RFR (M) 8207359: Make SymbolTable increment_refcount disallow zero

[Kim Barrett]

> On Jul 18, 2018, at 10:48 PM, coleen.phillimore at oracle.com wrote:
> 
> On 7/18/18 6:14 PM, Kim Barrett wrote:
>> src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/Symbol.java
>>   78   public long   getLength() {
>>   79     int i = (int)length.getValue(this.addr);
>>   80     return (i >> 16);
>>   81   }
>> 
>> Consider a symbol whose length is in the range [2^15, 2^16). The cast
>> result is implementation defined. The right shift of a negative value
>> is implemention defined, and might (or might not) return a negative value.
> 
> There is no unsigned int in Java, so I added back the & 0xffff which seems to work, maybe with less undefined behavior (?)
> 
>   public long getLength() {
>     long i = length.getValue(this.addr);
>     return (i >> 16) & 0xffff;
>   }

Oh, right, this is Java, so it’s all portably defined, and yes, the mask is needed and sufficient.