Use NonZeroU64 for Id<T> to enable niche optimization on Option<Id>#659
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Use NonZeroU64 for Id<T> to enable niche optimization on Option<Id>#659st0012 wants to merge 1 commit intooptimize-name-depth-sortingfrom
st0012 wants to merge 1 commit intooptimize-name-depth-sortingfrom
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Change Id<T> from u64 to NonZeroU64 so that Option<Id<T>> is 8 bytes instead of 16 bytes. Hash value 0 is remapped to u64::MAX (the collision probability is 2^-64, same as any other hash collision). This propagates savings through every struct with an Option<XxxId> field: - MethodRef: 40 → 32 bytes (receiver: Option<NameId>) - Name: 48 → 40 bytes (nesting: Option<NameId>) - ClassDefinition: 144 → 128 bytes (two Option<Id> fields) - All other definition structs: -8 bytes each (lexical_nesting_id) - Namespace declarations: 224 → 216 bytes (singleton_class_id) ## Benchmark | Metric | Before | After | Delta | |--------|--------|-------|-------| | Memory (peak footprint) | 4480 MB | 3978 MB | -11.2% | | Declarations | 895,844 | 895,844 | identical | | Definitions | 1,063,171 | 1,063,171 | identical |
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Summary
Id<T>fromu64toNonZeroU64so thatOption<Id<T>>is 8 bytes instead of 16 bytesu64::MAX(collision probability is 2^-64, same as any other hash collision)Option<XxxId>field:Benchmark