RFR(xs): 8244733: Add ResourceHashtable::compute_if_absent

[David Holmes]

Hi Thomas,

On 12/05/2020 2:25 pm, Thomas Stüfe wrote:
> Hi all,
> 
> May I please have reviews for this enhancement:
> 
> JBS: https://bugs.openjdk.java.net/browse/JDK-8244733
> Webrev:
> http://cr.openjdk.java.net/~stuefe/webrevs/8244733--add-resourcehashtable--compute_if_absent/webrev.00/webrev/
> 
> A common pattern when using ResourceHashTable is:
> 
> if (table.get(k) == NULL) table.add(k,v);
> 
> This runs the lookup code twice. By providing a new method which combines
> these two operations we would save one lookup.
> 
> The new method is named ResourceHashTable::compute_if_absent() and behaves
> very similar to the similarly named variant in j.u.Map.

The code to replace the pattern above would be putIfAbsent. The general 
motivation for computeIfAbsent is to avoid the execution of an expensive 
mapping function if the mapping already exists. Perhaps you need both?

David

> Note: Actually replacing caller code will happen in a separate RFE. The
> first caller to use this will be the ClassLoaderStats VM operation - I was
> able to significantly reduce runtime for that vm op. But since that is JFR
> territory and this is more of a runtime topic I wanted to keep the review
> separate.
> 
> Thanks, Thomas
>