Imported QtWebKit TP3 (git b57bc6801f1876c3220d5a4bfea33d620d477443)

Change-Id: I3b1d8a2808782c9f34d50240000e20cb38d3680f
Reviewed-by: Konstantin Tokarev <annulen@yandex.ru>

1 files changed, 318 insertions, 123 deletions

diff --git a/Source/JavaScriptCore/runtime/JSString.h b/Source/JavaScriptCore/runtime/JSString.h
index fc383b2f4..6dba78621 100644
--- a/Source/JavaScriptCore/runtime/JSString.h
+++ b/Source/JavaScriptCore/runtime/JSString.h
@@ -22,12 +22,15 @@
 
 #ifndef JSString_h
 #define JSString_h
+
 #include "CallFrame.h"
 #include "CommonIdentifiers.h"
 #include "Identifier.h"
 #include "PropertyDescriptor.h"
 #include "PropertySlot.h"
 #include "Structure.h"
+#include <array>
+#include <wtf/text/StringView.h>
 
 namespace JSC {
 
@@ -42,7 +45,6 @@ JSString* jsString(ExecState*, const String&); // returns empty string if passed
 
 JSString* jsSingleCharacterString(VM*, UChar);
 JSString* jsSingleCharacterString(ExecState*, UChar);
-JSString* jsSingleCharacterSubstring(ExecState*, const String&, unsigned offset);
 JSString* jsSubstring(VM*, const String&, unsigned offset, unsigned length);
 JSString* jsSubstring(ExecState*, const String&, unsigned offset, unsigned length);
 
@@ -50,6 +52,7 @@ JSString* jsSubstring(ExecState*, const String&, unsigned offset, unsigned lengt
 // These functions are faster than just calling jsString.
 JSString* jsNontrivialString(VM*, const String&);
 JSString* jsNontrivialString(ExecState*, const String&);
+JSString* jsNontrivialString(ExecState*, String&&);
 
 // Should be used for strings that are owned by an object that will
 // likely outlive the JSValue this makes, such as the parse tree or a
@@ -59,6 +62,15 @@ JSString* jsOwnedString(ExecState*, const String&);
 
 JSRopeString* jsStringBuilder(VM*);
 
+bool isJSString(JSCell*);
+bool isJSString(JSValue);
+JSString* asString(JSValue);
+
+struct StringViewWithUnderlyingString {
+    StringView view;
+    String underlyingString;
+};
+
 class JSString : public JSCell {
 public:
     friend class JIT;
@@ -70,9 +82,9 @@ public:
     friend struct ThunkHelpers;
 
     typedef JSCell Base;
+    static const unsigned StructureFlags = Base::StructureFlags | OverridesGetOwnPropertySlot | InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero | StructureIsImmortal;
 
     static const bool needsDestruction = true;
-    static const bool hasImmortalStructure = true;
     static void destroy(JSCell*);
 
 private:
@@ -95,7 +107,6 @@ private:
         Base::finishCreation(vm);
         m_length = length;
         setIs8Bit(m_value.impl()->is8Bit());
-        vm.m_newStringsSinceLastHashCons++;
     }
 
     void finishCreation(VM& vm, size_t length, size_t cost)
@@ -104,8 +115,7 @@ private:
         Base::finishCreation(vm);
         m_length = length;
         setIs8Bit(m_value.impl()->is8Bit());
-        Heap::heap(this)->reportExtraMemoryCost(cost);
-        vm.m_newStringsSinceLastHashCons++;
+        Heap::heap(this)->reportExtraMemoryAllocated(cost);
     }
 
 protected:
@@ -114,9 +124,8 @@ protected:
         Base::finishCreation(vm);
         m_length = 0;
         setIs8Bit(true);
-        vm.m_newStringsSinceLastHashCons++;
     }
-        
+
 public:
     static JSString* create(VM& vm, PassRefPtr<StringImpl> value)
     {
@@ -137,16 +146,25 @@ public:
         return newString;
     }
 
+    Identifier toIdentifier(ExecState*) const;
+    AtomicString toAtomicString(ExecState*) const;
+    RefPtr<AtomicStringImpl> toExistingAtomicString(ExecState*) const;
+
+    class SafeView;
+    SafeView view(ExecState*) const;
+    StringViewWithUnderlyingString viewWithUnderlyingString(ExecState&) const;
+
     const String& value(ExecState*) const;
     const String& tryGetValue() const;
-    unsigned length() { return m_length; }
+    const StringImpl* tryGetValueImpl() const;
+    unsigned length() const { return m_length; }
 
