RFR: 8305896: Alternative full GC forwarding [v20]

[Roman Kennke]

On Wed, 3 May 2023 11:08:04 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   More Thomas' comments
>
> src/hotspot/share/gc/shared/slidingForwarding.cpp line 153:
> 
>> 151:   // Set from and to in new or found entry.
>> 152:   entry->_from = from;
>> 153:   entry->_to   = to;
> 
> Why so complicated? Proposal:
> 
> while (entry != nullptr && entry->_from != from) {
>   entry = entry->_next;
> }
> if (entry == nullptr) {
>   FallbackTableEntry* new_entry = NEW_C_HEAP_OBJ(FallbackTableEntry, mtGC);
>   new_entry->next = head;
>   new_entry->_from = from;
>   head = entry = new_entry;
> }
> entry->_to = to;

Uhm, so this would not change the actual head

> Why so complicated? Proposal:
> 
> ```
> while (entry != nullptr && entry->_from != from) {
>   entry = entry->_next;
> }
> if (entry == nullptr) {
>   FallbackTableEntry* new_entry = NEW_C_HEAP_OBJ(FallbackTableEntry, mtGC);
>   new_entry->next = head;
>   new_entry->_from = from;
>   head = entry = new_entry;
> }
> entry->_to = to;
> ```

Remember that head points into the array. We cannot actually prepend the new entry, we can only insert it as the first linked entry after head. If I see it correctly, it would not actually change the head-entry (the stuff in the array) except for its _to field. Also, the new_entry would not get linked anywhere. Or what am I missing?

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PR Review Comment: https://git.openjdk.org/jdk/pull/13582#discussion_r1183550376