Bug 1334744 - Add a faster API for working with external strings. r=luke

--HG--
extra : rebase_source : 99b93a6cb0dde0960d1d4349498dd3e61fefec56

js/src/jsfriendapi.h

@@ -567,6 +567,7 @@ struct String
     static const uint32_t INLINE_CHARS_BIT = JS_BIT(2);
     static const uint32_t LATIN1_CHARS_BIT = JS_BIT(6);
     static const uint32_t ROPE_FLAGS       = 0;
+    static const uint32_t EXTERNAL_FLAGS   = JS_BIT(5);
     static const uint32_t TYPE_FLAGS_MASK  = JS_BIT(6) - 1;
     uint32_t flags;
     uint32_t length;
@@ -576,6 +577,7 @@ struct String
         JS::Latin1Char inlineStorageLatin1[1];
         char16_t inlineStorageTwoByte[1];
     };
+    const JSStringFinalizer* externalFinalizer;
 };
 
 } /* namespace shadow */
@@ -830,6 +832,21 @@ GetTwoByteAtomChars(const JS::AutoCheckCannotGC& nogc, JSAtom* atom)
     return GetTwoByteLinearStringChars(nogc, AtomToLinearString(atom));
 }
 
+MOZ_ALWAYS_INLINE bool
+IsExternalString(JSString* str, const JSStringFinalizer** fin, const char16_t** chars)
+{
+    using shadow::String;
+    String* s = reinterpret_cast<String*>(str);
+
+    if ((s->flags & String::TYPE_FLAGS_MASK) != String::EXTERNAL_FLAGS)
+        return false;
+
+    MOZ_ASSERT(JS_IsExternalString(str));
+    *fin = s->externalFinalizer;
+    *chars = s->nonInlineCharsTwoByte;
+    return true;
+}
+
 JS_FRIEND_API(JSLinearString*)
 StringToLinearStringSlow(JSContext* cx, JSString* str);
 

js/src/vm/String.h

@@ -302,6 +302,8 @@ class JSString : public js::gc::TenuredCell
                       "shadow::String nonInlineChars offset must match JSString");
         static_assert(offsetof(JSString, d.s.u2.nonInlineCharsTwoByte) == offsetof(String, nonInlineCharsTwoByte),
                       "shadow::String nonInlineChars offset must match JSString");
+        static_assert(offsetof(JSString, d.s.u3.externalFinalizer) == offsetof(String, externalFinalizer),
+                      "shadow::String externalFinalizer offset must match JSString");
         static_assert(offsetof(JSString, d.inlineStorageLatin1) == offsetof(String, inlineStorageLatin1),
                       "shadow::String inlineStorage offset must match JSString");
         static_assert(offsetof(JSString, d.inlineStorageTwoByte) == offsetof(String, inlineStorageTwoByte),
@@ -314,6 +316,8 @@ class JSString : public js::gc::TenuredCell
                       "shadow::String::TYPE_FLAGS_MASK must match JSString::TYPE_FLAGS_MASK");
         static_assert(ROPE_FLAGS == String::ROPE_FLAGS,
                       "shadow::String::ROPE_FLAGS must match JSString::ROPE_FLAGS");
+        static_assert(EXTERNAL_FLAGS == String::EXTERNAL_FLAGS,
+                      "shadow::String::EXTERNAL_FLAGS must match JSString::EXTERNAL_FLAGS");
     }
 
     /* Avoid lame compile errors in JSRope::flatten */