     JSValue toPrimitive(ExecState*, PreferredPrimitiveType) const;
-    JS_EXPORT_PRIVATE bool toBoolean() const;
+    bool toBoolean() const { return !!m_length; }
     bool getPrimitiveNumber(ExecState*, double& number, JSValue&) const;
     JSObject* toObject(ExecState*, JSGlobalObject*) const;
     double toNumber(ExecState*) const;
-        
+
     bool getStringPropertySlot(ExecState*, PropertyName, PropertySlot&);
     bool getStringPropertySlot(ExecState*, unsigned propertyName, PropertySlot&);
     bool getStringPropertyDescriptor(ExecState*, PropertyName, PropertyDescriptor&);
@@ -154,70 +172,60 @@ public:
     bool canGetIndex(unsigned i) { return i < m_length; }
     JSString* getIndex(ExecState*, unsigned);
 
-    static Structure* createStructure(VM& vm, JSGlobalObject* globalObject, JSValue proto)
-    {
-        return Structure::create(vm, globalObject, proto, TypeInfo(StringType, OverridesGetOwnPropertySlot | InterceptsGetOwnPropertySlotByIndexEvenWhenLengthIsNotZero), &s_info);
-    }
+    static Structure* createStructure(VM&, JSGlobalObject*, JSValue);
 
     static size_t offsetOfLength() { return OBJECT_OFFSETOF(JSString, m_length); }
     static size_t offsetOfFlags() { return OBJECT_OFFSETOF(JSString, m_flags); }
     static size_t offsetOfValue() { return OBJECT_OFFSETOF(JSString, m_value); }
 
-    static JS_EXPORTDATA const ClassInfo s_info;
+    DECLARE_EXPORT_INFO;
 
+    static void dumpToStream(const JSCell*, PrintStream&);
+    static size_t estimatedSize(JSCell*);
     static void visitChildren(JSCell*, SlotVisitor&);
 
     enum {
-        HashConsLock = 1u << 2,
-        IsHashConsSingleton = 1u << 1,
         Is8Bit = 1u
     };
 
 protected:
     friend class JSValue;
-        
+
     bool isRope() const { return m_value.isNull(); }
+    bool isSubstring() const;
     bool is8Bit() const { return m_flags & Is8Bit; }
-    void setIs8Bit(bool flag)
+    void setIs8Bit(bool flag) const
     {
         if (flag)
             m_flags |= Is8Bit;
         else
             m_flags &= ~Is8Bit;
     }
-    bool shouldTryHashCons();
-    bool isHashConsSingleton() const { return m_flags & IsHashConsSingleton; }
-    void clearHashConsSingleton() { m_flags &= ~IsHashConsSingleton; }
-    void setHashConsSingleton() { m_flags |= IsHashConsSingleton; }
-    bool tryHashConsLock();
-    void releaseHashConsLock();
 
-    unsigned m_flags;
-        
+    mutable unsigned m_flags;
+
     // A string is represented either by a String or a rope of fibers.
     unsigned m_length;
     mutable String m_value;
 
 private:
     friend class LLIntOffsetsExtractor;
-        
-    static JSObject* toThisObject(JSCell*, ExecState*);
 
-    // Actually getPropertySlot, not getOwnPropertySlot (see JSCell).
-    static bool getOwnPropertySlot(JSCell*, ExecState*, PropertyName, PropertySlot&);
-    static bool getOwnPropertySlotByIndex(JSCell*, ExecState*, unsigned propertyName, PropertySlot&);
+    static JSValue toThis(JSCell*, ExecState*, ECMAMode);
 
     String& string() { ASSERT(!isRope()); return m_value; }
+    StringView unsafeView(ExecState&) const;
 
     friend JSValue jsString(ExecState*, JSString*, JSString*);
     friend JSString* jsSubstring(ExecState*, JSString*, unsigned offset, unsigned length);
 };
 
-class JSRopeString : public JSString {
+class JSRopeString final : public JSString {
     friend class JSString;
 
     friend JSRopeString* jsStringBuilder(VM*);
 
