Changeset 185486 in webkit for trunk/Source/JavaScriptCore/runtime/JSString.h

Timestamp:

Jun 11, 2015, 9:09:11 PM (10 years ago)

Author:

[email protected]

Message:

jsSubstring() should support creating substrings from substrings.
<​https://webkit.org/b/145427>

Reviewed by Geoffrey Garen

Tweak jsSubstring() to support base strings that are themselves substrings.
They will now share the same grandparent base. This avoids creating a new StringImpl.

• runtime/JSString.h:

(JSC::jsSubstring): Don't force rope resolution here. Instead do that in finishCreation()
if the base string is a non-substring rope. Note that resolveRope() is the very last thing
called, since it may allocate and the JSRopeString needs to be ready for marking.

(JSC::JSString::isSubstring): Added a helper to find out if a JSString is
a substring. This is just for internal use, so you don't have to cast to
JSRopeString for the real substringness flag.

File:

• trunk/Source/JavaScriptCore/runtime/JSString.h (modified) (5 diffs)

trunk/Source/JavaScriptCore/runtime/JSString.h

@@ -187 +187 @@
 
     bool isRope() const { return m_value.isNull(); }
+    bool isSubstring() const;
     bool is8Bit() const { return m_flags & Is8Bit; }
     void setIs8Bit(bool flag) const
@@ -292 +293 @@
     }
 
-    void finishCreation(VM& vm, JSString* base, unsigned offset, unsigned length)
-    {
+    void finishCreation(ExecState& exec, JSString& base, unsigned offset, unsigned length)
+    {
+        VM& vm = exec.vm();
         Base::finishCreation(vm);
-        ASSERT(!base->isRope());
         ASSERT(!sumOverflows<int32_t>(offset, length));
-        ASSERT(offset + length <= base->length());
+        ASSERT(offset + length <= base.length());
         m_length = length;
-        setIs8Bit(base->is8Bit());
+        setIs8Bit(base.is8Bit());
         setIsSubstring(true);
-        substringBase().set(vm, this, base);
-        substringOffset() = offset;
+        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);
+        }
     }
 
@@ -343 +356 @@
     }
 
-    static JSString* create(VM& vm, JSString* base, unsigned offset, unsigned length)
-    {
-        JSRopeString* newString = new (NotNull, allocateCell<JSRopeString>(vm.heap)) JSRopeString(vm);
-        newString->finishCreation(vm, base, offset, length);
+    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;
     }
@@ -516 +529 @@
     if (!length)
         return vm.smallStrings.emptyString();
-    s->value(exec); // For effect. We need to ensure that any string that is used as a substring base is not a rope.
-    return JSRopeString::create(vm, s, offset, length);
+    return JSRopeString::create(*exec, *s, offset, length);
 }
 
@@ -704 +716 @@
 }
 
+inline bool JSString::isSubstring() const
+{
+    return isRope() && static_cast<const JSRopeString*>(this)->isSubstring();
+}
+
 } // namespace JSC