Cherry-picks a WebKit change to revert recent changes to String addition for JSC.

WebKit change http://trac.webkit.org/changeset/51975 modified String addition in
JSC to use ropes. However, the change was incorrect, so was rolled back in
http://trac.webkit.org/changeset/51978.

The last WebKit merge was to revision r51976, so picked up the initial, broken
change only.

This change cherry-picks http://trac.webkit.org/changeset/51978 to revert the
original change.

Bug: 2336856
Change-Id: I7f48bce6a7f605779424ba0a7601524ab3a71990

3 files changed, 28 insertions, 87 deletions

diff --git a/JavaScriptCore/runtime/JSString.h b/JavaScriptCore/runtime/JSString.h
index 1867b17..93b11f1 100644
--- a/JavaScriptCore/runtime/JSString.h
+++ b/JavaScriptCore/runtime/JSString.h
@@ -203,32 +203,6 @@ namespace JSC {
             appendStringInConstruct(index, s2);
             ASSERT(ropeLength == index);
         }
-        // This constructor constructs a new string by concatenating s1 & s2.
-        // This should only be called with ropeLength <= 3.
-        JSString(JSGlobalData* globalData, unsigned ropeLength, JSString* s1, const UString& u2)
-            : JSCell(globalData->stringStructure.get())
-            , m_stringLength(s1->length() + u2.size())
-            , m_ropeLength(ropeLength)
-        {
-            ASSERT(ropeLength <= s_maxInternalRopeLength);
-            unsigned index = 0;
-            appendStringInConstruct(index, s1);
-            appendStringInConstruct(index, u2);
-            ASSERT(ropeLength == index);
-        }
-        // This constructor constructs a new string by concatenating s1 & s2.
-        // This should only be called with ropeLength <= 3.
-        JSString(JSGlobalData* globalData, unsigned ropeLength, const UString& u1, JSString* s2)
-            : JSCell(globalData->stringStructure.get())
-            , m_stringLength(u1.size() + s2->length())
-            , m_ropeLength(ropeLength)
-        {
-            ASSERT(ropeLength <= s_maxInternalRopeLength);
-            unsigned index = 0;
-            appendStringInConstruct(index, u1);
-            appendStringInConstruct(index, s2);
-            ASSERT(ropeLength == index);
-        }
         // This constructor constructs a new string by concatenating v1, v2 & v3.
         // This should only be called with ropeLength <= 3 ... which since every
         // value must require a ropeLength of at least one implies that the length
@@ -284,18 +258,13 @@ namespace JSC {
 
         void resolveRope(ExecState*) const;
 
-        void appendStringInConstruct(unsigned& index, const UString& string)
-        {
-            m_fibers[index++] = Rope::Fiber(string.rep()->ref());
-        }
-
         void appendStringInConstruct(unsigned& index, JSString* jsString)
         {
             if (jsString->isRope()) {
                 for (unsigned i = 0; i < jsString->m_ropeLength; ++i)
                     m_fibers[index++] = jsString->m_fibers[i].ref();
             } else
-                appendStringInConstruct(index, jsString->string());
+                m_fibers[index++] = Rope::Fiber(jsString->string().rep()->ref());
         }
 
         void appendValueInConstructAndIncrementLength(ExecState* exec, unsigned& index, JSValue v)
@@ -342,8 +311,6 @@ namespace JSC {
         unsigned ropeLength() { return m_ropeLength ? m_ropeLength : 1; }
 
         friend JSValue jsString(ExecState* exec, JSString* s1, JSString* s2);
-        friend JSValue jsString(ExecState* exec, const UString& u1, JSString* s2);
-        friend JSValue jsString(ExecState* exec, JSString* s1, const UString& u2);
         friend JSValue jsString(ExecState* exec, Register* strings, unsigned count);
     };
 
diff --git a/JavaScriptCore/runtime/Operations.cpp b/JavaScriptCore/runtime/Operations.cpp
index 0e1887c..139c7b8 100644
--- a/JavaScriptCore/runtime/Operations.cpp
+++ b/JavaScriptCore/runtime/Operations.cpp
@@ -54,13 +54,12 @@ NEVER_INLINE JSValue jsAddSlowCase(CallFrame* callFrame, JSValue v1, JSValue v2)
     JSValue p1 = v1.toPrimitive(callFrame);
     JSValue p2 = v2.toPrimitive(callFrame);
 
