return sizeof(String) + (sizeof(uint16_t) * GetLength<kVerifyFlags>());
 }
 
-inline void String::SetArray(CharArray* new_array) {
-  // Array is invariant so use non-transactional mode. Also disable check as we may run inside
-  // a transaction.
-  DCHECK(new_array != nullptr);
-  SetFieldObject<false, false>(OFFSET_OF_OBJECT_MEMBER(String, array_), new_array);
+template <bool kIsInstrumented, typename PreFenceVisitor>
+inline String* String::Alloc(Thread* self, int32_t utf16_length, gc::AllocatorType allocator_type,
+                             const PreFenceVisitor& pre_fence_visitor) {
+  size_t header_size = sizeof(String);
+  size_t data_size = sizeof(uint16_t) * utf16_length;
+  size_t size = header_size + data_size;
+  Class* string_class = GetJavaLangString();
+
+  // Check for overflow and throw OutOfMemoryError if this was an unreasonable request.
+  if (UNLIKELY(size < data_size)) {
+    self->ThrowOutOfMemoryError(StringPrintf("%s of length %d would overflow",
+                                             PrettyDescriptor(string_class).c_str(),
+                                             utf16_length).c_str());
+    return nullptr;
+  }
+  gc::Heap* heap = Runtime::Current()->GetHeap();
+  return down_cast<String*>(
+      heap->AllocObjectWithAllocator<kIsInstrumented, false>(self, string_class, size,
+                                                             allocator_type, pre_fence_visitor));
 }
 
-inline String* String::Intern() {
-  return Runtime::Current()->GetInternTable()->InternWeak(this);
+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) {
+  uint8_t* data = reinterpret_cast<uint8_t*>(array->GetData()) + offset;
+  SetStringCountAndBytesVisitor visitor(byte_length, data, high_byte << 8);
+  String* string = Alloc<kIsInstrumented>(self, byte_length, allocator_type, visitor);
+  return string;
+}
+
+template <bool kIsInstrumented>
+inline String* String::AllocFromCharArray(Thread* self, int32_t array_length,
+                                          Handle<CharArray> array, int32_t offset,
+                                          gc::AllocatorType allocator_type) {
+  uint16_t* data = array->GetData() + offset;
+  SetStringCountAndValueVisitor visitor(array_length, data);
+  String* new_string = Alloc<kIsInstrumented>(self, array_length, 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) {
+  uint16_t* data = string->GetValue() + offset;
+  SetStringCountAndValueVisitor visitor(string_length, data);
+  String* new_string = Alloc<kIsInstrumented>(self, string_length, allocator_type, visitor);
+  return new_string;
 }
 
 inline int32_t String::GetHashCode() {
@@ -63,7 +170,7 @@ inline int32_t String::GetHashCode() {
   if (UNLIKELY(result == 0)) {
     result = ComputeHashCode();
   }
-  DCHECK(result != 0 || ComputeUtf16Hash(GetCharArray(), GetOffset(), GetLength()) == 0)
+  DCHECK(result != 0 || ComputeUtf16Hash(GetValue(), GetLength()) == 0)
       << ToModifiedUtf8() << " " << result;
   return result;
 }
diff --git a/runtime/mirror/string.cc b/runtime/mirror/string.cc
index b7fd240..b6236b1 100644
--- a/runtime/mirror/string.cc
+++ b/runtime/mirror/string.cc
@@ -20,10 +20,11 @@
 #include "array.h"
 #include "class-inl.h"
 #include "gc/accounting/card_table-inl.h"
+#include "handle_scope-inl.h"
 #include "intern_table.h"
 #include "object-inl.h"
 #include "runtime.h"
-#include "handle_scope-inl.h"
+#include "string-inl.h"
 #include "thread.h"
 #include "utf-inl.h"
 
@@ -40,7 +41,7 @@ int32_t String::FastIndexOf(int32_t ch, int32_t start) {
   } else if (start > count) {
     start = count;
   }
-  const uint16_t* chars = GetCharArray()->GetData() + GetOffset();
+  const uint16_t* chars = GetValue();
   const uint16_t* p = chars + start;
   const uint16_t* end = chars + count;
   while (p < end) {
@@ -62,36 +63,46 @@ void String::ResetClass() {
   java_lang_String_ = GcRoot<Class>(nullptr);
 }
 