+public:
     class RopeBuilder {
     public:
         RopeBuilder(VM& vm)
@@ -232,7 +240,7 @@ class JSRopeString : public JSString {
             if (m_index == JSRopeString::s_maxInternalRopeLength)
                 expand();
             if (static_cast<int32_t>(m_jsString->length() + jsString->length()) < 0) {
-                m_jsString = 0;
+                m_jsString = nullptr;
                 return false;
             }
             m_jsString->append(m_vm, m_index++, jsString);
@@ -247,16 +255,16 @@ class JSRopeString : public JSString {
             return tmp;
         }
 
-        unsigned length() { return m_jsString->m_length; }
+        unsigned length() const { return m_jsString->m_length; }
 
     private:
         void expand();
-            
+
         VM& m_vm;
         JSRopeString* m_jsString;
         size_t m_index;
     };
-        
+
 private:
     JSRopeString(VM& vm)
         : JSString(vm)
@@ -268,28 +276,60 @@ private:
         Base::finishCreation(vm);
         m_length = s1->length() + s2->length();
         setIs8Bit(s1->is8Bit() && s2->is8Bit());
-        m_fibers[0].set(vm, this, s1);
-        m_fibers[1].set(vm, this, s2);
+        setIsSubstring(false);
+        fiber(0).set(vm, this, s1);
+        fiber(1).set(vm, this, s2);
+        fiber(2).clear();
     }
-        
+
     void finishCreation(VM& vm, JSString* s1, JSString* s2, JSString* s3)
     {
         Base::finishCreation(vm);
         m_length = s1->length() + s2->length() + s3->length();
         setIs8Bit(s1->is8Bit() && s2->is8Bit() &&  s3->is8Bit());
-        m_fibers[0].set(vm, this, s1);
-        m_fibers[1].set(vm, this, s2);
-        m_fibers[2].set(vm, this, s3);
+        setIsSubstring(false);
+        fiber(0).set(vm, this, s1);
+        fiber(1).set(vm, this, s2);
+        fiber(2).set(vm, this, s3);
+    }
+
+    void finishCreation(ExecState& exec, JSString& base, unsigned offset, unsigned length)
+    {
+        VM& vm = exec.vm();
+        Base::finishCreation(vm);
+        ASSERT(!sumOverflows<int32_t>(offset, length));
+        ASSERT(offset + length <= base.length());
+        m_length = length;
+        setIs8Bit(base.is8Bit());
+        setIsSubstring(true);
+        if (base.isSubstring()) {
+            JSRopeString& baseRope = static_cast<JSRopeString&>(base);
+            substringBase().set(vm, this, baseRope.substringBase().get());
+            substringOffset() = baseRope.substringOffset() + offset;
+        } else {
+            substringBase().set(vm, this, &base);
+            substringOffset() = offset;
+
+            // For now, let's not allow substrings with a rope base.
+            // Resolve non-substring rope bases so we don't have to deal with it.
+            // FIXME: Evaluate if this would be worth adding more branches.
+            if (base.isRope())
+                static_cast<JSRopeString&>(base).resolveRope(&exec);
+        }
     }
 
     void finishCreation(VM& vm)
     {
         JSString::finishCreation(vm);
+        setIsSubstring(false);
+        fiber(0).clear();
+        fiber(1).clear();
+        fiber(2).clear();
     }
 
     void append(VM& vm, size_t index, JSString* jsString)
     {
-        m_fibers[index].set(vm, this, jsString);
+        fiber(index).set(vm, this, jsString);
         m_length += jsString->m_length;
         RELEASE_ASSERT(static_cast<int32_t>(m_length) >= 0);
         setIs8Bit(is8Bit() && jsString->is8Bit());
@@ -316,27 +356,107 @@ public:
         return newString;
     }
 
+    static JSString* create(ExecState& exec, JSString& base, unsigned offset, unsigned length)
+    {
+        JSRopeString* newString = new (NotNull, allocateCell<JSRopeString>(exec.vm().heap)) JSRopeString(exec.vm());
+        newString->finishCreation(exec, base, offset, length);
+        return newString;
+    }
+
     void visitFibers(SlotVisitor&);
-        
-    static ptrdiff_t offsetOfFibers() { return OBJECT_OFFSETOF(JSRopeString, m_fibers); }
+
+    static ptrdiff_t offsetOfFibers() { return OBJECT_OFFSETOF(JSRopeString, u); }
 
     static const unsigned s_maxInternalRopeLength = 3;
-        
+
 private:
-    friend JSValue jsString(ExecState*, Register*, unsigned);
+    friend JSValue jsStringFromRegisterArray(ExecState*, Register*, unsigned);
     friend JSValue jsStringFromArguments(ExecState*, JSValue);
 
