What's the purpose of hashtable::reverse()

[Ioi Lam]

Thanks Mikael for the detective work. The bug comment says:

"the String and symbol tables are hashtables where the hashbuckets
are linked lists -- in reverse chronological order of there creation. This
means that, in some circumstances, some symbols are unnecessarily touched
when looking for older most likely needed symbols. This can be addressed by
simply reversing the order of the lists before creating the archive."

Since JDK9, CDS no longer stores the string and symbol tables using the 
"regular" hashtable.

The only "regular" hashtable use by CDS is the shared dictionary (for 
name->class lookup), but this table is rehashed at some point, so 
calling reverse() will no longer has the desired effect of moving 
popular Klasses (such as java/lang/Object) to the front of a linked-list.

So this optimization is irrelevant now. I've created an RFE to remove it.

https://bugs.openjdk.java.net/browse/JDK-8176863

Thanks
- Ioi

On 3/16/17 5:17 PM, Mikael Gerdin wrote:
> Hi Ioi,
>
> I found this old bug where I think the reversing was added:
> https://bugs.openjdk.java.net/browse/JDK-4994483
>
> /Mikael
>
> On 2017-03-16 07:48, Ioi Lam wrote:
>> This seems to be used only by CDS dumping, and if I remove the call
>> nothing bad seems to happen. Does anyone know what hashtable::reverse()
>> could possibly achieve?
>>
>> Also, there's another variant that seems to be dead code that no one 
>> calls.
>>
>>     template <class T, MEMFLAGS F> void Hashtable<T, F>::reverse(void*
>> boundary)
>>
>> - Ioi
>>
>>