-int32_t String::ComputeHashCode() {
-  const int32_t hash_code = ComputeUtf16Hash(GetCharArray(), GetOffset(), GetLength());
+int String::ComputeHashCode() {
+  const int32_t hash_code = ComputeUtf16Hash(GetValue(), GetLength());
   SetHashCode(hash_code);
   return hash_code;
 }
 
 int32_t String::GetUtfLength() {
-  return CountUtf8Bytes(GetCharArray()->GetData() + GetOffset(), GetLength());
+  return CountUtf8Bytes(GetValue(), GetLength());
 }
 
-String* String::AllocFromUtf16(Thread* self,
-                               int32_t utf16_length,
-                               const uint16_t* utf16_data_in,
-                               int32_t hash_code) {
-  CHECK(utf16_data_in != nullptr || utf16_length == 0);
-  String* string = Alloc(self, utf16_length);
-  if (UNLIKELY(string == nullptr)) {
+void String::SetCharAt(int32_t index, uint16_t c) {
+  DCHECK((index >= 0) && (index < count_));
+  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);
+  if (UNLIKELY(new_string == nullptr)) {
     return nullptr;
   }
-  CharArray* array = const_cast<CharArray*>(string->GetCharArray());
-  if (UNLIKELY(array == 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));
+  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);
+  if (UNLIKELY(string == nullptr)) {
     return nullptr;
   }
-  memcpy(array->GetData(), utf16_data_in, utf16_length * sizeof(uint16_t));
-  if (hash_code != 0) {
-    DCHECK_EQ(hash_code, ComputeUtf16Hash(utf16_data_in, utf16_length));
-    string->SetHashCode(hash_code);
-  } else {
-    string->ComputeHashCode();
-  }
+  uint16_t* array = string->GetValue();
+  memcpy(array, utf16_data_in, utf16_length * sizeof(uint16_t));
   return string;
 }
 
@@ -103,33 +114,14 @@ String* String::AllocFromModifiedUtf8(Thread* self, const char* utf) {
 
 String* String::AllocFromModifiedUtf8(Thread* self, int32_t utf16_length,
                                       const char* utf8_data_in) {
-  String* string = Alloc(self, utf16_length);
+  gc::AllocatorType allocator_type = Runtime::Current()->GetHeap()->GetCurrentAllocator();
+  SetStringCountVisitor visitor(utf16_length);
+  String* string = Alloc<true>(self, utf16_length, allocator_type, visitor);
   if (UNLIKELY(string == nullptr)) {
     return nullptr;
   }
-  uint16_t* utf16_data_out =
-      const_cast<uint16_t*>(string->GetCharArray()->GetData());
+  uint16_t* utf16_data_out = string->GetValue();
   ConvertModifiedUtf8ToUtf16(utf16_data_out, utf8_data_in);
-  string->ComputeHashCode();
-  return string;
-}
-
-String* String::Alloc(Thread* self, int32_t utf16_length) {
-  StackHandleScope<1> hs(self);
-  Handle<CharArray> array(hs.NewHandle(CharArray::Alloc(self, utf16_length)));
-  if (UNLIKELY(array.Get() == nullptr)) {
-    return nullptr;
-  }
-  return Alloc(self, array);
-}
-
-String* String::Alloc(Thread* self, Handle<CharArray> array) {
-  // Hold reference in case AllocObject causes GC.
-  String* string = down_cast<String*>(GetJavaLangString()->AllocObject(self));
-  if (LIKELY(string != nullptr)) {
-    string->SetArray(array.Get());
-    string->SetCount(array->GetLength());
-  }
   return string;
 }
 
@@ -147,7 +139,7 @@ bool String::Equals(String* that) {
     // Note: don't short circuit on hash code as we're presumably here as the
     // hash code was already equal
     for (int32_t i = 0; i < that->GetLength(); ++i) {
-      if (this->UncheckedCharAt(i) != that->UncheckedCharAt(i)) {
+      if (this->CharAt(i) != that->CharAt(i)) {
         return false;
       }
     }
@@ -160,7 +152,7 @@ bool String::Equals(const uint16_t* that_chars, int32_t that_offset, int32_t tha
     return false;
   } else {
     for (int32_t i = 0; i < that_length; ++i) {
-      if (this->UncheckedCharAt(i) != that_chars[that_offset + i]) {
+      if (this->CharAt(i) != that_chars[that_offset + i]) {
         return false;
       }
     }
@@ -177,7 +169,7 @@ bool String::Equals(const char* modified_utf8) {
       return false;
     }
 