     JS_EXPORT_PRIVATE void resolveRope(ExecState*) const;
+    JS_EXPORT_PRIVATE void resolveRopeToAtomicString(ExecState*) const;
+    JS_EXPORT_PRIVATE RefPtr<AtomicStringImpl> resolveRopeToExistingAtomicString(ExecState*) const;
     void resolveRopeSlowCase8(LChar*) const;
     void resolveRopeSlowCase(UChar*) const;
     void outOfMemory(ExecState*) const;
-        
-    JSString* getIndexSlowCase(ExecState*, unsigned);
+    void resolveRopeInternal8(LChar*) const;
+    void resolveRopeInternal8NoSubstring(LChar*) const;
+    void resolveRopeInternal16(UChar*) const;
+    void resolveRopeInternal16NoSubstring(UChar*) const;
+    void clearFibers() const;
+    StringView unsafeView(ExecState&) const;
+    StringViewWithUnderlyingString viewWithUnderlyingString(ExecState&) const;
+
+    WriteBarrierBase<JSString>& fiber(unsigned i) const
+    {
+        ASSERT(!isSubstring());
+        ASSERT(i < s_maxInternalRopeLength);
+        return u[i].string;
+    }
+
+    WriteBarrierBase<JSString>& substringBase() const
+    {
+        return u[1].string;
+    }
 
-    mutable FixedArray<WriteBarrier<JSString>, s_maxInternalRopeLength> m_fibers;
+    uintptr_t& substringOffset() const
+    {
+        return u[2].number;
+    }
+
+    static uintptr_t notSubstringSentinel()
+    {
+        return 0;
+    }
+
+    static uintptr_t substringSentinel()
+    {
+        return 1;
+    }
+
+    bool isSubstring() const
+    {
+        return u[0].number == substringSentinel();
+    }
+
+    void setIsSubstring(bool isSubstring)
+    {
+        u[0].number = isSubstring ? substringSentinel() : notSubstringSentinel();
+    }
+
+    mutable union {
+        uintptr_t number;
+        WriteBarrierBase<JSString> string;
+    } u[s_maxInternalRopeLength];
 };
 
-JSString* asString(JSValue);
+class JSString::SafeView {
+public:
+    explicit SafeView(ExecState&, const JSString&);
+    StringView get() const;
+
+    bool is8Bit() const { return m_string->is8Bit(); }
+    unsigned length() const { return m_string->length(); }
+    const LChar* characters8() const { return get().characters8(); }
+    const UChar* characters16() const { return get().characters16(); }
+    UChar operator[](unsigned index) const { return get()[index]; }
+
+private:
+    ExecState& m_state;
+
+    // The following pointer is marked "volatile" to make the compiler leave it on the stack
+    // or in a register as long as this object is alive, even after the last use of the pointer.
+    // That's needed to prevent garbage collecting the string and possibly deleting the block
+    // with the characters in it, and then using the StringView after that.
+    const JSString* volatile m_string;
+};
+
+JS_EXPORT_PRIVATE JSString* jsStringWithCacheSlowCase(VM&, StringImpl&);
+
+inline const StringImpl* JSString::tryGetValueImpl() const
+{
+    return m_value.impl();
+}
 
 inline JSString* asString(JSValue value)
 {
@@ -352,24 +472,41 @@ inline JSString* jsEmptyString(VM* vm)
 ALWAYS_INLINE JSString* jsSingleCharacterString(VM* vm, UChar c)
 {
     if (c <= maxSingleCharacterString)
-        return vm->smallStrings.singleCharacterString(vm, c);
+        return vm->smallStrings.singleCharacterString(c);
     return JSString::create(*vm, String(&c, 1).impl());
 }
 
-ALWAYS_INLINE JSString* jsSingleCharacterSubstring(ExecState* exec, const String& s, unsigned offset)
+inline JSString* jsNontrivialString(VM* vm, const String& s)
 {
-    VM* vm = &exec->vm();
-    ASSERT(offset < static_cast<unsigned>(s.length()));
-    UChar c = s.characterAt(offset);
-    if (c <= maxSingleCharacterString)
-        return vm->smallStrings.singleCharacterString(vm, c);
-    return JSString::create(*vm, StringImpl::create(s.impl(), offset, 1));
+    ASSERT(s.length() > 1);
+    return JSString::create(*vm, s.impl());
 }
 
