> # 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 > Directly iterates over `src.table` to eliminate entrySet() allocation and > polymorphic iterator calls, using the internal Node structure for maximum > efficiency. > > ### 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 direct access. > > ## Performance Impact > > | Source Type | Size | Poisoned | Baseline | Optimized | Improvement | > |-------------|------|----------|----------|-----------|-------------| > | HashMap | 0 | true | 9.429 ns/op | 8.875 ns/op |...
jengebr has updated the pull request incrementally with one additional commit since the last revision: Unit test revisions ------------- Changes: - all: https://git.openjdk.org/jdk/pull/28243/files - new: https://git.openjdk.org/jdk/pull/28243/files/05891484..edfc6a2f Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=28243&range=01 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28243&range=00-01 Stats: 27 lines in 2 files changed: 7 ins; 11 del; 9 mod Patch: https://git.openjdk.org/jdk/pull/28243.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/28243/head:pull/28243 PR: https://git.openjdk.org/jdk/pull/28243