-    if (GetLeadingUtf16Char(ch) != UncheckedCharAt(i++)) {
+    if (GetLeadingUtf16Char(ch) != CharAt(i++)) {
       return false;
     }
 
@@ -187,7 +179,7 @@ bool String::Equals(const char* modified_utf8) {
         return false;
       }
 
-      if (UncheckedCharAt(i++) != trailing) {
+      if (CharAt(i++) != trailing) {
         return false;
       }
     }
@@ -201,7 +193,7 @@ bool String::Equals(const StringPiece& modified_utf8) {
   for (int32_t i = 0; i < length; ++i) {
     uint32_t ch = GetUtf16FromUtf8(&p);
 
-    if (GetLeadingUtf16Char(ch) != UncheckedCharAt(i)) {
+    if (GetLeadingUtf16Char(ch) != CharAt(i)) {
       return false;
     }
 
@@ -211,7 +203,7 @@ bool String::Equals(const StringPiece& modified_utf8) {
         return false;
       }
 
-      if (UncheckedCharAt(++i) != trailing) {
+      if (CharAt(++i) != trailing) {
         return false;
       }
     }
@@ -221,7 +213,7 @@ 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 = GetCharArray()->GetData() + GetOffset();
+  const uint16_t* chars = GetValue();
   size_t byte_count = GetUtfLength();
   std::string result(byte_count, static_cast<char>(0));
   ConvertUtf16ToModifiedUtf8(&result[0], chars, GetLength());
@@ -244,8 +236,8 @@ 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->GetCharArray()->GetData() + lhs->GetOffset();
-  const uint16_t* rhsChars = rhs->GetCharArray()->GetData() + rhs->GetOffset();
+  const uint16_t* lhsChars = lhs->GetValue();
+  const uint16_t* rhsChars = rhs->GetValue();
   int32_t otherRes = MemCmp16(lhsChars, rhsChars, minCount);
   if (otherRes != 0) {
     return otherRes;
@@ -257,5 +249,19 @@ void String::VisitRoots(RootVisitor* visitor) {
   java_lang_String_.VisitRootIfNonNull(visitor, RootInfo(kRootStickyClass));
 }
 
+CharArray* String::ToCharArray(Thread* self) {
+  StackHandleScope<1> hs(self);
+  Handle<String> string(hs.NewHandle(this));
+  CharArray* result = CharArray::Alloc(self, GetLength());
+  memcpy(result->GetData(), string->GetValue(), string->GetLength() * sizeof(uint16_t));
+  return result;
+}
+
+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));
+}
+
 }  // namespace mirror
 }  // namespace art
diff --git a/runtime/mirror/string.h b/runtime/mirror/string.h
index 0670d0b..fcfe976 100644
--- a/runtime/mirror/string.h
+++ b/runtime/mirror/string.h
@@ -18,6 +18,7 @@
 #define ART_RUNTIME_MIRROR_STRING_H_
 
 #include "gc_root.h"
+#include "gc/allocator_type.h"
 #include "object.h"
 #include "object_callbacks.h"
 
@@ -45,22 +46,27 @@ class MANAGED String FINAL : public Object {
   }
 
   static MemberOffset ValueOffset() {
-    return OFFSET_OF_OBJECT_MEMBER(String, array_);
+    return OFFSET_OF_OBJECT_MEMBER(String, value_);
   }
 
-  static MemberOffset OffsetOffset() {
-    return OFFSET_OF_OBJECT_MEMBER(String, offset_);
+  uint16_t* GetValue() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    return &value_[0];
   }
 
-  CharArray* GetCharArray() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
+  size_t SizeOf() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  int32_t GetOffset() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    int32_t result = GetField32(OffsetOffset());
-    DCHECK_LE(0, result);
-    return result;
+  template<VerifyObjectFlags kVerifyFlags = kDefaultVerifyFlags>
+  int32_t GetLength() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    return GetField32<kVerifyFlags>(OFFSET_OF_OBJECT_MEMBER(String, count_));
   }
 