-inline JSString* jsNontrivialString(VM* vm, const String& s)
+inline JSString* jsNontrivialString(VM* vm, String&& s)
 {
     ASSERT(s.length() > 1);
-    return JSString::create(*vm, s.impl());
+    return JSString::create(*vm, s.releaseImpl());
+}
+
+ALWAYS_INLINE Identifier JSString::toIdentifier(ExecState* exec) const
+{
+    return Identifier::fromString(exec, toAtomicString(exec));
+}
+
+ALWAYS_INLINE AtomicString JSString::toAtomicString(ExecState* exec) const
+{
+    if (isRope())
+        static_cast<const JSRopeString*>(this)->resolveRopeToAtomicString(exec);
+    return AtomicString(m_value);
+}
+
+ALWAYS_INLINE RefPtr<AtomicStringImpl> JSString::toExistingAtomicString(ExecState* exec) const
+{
+    if (isRope())
+        return static_cast<const JSRopeString*>(this)->resolveRopeToExistingAtomicString(exec);
+    if (m_value.impl()->isAtomic())
+        return static_cast<AtomicStringImpl*>(m_value.impl());
+    return AtomicStringImpl::lookUp(m_value.impl());
 }
 
 inline const String& JSString::value(ExecState* exec) const
@@ -389,10 +526,7 @@ inline const String& JSString::tryGetValue() const
 inline JSString* JSString::getIndex(ExecState* exec, unsigned i)
 {
     ASSERT(canGetIndex(i));
-    if (isRope())
-        return static_cast<JSRopeString*>(this)->getIndexSlowCase(exec, i);
-    ASSERT(i < m_value.length());
-    return jsSingleCharacterSubstring(exec, m_value, i);
+    return jsSingleCharacterString(exec, unsafeView(*exec)[i]);
 }
 
 inline JSString* jsString(VM* vm, const String& s)
@@ -403,7 +537,7 @@ inline JSString* jsString(VM* vm, const String& s)
     if (size == 1) {
         UChar c = s.characterAt(0);
         if (c <= maxSingleCharacterString)
-            return vm->smallStrings.singleCharacterString(vm, c);
+            return vm->smallStrings.singleCharacterString(c);
     }
     return JSString::create(*vm, s.impl());
 }
@@ -413,25 +547,12 @@ inline JSString* jsSubstring(ExecState* exec, JSString* s, unsigned offset, unsi
     ASSERT(offset <= static_cast<unsigned>(s->length()));
     ASSERT(length <= static_cast<unsigned>(s->length()));
     ASSERT(offset + length <= static_cast<unsigned>(s->length()));
-    VM* vm = &exec->vm();
-    if (!length)
-        return vm->smallStrings.emptyString();
-    return jsSubstring(vm, s->value(exec), offset, length);
-}
-
-inline JSString* jsSubstring8(VM* vm, const String& s, unsigned offset, unsigned length)
-{
-    ASSERT(offset <= static_cast<unsigned>(s.length()));
-    ASSERT(length <= static_cast<unsigned>(s.length()));
-    ASSERT(offset + length <= static_cast<unsigned>(s.length()));
+    VM& vm = exec->vm();
     if (!length)
-        return vm->smallStrings.emptyString();
-    if (length == 1) {
-        UChar c = s.characterAt(offset);
-        if (c <= maxSingleCharacterString)
-            return vm->smallStrings.singleCharacterString(vm, c);
-    }
-    return JSString::createHasOtherOwner(*vm, StringImpl::create8(s.impl(), offset, length));
+        return vm.smallStrings.emptyString();
+    if (!offset && length == s->length())
+        return s;
+    return JSRopeString::create(*exec, *s, offset, length);
 }
 
 inline JSString* jsSubstring(VM* vm, const String& s, unsigned offset, unsigned length)
@@ -444,9 +565,9 @@ inline JSString* jsSubstring(VM* vm, const String& s, unsigned offset, unsigned
     if (length == 1) {
         UChar c = s.characterAt(offset);
         if (c <= maxSingleCharacterString)
-            return vm->smallStrings.singleCharacterString(vm, c);
+            return vm->smallStrings.singleCharacterString(c);
     }
-    return JSString::createHasOtherOwner(*vm, StringImpl::create(s.impl(), offset, length));
+    return JSString::createHasOtherOwner(*vm, StringImpl::createSubstringSharingImpl(s.impl(), offset, length));
 }
 
