Correctly recognize external strings.

Correctly recognize external strings.

Since r15906, cons strings in old pointer space that are made external
remain cons strings, and point to a newly-allocated external string. This
is because external strings are not allow in the pointer space.

However, v8::String::IsExternal and related methods do not reflect this.
The consequence is that the embedder cannot take advantage of external
strings.

Note that v8::String::CanMakeExternal still returns false for cons strings
due to AccessCheck not being GC safe (see r17023).

BUG=268686
LOG=N

[Yang, 2014-02-04 14:07:37]

Please take a look.

[dcarney, 2014-02-04 15:10:05]

lgtm

https://codereview.chromium.org/153803003/diff/1/include/v8.h
File include/v8.h (right):

https://codereview.chromium.org/153803003/diff/1/include/v8.h#newcode6011
include/v8.h:6011: String::ExternalStringResourceBase*
String::GetExternalStringResourceBase(
this function is ridiculously hot in some dromaeo benchmark, so you can't land
it like this.
maybe try doing some reordering

if (string_type == I::kExternalRepresentationTag) {
  return ...
} else if (string_type != I::kConsRepresentationTag) {
  return NULL;
}
return slow_case()

https://codereview.chromium.org/153803003/diff/1/src/api.cc
File src/api.cc (right):

https://codereview.chromium.org/153803003/diff/1/src/api.cc#newcode4714
src/api.cc:4714: if (i::StringShape(str).IsCons()) {
extract these 4 lines into a static function

[Yang, 2014-02-05 09:01:26]

On 2014/02/04 15:10:05, dcarney wrote:
> lgtm
> 
> https://codereview.chromium.org/153803003/diff/1/include/v8.h
> File include/v8.h (right):
> 
> https://codereview.chromium.org/153803003/diff/1/include/v8.h#newcode6011
> include/v8.h:6011: String::ExternalStringResourceBase*
> String::GetExternalStringResourceBase(
> this function is ridiculously hot in some dromaeo benchmark, so you can't land
> it like this.
> maybe try doing some reordering
> 
> if (string_type == I::kExternalRepresentationTag) {
>   return ...
> } else if (string_type != I::kConsRepresentationTag) {
>   return NULL;
> }
> return slow_case()
> 
> https://codereview.chromium.org/153803003/diff/1/src/api.cc
> File src/api.cc (right):
> 
> https://codereview.chromium.org/153803003/diff/1/src/api.cc#newcode4714
> src/api.cc:4714: if (i::StringShape(str).IsCons()) {
> extract these 4 lines into a static function

Scrapping this since we have a better solution now.