-    if (p1.isString()) {
-        return p2.isString()
-            ? jsString(callFrame, asString(p1), asString(p2))
-            : jsString(callFrame, asString(p1), p2.toString(callFrame));
+    if (p1.isString() || p2.isString()) {
+        RefPtr<UString::Rep> value = concatenate(p1.toString(callFrame).rep(), p2.toString(callFrame).rep());
+        if (!value)
+            return throwOutOfMemoryError(callFrame);
+        return jsString(callFrame, value.release());
     }
-    if (p2.isString())
-        return jsString(callFrame, p1.toString(callFrame), asString(p2));
 
     return jsNumber(callFrame, p1.toNumber(callFrame) + p2.toNumber(callFrame));
 }
diff --git a/JavaScriptCore/runtime/Operations.h b/JavaScriptCore/runtime/Operations.h
index 0289ac2..1cab06d 100644
--- a/JavaScriptCore/runtime/Operations.h
+++ b/JavaScriptCore/runtime/Operations.h
@@ -37,11 +37,6 @@ namespace JSC {
 
     ALWAYS_INLINE JSValue jsString(ExecState* exec, JSString* s1, JSString* s2)
     {
-        if (!s1->length())
-            return s2;
-        if (!s2->length())
-            return s1;
-
         unsigned ropeLength = s1->ropeLength() + s2->ropeLength();
         JSGlobalData* globalData = &exec->globalData();
 
@@ -58,42 +53,6 @@ namespace JSC {
         return new (globalData) JSString(globalData, rope.release());
     }
 
-    ALWAYS_INLINE JSValue jsString(ExecState* exec, const UString& u1, JSString* s2)
-    {
-        unsigned ropeLength = 1 + s2->ropeLength();
-        JSGlobalData* globalData = &exec->globalData();
-
-        if (ropeLength <= JSString::s_maxInternalRopeLength)
-            return new (globalData) JSString(globalData, ropeLength, u1, s2);
-
-        unsigned index = 0;
-        RefPtr<JSString::Rope> rope = JSString::Rope::createOrNull(ropeLength);
-        if (UNLIKELY(!rope))
-            return throwOutOfMemoryError(exec);
-        rope->append(index, u1);
-        rope->append(index, s2);
-        ASSERT(index == ropeLength);
-        return new (globalData) JSString(globalData, rope.release());
-    }
-
-    ALWAYS_INLINE JSValue jsString(ExecState* exec, JSString* s1, const UString& u2)
-    {
-        unsigned ropeLength = s1->ropeLength() + 1;
-        JSGlobalData* globalData = &exec->globalData();
-
-        if (ropeLength <= JSString::s_maxInternalRopeLength)
-            return new (globalData) JSString(globalData, ropeLength, s1, u2);
-
-        unsigned index = 0;
-        RefPtr<JSString::Rope> rope = JSString::Rope::createOrNull(ropeLength);
-        if (UNLIKELY(!rope))
-            return throwOutOfMemoryError(exec);
-        rope->append(index, s1);
-        rope->append(index, u2);
-        ASSERT(index == ropeLength);
-        return new (globalData) JSString(globalData, rope.release());
-    }
-
     ALWAYS_INLINE JSValue jsString(ExecState* exec, Register* strings, unsigned count)
     {
         ASSERT(count >= 3);
@@ -288,14 +247,30 @@ namespace JSC {
 
     ALWAYS_INLINE JSValue jsAdd(CallFrame* callFrame, JSValue v1, JSValue v2)
     {
-        double left = 0.0, right;
-        if (v1.getNumber(left), v2.getNumber(right))
+        double left;
+        double right = 0.0;
+
+        bool rightIsNumber = v2.getNumber(right);
+        if (rightIsNumber && v1.getNumber(left))
             return jsNumber(callFrame, left + right);
         
-        if (v1.isString()) {
-            return v2.isString()
-                ? jsString(callFrame, asString(v1), asString(v2))
-                : jsString(callFrame, asString(v1), v2.toString(callFrame));
+        bool leftIsString = v1.isString();
+        if (leftIsString && v2.isString()) {
+            if (!asString(v1)->length())
+                return asString(v2);
+            if (!asString(v2)->length())
+                return asString(v1);
+            return jsString(callFrame, asString(v1), asString(v2));
+        }
+
+        if (rightIsNumber & leftIsString) {
+            RefPtr<UString::Rep> value = v2.isInt32() ?
+                concatenate(asString(v1)->value(callFrame).rep(), v2.asInt32()) :
+                concatenate(asString(v1)->value(callFrame).rep(), right);
+
+            if (!value)
+                return throwOutOfMemoryError(callFrame);
+            return jsString(callFrame, value.release());
         }
 
         // All other cases are pretty uncommon