 inline JSString* jsOwnedString(VM* vm, const String& s)
@@ -457,7 +578,7 @@ inline JSString* jsOwnedString(VM* vm, const String& s)
     if (size == 1) {
         UChar c = s.characterAt(0);
         if (c <= maxSingleCharacterString)
-            return vm->smallStrings.singleCharacterString(vm, c);
+            return vm->smallStrings.singleCharacterString(c);
     }
     return JSString::createHasOtherOwner(*vm, s.impl());
 }
@@ -470,42 +591,133 @@ inline JSRopeString* jsStringBuilder(VM* vm)
 inline JSString* jsEmptyString(ExecState* exec) { return jsEmptyString(&exec->vm()); }
 inline JSString* jsString(ExecState* exec, const String& s) { return jsString(&exec->vm(), s); }
 inline JSString* jsSingleCharacterString(ExecState* exec, UChar c) { return jsSingleCharacterString(&exec->vm(), c); }
-inline JSString* jsSubstring8(ExecState* exec, const String& s, unsigned offset, unsigned length) { return jsSubstring8(&exec->vm(), s, offset, length); }
 inline JSString* jsSubstring(ExecState* exec, const String& s, unsigned offset, unsigned length) { return jsSubstring(&exec->vm(), s, offset, length); }
 inline JSString* jsNontrivialString(ExecState* exec, const String& s) { return jsNontrivialString(&exec->vm(), s); }
+inline JSString* jsNontrivialString(ExecState* exec, String&& s) { return jsNontrivialString(&exec->vm(), WTFMove(s)); }
 inline JSString* jsOwnedString(ExecState* exec, const String& s) { return jsOwnedString(&exec->vm(), s); }
 
+ALWAYS_INLINE JSString* jsStringWithCache(ExecState* exec, const String& s)
+{
+    VM& vm = exec->vm();
+    StringImpl* stringImpl = s.impl();
+    if (!stringImpl || !stringImpl->length())
+        return jsEmptyString(&vm);
+
+    if (stringImpl->length() == 1) {
+        UChar singleCharacter = (*stringImpl)[0u];
+        if (singleCharacter <= maxSingleCharacterString)
+            return vm.smallStrings.singleCharacterString(static_cast<unsigned char>(singleCharacter));
+    }
+
+    if (JSString* lastCachedString = vm.lastCachedString.get()) {
+        if (lastCachedString->tryGetValueImpl() == stringImpl)
+            return lastCachedString;
+    }
+
+    return jsStringWithCacheSlowCase(vm, *stringImpl);
+}
+
+ALWAYS_INLINE JSString* jsStringWithCache(ExecState* exec, const AtomicString& s)
+{
+    return jsStringWithCache(exec, s.string());
+}
+
 ALWAYS_INLINE bool JSString::getStringPropertySlot(ExecState* exec, PropertyName propertyName, PropertySlot& slot)
 {
     if (propertyName == exec->propertyNames().length) {
-        slot.setValue(jsNumber(m_length));
+        slot.setValue(this, DontEnum | DontDelete | ReadOnly, jsNumber(m_length));
         return true;
     }
 
-    unsigned i = propertyName.asIndex();
-    if (i < m_length) {
-        ASSERT(i != PropertyName::NotAnIndex); // No need for an explicit check, the above test would always fail!
-        slot.setValue(getIndex(exec, i));
+    Optional<uint32_t> index = parseIndex(propertyName);
+    if (index && index.value() < m_length) {
+        slot.setValue(this, DontDelete | ReadOnly, getIndex(exec, index.value()));
         return true;
     }
 
     return false;
 }
-        
+
 ALWAYS_INLINE bool JSString::getStringPropertySlot(ExecState* exec, unsigned propertyName, PropertySlot& slot)
 {
     if (propertyName < m_length) {
-        slot.setValue(getIndex(exec, propertyName));
+        slot.setValue(this, DontDelete | ReadOnly, getIndex(exec, propertyName));
         return true;
     }
 
