RFR (L) JDK-8195100: Use a low latency hashtable for SymbolTable

[Ioi Lam]

On 8/2/18 8:44 AM, Gerard Ziemski wrote:
>> On Aug 2, 2018, at 10:40 AM, Ioi Lam <ioi.lam at oracle.com> wrote:
>>
>>
>>
>> On 8/2/18 8:26 AM, Gerard Ziemski wrote:
>>>> symbolTable.cpp:
>>>>
>>>>   342   // Otherwise, add the symbol to table. Copy to a C string first.
>>>>   343   char stack_buf[ON_STACK_BUFFER_LENGTH];
>>>>   344   ResourceMark rm(THREAD);
>>>>   345   if (len <= ON_STACK_BUFFER_LENGTH) {
>>>>   346     buffer = stack_buf;
>>>>   347   } else {
>>>>   348     buffer = NEW_RESOURCE_ARRAY_IN_THREAD(THREAD, char, len);
>>>>   349   }
>>>>   350   for (int i=0; i<len; i++) {
>>>>   351     buffer[i] = name[i];
>>>>   352   }
>>>>   353   // Make sure there is no safepoint in the code above since name can't move.
>>>>   354   // We can't include the code in NoSafepointVerifier because of the
>>>>   355   // ResourceMark.
>>>>
>>>> The above is dead code and should be removed.
>>> That’s not dead code, but in fact you did find an issue here - we should be using the “buffer”, not “name” when we add to the table. Thanks!
>>>
>>>
>> Why do you need to make a copy here? buffer and name seems to have the exact same contents.
> I didn’t write that code, but I assume the name comes in some temp memory that can go away at any time, so we need a solid copy.
>
>
> cheers
>
This function is called only from two places. In both cases, the input 
Symbol won't go away

Symbol* SignatureStream::as_symbol(TRAPS) {
   // Create a symbol from for string _begin _end
   int begin = _begin;
   int end   = _end;

   if (   _signature->byte_at(_begin) == 'L'
       && _signature->byte_at(_end-1) == ';') {
     begin++;
     end--;
   }

   // Save names for cleaning up reference count at the end of
   // SignatureStream scope.
   Symbol* name = SymbolTable::new_symbol(_signature, begin, end, 
CHECK_NULL);
   _names->push(name);  // save new symbol for decrementing later
   return name;
}

Symbol* ClassVerifier::create_temporary_symbol(const Symbol *s, int begin,
                                                int end, TRAPS) {
   Symbol* sym = SymbolTable::new_symbol(s, begin, end, CHECK_NULL);
   _symbols->push(sym);
   return sym;
}

Also, the "name" was used just a few lines above in the lookup, so if 
this was ever a issue, that lookup would fail as well.

Mercurial history shows that this function has not had any meaningful 
change since it was imported from SCCS in 2007, so I think it's just a 
case of bit rot and the dead code can be safely removed.

Thanks
- Ioi