RFR (S): 8192003: Refactor weak references in StringTable to use the Access API

[Per Liden]

Hi Erik,

On 2017-11-28 17:50, Erik Österlund wrote:
> Hi,
>
> The StringTable contains weak references to oops. Today the weak
> semantics is managed using explicit calls to G1 SATB enqueue barriers.
>
> Now that the Access API is available, it should be used instead as it is
> more modular.
>
> This change fixes that by making these oops ON_PHANTOM_OOP_REF with a
> corresponding AS_NO_KEEPALIVE accessor when we do not want to keep it
> alive, much like my previous changes to other weak tables.
>
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8192003/webrev.00/

share/classfile/javaClasses.inline.hpp
--------------------------------------

   57 typeArrayOop java_lang_String::value_no_keepalive(oop java_string) {
   58   assert(initialized && (value_offset > 0), "Must be initialized");
   59   assert(is_instance(java_string), "must be java_string");
   60   oop value = 
HeapAccess<AS_NO_KEEPALIVE>::oop_load_at(java_string, value_offset);
   61   return (typeArrayOop)value;
   62 }

How about pushing this barrier down to oopDesc, with a 
oopDesc::obj_field_special_access<DecoratorSet D>(...) function?


   76   typeArrayOop value = 
java_lang_String::value_no_keepalive(java_string);
   77   assert(initialized, "Must be initialized");
   78   assert(is_instance(java_string), "must be java_string");

Looks like you accidentally moved the value_no_keepalive() call above 
the asserts?


share/classfile/stringTable.cpp
-------------------------------

  155       oop string = string_object_no_keepalive(l);
  156       if (java_lang_String::equals(string, name, len)) {
  157         return string_object(l);
  158       }

Can we please add a comment here, saying that returning "string" would 
be very bad, or maybe even fold this a bit, so that no one will be 
tempted to ever return that value, something like:

if (java_lang_String::equals(string_object_no_keepalive(l), name, len)) {
     // Comment saying why we must call string_object() here...
     return string_object(l);
}

cheers,
Per

>
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8192003
>
> Thanks,
> /Erik