     return false;
 }
 
-inline bool isJSString(JSValue v) { return v.isCell() && v.asCell()->classInfo() == &JSString::s_info; }
+inline bool isJSString(JSCell* cell)
+{
+    return cell->type() == StringType;
+}
+
+inline bool isJSString(JSValue v)
+{
+    return v.isCell() && isJSString(v.asCell());
+}
+
+ALWAYS_INLINE StringView JSRopeString::unsafeView(ExecState& state) const
+{
+    if (isSubstring()) {
+        if (is8Bit())
+            return StringView(substringBase()->m_value.characters8() + substringOffset(), m_length);
+        return StringView(substringBase()->m_value.characters16() + substringOffset(), m_length);
+    }
+    resolveRope(&state);
+    return m_value;
+}
+
+ALWAYS_INLINE StringViewWithUnderlyingString JSRopeString::viewWithUnderlyingString(ExecState& state) const
+{
+    if (isSubstring()) {
+        auto& base = substringBase()->m_value;
+        if (is8Bit())
+            return { { base.characters8() + substringOffset(), m_length }, base };
+        return { { base.characters16() + substringOffset(), m_length }, base };
+    }
+    resolveRope(&state);
+    return { m_value, m_value };
+}
+
+ALWAYS_INLINE StringView JSString::unsafeView(ExecState& state) const
+{
+    if (isRope())
+        return static_cast<const JSRopeString*>(this)->unsafeView(state);
+    return m_value;
+}
+
+ALWAYS_INLINE StringViewWithUnderlyingString JSString::viewWithUnderlyingString(ExecState& state) const
+{
+    if (isRope())
+        return static_cast<const JSRopeString&>(*this).viewWithUnderlyingString(state);
+    return { m_value, m_value };
+}
+
+inline bool JSString::isSubstring() const
+{
+    return isRope() && static_cast<const JSRopeString*>(this)->isSubstring();
+}
+
+inline JSString::SafeView::SafeView(ExecState& state, const JSString& string)
+    : m_state(state)
+    , m_string(&string)
+{
+}
+
+inline StringView JSString::SafeView::get() const
+{
+    return m_string->unsafeView(m_state);
+}
+
+ALWAYS_INLINE JSString::SafeView JSString::view(ExecState* exec) const
+{
+    return SafeView(*exec, *this);
+}
 
 // --- JSValue inlines ----------------------------
-    
+
 inline bool JSValue::toBoolean(ExecState* exec) const
 {
     if (isInt32())
@@ -521,40 +733,23 @@ inline JSString* JSValue::toString(ExecState* exec) const
 {
     if (isString())
         return jsCast<JSString*>(asCell());
-    return toStringSlowCase(exec);
+    bool returnEmptyStringOnError = true;
+    return toStringSlowCase(exec, returnEmptyStringOnError);
 }
 
-inline String JSValue::toWTFString(ExecState* exec) const
+inline JSString* JSValue::toStringOrNull(ExecState* exec) const
 {
     if (isString())
-        return static_cast<JSString*>(asCell())->value(exec);
-    return toWTFStringSlowCase(exec);
+        return jsCast<JSString*>(asCell());
+    bool returnEmptyStringOnError = false;
+    return toStringSlowCase(exec, returnEmptyStringOnError);
 }
 
-ALWAYS_INLINE String inlineJSValueNotStringtoString(const JSValue& value, ExecState* exec)
-{
-    VM& vm = exec->vm();
-    if (value.isInt32())
-        return vm.numericStrings.add(value.asInt32());
-    if (value.isDouble())
-        return vm.numericStrings.add(value.asDouble());
-    if (value.isTrue())
-        return vm.propertyNames->trueKeyword.string();
-    if (value.isFalse())
-        return vm.propertyNames->falseKeyword.string();
-    if (value.isNull())
-        return vm.propertyNames->nullKeyword.string();
-    if (value.isUndefined())
-        return vm.propertyNames->undefinedKeyword.string();
-    return value.toString(exec)->value(exec);
-}
-
-ALWAYS_INLINE String JSValue::toWTFStringInline(ExecState* exec) const
+inline String JSValue::toWTFString(ExecState* exec) const
 {
     if (isString())
         return static_cast<JSString*>(asCell())->value(exec);
-
-    return inlineJSValueNotStringtoString(*this, exec);
+    return toWTFStringSlowCase(exec);
 }
 
 } // namespace JSC