-  int32_t GetLength() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  void SetCount(int32_t new_count) SHARED_LOCKS_REQUIRED(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);
+    SetField32<false, false>(OFFSET_OF_OBJECT_MEMBER(String, count_), new_count);
+  }
 
   int32_t GetHashCode() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
@@ -69,19 +75,47 @@ class MANAGED String FINAL : public Object {
 
   int32_t GetUtfLength() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
+  uint16_t CharAt(int32_t index) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  void SetCharAt(int32_t index, uint16_t c) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
   String* Intern() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  static String* AllocFromUtf16(Thread* self,
-                                int32_t utf16_length,
-                                const uint16_t* utf16_data_in,
-                                int32_t hash_code = 0)
+  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_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  template <bool kIsInstrumented>
+  ALWAYS_INLINE static String* AllocFromByteArray(Thread* self, int32_t byte_length,
+                                                  Handle<ByteArray> array, int32_t offset,
+                                                  int32_t high_byte,
+                                                  gc::AllocatorType allocator_type)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  template <bool kIsInstrumented>
+  ALWAYS_INLINE static String* AllocFromCharArray(Thread* self, int32_t array_length,
+                                                  Handle<CharArray> array, int32_t offset,
+                                                  gc::AllocatorType allocator_type)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  template <bool kIsInstrumented>
+  ALWAYS_INLINE static String* AllocFromString(Thread* self, int32_t string_length,
+                                               Handle<String> string, int32_t offset,
+                                               gc::AllocatorType allocator_type)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  static String* AllocFromStrings(Thread* self, Handle<String> string, Handle<String> string2)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
+  static String* AllocFromUtf16(Thread* self, int32_t utf16_length, const uint16_t* utf16_data_in)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   static String* AllocFromModifiedUtf8(Thread* self, const char* utf)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  static String* AllocFromModifiedUtf8(Thread* self, int32_t utf16_length,
-                                       const char* utf8_data_in)
+  static String* AllocFromModifiedUtf8(Thread* self, int32_t utf16_length, const char* utf8_data_in)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   // TODO: This is only used in the interpreter to compare against
@@ -112,13 +146,10 @@ class MANAGED String FINAL : public Object {
 
   int32_t CompareTo(String* other) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  void SetOffset(int32_t new_offset) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    // Offset is only used during testing so use non-transactional mode.
-    DCHECK_LE(0, new_offset);
-    SetField32<false>(OFFSET_OF_OBJECT_MEMBER(String, offset_), new_offset);
-  }
+  CharArray* ToCharArray(Thread* self) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  void SetArray(CharArray* new_array) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  void GetChars(int32_t start, int32_t end, Handle<CharArray> array, int32_t index)
+      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   static Class* GetJavaLangString() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     DCHECK(!java_lang_String_.IsNull());
@@ -130,9 +161,6 @@ class MANAGED String FINAL : public Object {
   static void VisitRoots(RootVisitor* visitor)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  // TODO: Make this private. It's only used on ObjectTest at the moment.
-  uint16_t UncheckedCharAt(int32_t index) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
-
  private:
   void SetHashCode(int32_t new_hash_code) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     // Hash code is invariant so use non-transactional mode. Also disable check as we may run inside
@@ -141,27 +169,12 @@ class MANAGED String FINAL : public Object {
     SetField32<false, false>(OFFSET_OF_OBJECT_MEMBER(String, hash_code_), new_hash_code);
   }
 
-  void SetCount(int32_t new_count) SHARED_LOCKS_REQUIRED(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);
-    SetField32<false, false>(OFFSET_OF_OBJECT_MEMBER(String, count_), new_count);
-  }
-
-  static String* Alloc(Thread* self, int32_t utf16_length)
-      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
-
-  static String* Alloc(Thread* self, Handle<CharArray> array)
-      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
-
   // Field order required by test "ValidateFieldOrderOfJavaCppUnionClasses".
-  HeapReference<CharArray> array_;
-
   int32_t count_;
 
   uint32_t hash_code_;
 
-  int32_t offset_;
+  uint16_t value_[0];
 
   static GcRoot<Class> java_lang_String_;