RFR: 8371656: HashMap.putAll() optimizations [v3]

[Rémi Forax]

On Thu, 4 Dec 2025 14:59:33 GMT, jengebr <duke at openjdk.org> wrote:

>> # HashMap.putAll() optimizations: Eliminating Megamorphic Call Site Bottlenecks
>> 
>> ## Summary
>> 
>> This PR addresses performance bottlenecks in `HashMap.putMapEntries()` by implementing direct optimizations for specific input types: `j.u.HashMap` and `j.u.Collections$UnmodifiableMap`. The optimizations target `HashMap(Map)` constructor and `putAll()` operations based on the real-world megamorphic behavior identified in [JDK-8368292](https://bugs.openjdk.org/browse/JDK-8368292), delivering significant performance improvements when multiple `Map` subtypes are used.
>> 
>> ## Problem Context
>> 
>> ### Megamorphic Call Site Overhead in Map Iteration
>> `HashMap.putMapEntries()` currently uses a generic approach that suffers from megamorphic call site overhead when applications perform bulk creation or population of HashMaps from various source map types:
>> 
>> 1. `m.entrySet()` becomes megamorphic across different map implementations
>> 2. `entrySet().iterator()` creates different iterator types
>> 3. `entry.getKey()` and `entry.getValue()` calls vary by map type
>> 4. Individual `putVal()` calls for each entry
>> 
>> When the source is `Collections$UnmodifiableMap`, the problem is compounded by megamorphic wrappers around the already-megamorphic iteration methods. In cases where the unwrapped map is also a HashMap, both the wrapper overhead and the iteration overhead can be eliminated with a single optimization.
>> 
>> ## Optimized Methods
>> 
>> ### HashMap
>> - **`putMapEntries(Map<? extends K, ? extends V> m, boolean evict)`**: Added fast paths for UnmodifiableMap unwrapping and HashMap-to-HashMap copying
>> - **`putMapEntries(HashMap<? extends K, ? extends V> src, boolean evict)`**: copies HashMap-to-HashMap via direct Node processing.  Avoids polymorphic issues and eliminates redundant calls to HashMap.hash().
>> 
>> ## Implementation Details
>> 
>> ### HashMap-to-HashMap Fast Path
>> Eliminates megamorphic iteration by targeting internal Node structure - and also reuses the pre-calculated hash code, thus avoiding megamorphic calls to Object.hashCode() and the sometimes-expensive recalculation (depending on key type).  This also eliminates direct reads from the key, thus reducing the set of objects accessed.
>> 
>> ### UnmodifiableMap Unwrapping
>> Detects UnmodifiableMap instances and accesses the underlying map directly via the `m` field, eliminating wrapper-induced megamorphic call sites. UnmodifiableMap visibility changed from `private` to package-private to enable this...
>
> jengebr has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Merge branch 'openjdk:master' into putMapEntriesOptimizations
>  - Unit test revisions
>  - Bug fix & unit test
>  - fixing whitespace
>  - Optimizing HashMap.putAll() and .<init> for HashMap and C$UM

src/java.base/share/classes/java/util/HashMap.java line 541:

> 539:                 HashMap<K, V> hashMap = (HashMap<K, V>) m;
> 540:                 putMapEntries(hashMap, evict);
> 541:             } else {

I would add a return here and avoid the "else" because this is an optimization, the nominal case (below) should be aligned on the left.

so instead of
if (...) { ... } else { ... }
use
if (...) { ... return; }
...

to emphasis that this is an optimization.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28243#discussion_r2765998998