Use StringView for v8String().

third_party/WebKit/Source/bindings/core/v8/V8Binding.h

 /*
 * Copyright (C) 2009 Google Inc. All rights reserved.
 * Copyright (C) 2012 Ericsson AB. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
@@ skipping 379 matching lines @@
 // It will not call ToString() on the v8::Value. If you want ToString() to be called,
 // please use the TONATIVE_FOR_V8STRINGRESOURCE_*() macros instead.
 inline String toCoreStringWithUndefinedOrNullCheck(v8::Local<v8::Value> value)
 {
     if (value.IsEmpty() || !value->IsString())
         return String();
     return toCoreString(value.As<v8::String>());
 }
 
 // Convert a string to a V8 string.
-// Return a V8 external string that shares the underlying buffer with the given
-// WebCore string. The reference counting mechanism is used to keep the
-// underlying buffer alive while the string is still live in the V8 engine.
-inline v8::Local<v8::String> v8String(v8::Isolate* isolate, const String& string)
+
+inline v8::Local<v8::String> v8String(v8::Isolate* isolate, const StringView& string)
 {
+    DCHECK(isolate);
     if (string.isNull())
         return v8::String::Empty(isolate);
-    return V8PerIsolateData::from(isolate)->getStringCache()->v8ExternalString(isolate, string.impl());
+    if (StringImpl* impl = string.sharedImpl())
+        return V8PerIsolateData::from(isolate)->getStringCache()->v8ExternalString(isolate, impl);
+    if (string.is8Bit())
+        return v8::String::NewFromOneByte(isolate, reinterpret_cast<const uint8_t*>(string.characters8()), v8::NewStringType::kNormal, static_cast<int>(string.length())).ToLocalChecked();

[haraken, 2016/08/27 01:21:46]

Don't you want to use:

  V8PerIsolateData::from(isolate)->getStringCache()->v8ExternalString(isolate,
string.impl());

?

If we're highly likely to hit the branch at line 406, I'm fine with not using a
StringCache here though.

[esprehn, 2016/08/27 02:18:57]

Hmm? There's no such thing as string.impl() on a StringView, just sharedImpl()
which is already above.
We hit that branch for things like toV8 in a bunch of cases, we hit the NewFrom*
cases for literal strings.

StringView makes it so that if you pass an AtomicString/String we'll
externalize, but if you pass a literal string we'll just allocate a string and
not create a WTF string + StringResource and then externalize which is much more
expensive.

[esprehn, 2016/08/27 02:24:03]

ex. 

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/bindings/core/...

that hits the sharedImpl() case which will then externalize the string using the
cache.

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/bindings/core/...

that will return null from sharedImpl() and so it'll go down the NewFromOneByte
path which is better since we don't waste time externalizing the temporary
string.

+    return v8::String::NewFromTwoByte(isolate, reinterpret_cast<const uint16_t*>(string.characters16()), v8::NewStringType::kNormal, static_cast<int>(string.length())).ToLocalChecked();
 }
 
 inline v8::Local<v8::Value> v8StringOrNull(v8::Isolate* isolate, const AtomicString& string)
 {
     if (string.isNull())
         return v8::Null(isolate);
     return V8PerIsolateData::from(isolate)->getStringCache()->v8ExternalString(isolate, string.impl());
 }
 
 inline v8::Local<v8::String> v8AtomicString(v8::Isolate* isolate, const StringView& string)
@@ skipping 541 matching lines @@
 typedef void (*InstallTemplateFunction)(v8::Isolate*, const DOMWrapperWorld&, v8::Local<v8::FunctionTemplate> interfaceTemplate);
 
 // Freeze a V8 object. The type of the first parameter and the return value is
 // intentionally v8::Value so that this function can wrap toV8().
 // If the argument isn't an object, this will crash.
 CORE_EXPORT v8::Local<v8::Value> freezeV8Object(v8::Local<v8::Value>, v8::Isolate*);
 
 } // namespace blink
 
 #endif // V8Binding_h