RFR: 8320649: C2: Optimize scoped values [v16]

[Andrew Haley]

On Fri, 19 Apr 2024 13:09:22 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> This change implements C2 optimizations for calls to
>> ScopedValue.get(). Indeed, in:
>> 
>> 
>> v1 = scopedValue.get();
>> ...
>> v2 = scopedValue.get();
>> 
>> 
>> `v2` can be replaced by `v1` and the second call to `get()` can be
>> optimized out. That's true whatever is between the 2 calls unless a
>> new mapping for `scopedValue` is created in between (when that happens
>> no optimizations is performed for the method being compiled). Hoisting
>> a `get()` call out of loop for a loop invariant `scopedValue` should
>> also be legal in most cases.
>> 
>> `ScopedValue.get()` is implemented in java code as a 2 step process. A
>> cache is attached to the current thread object. If the `ScopedValue`
>> object is in the cache then the result from `get()` is read from
>> there. Otherwise a slow call is performed that also inserts the
>> mapping in the cache. The cache itself is lazily allocated. One
>> `ScopedValue` can be hashed to 2 different indexes in the cache. On a
>> cache probe, both indexes are checked. As a consequence, the process
>> of probing the cache is a multi step process (check if the cache is
>> present, check first index, check second index if first index
>> failed). If the cache is populated early on, then when the method that
>> calls `ScopedValue.get()` is compiled, profile reports the slow path
>> as never taken and only the read from the cache is compiled.
>> 
>> To perform the optimizations, I added 3 new node types to C2:
>> 
>> - the pair
>>   ScopedValueGetHitsInCacheNode/ScopedValueGetLoadFromCacheNode for
>>   the cache probe
>>   
>> - a cfg node ScopedValueGetResultNode to help locate the result of the
>>   `get()` call in the IR graph.
>> 
>> In pseudo code, once the nodes are inserted, the code of a `get()` is:
>> 
>> 
>> hits_in_the_cache = ScopedValueGetHitsInCache(scopedValue)
>> if (hits_in_the_cache) {
>>   res = ScopedValueGetLoadFromCache(hits_in_the_cache);
>> } else {
>>   res = ..; //slow call possibly inlined. Subgraph can be arbitray complex
>> }
>> res = ScopedValueGetResult(res)
>> 
>> 
>> In the snippet:
>> 
>> 
>> v1 = scopedValue.get();
>> ...
>> v2 = scopedValue.get();
>> 
>> 
>> Replacing `v2` by `v1` is then done by starting from the
>> `ScopedValueGetResult` node for the second `get()` and looking for a
>> dominating `ScopedValueGetResult` for the same `ScopedValue`
>> object. When one is found, it is used as a replacement. Eliminating
>> the second `get()` call is achieved by making
>> `ScopedValueGetHitsInCache` always successful if there's a dominating
>> `Scoped...
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update src/hotspot/share/opto/loopnode.cpp
>   
>   Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>

How are we doing here? I've just submitted the Scoped Values (Third Preview) JEP, and it would be helpful to get this work committed in time for the next JDK release.

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PR Comment: https://git.openjdk.org/jdk/pull/16966#issuecomment-2080423705