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HashSet iteration on drop not elided for Copy elements #31711

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bluss opened this issue Feb 16, 2016 · 2 comments
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HashSet iteration on drop not elided for Copy elements #31711

bluss opened this issue Feb 16, 2016 · 2 comments
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A-collections Area: `std::collections` I-slow Issue: Problems and improvements with respect to performance of generated code.

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@bluss
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bluss commented Feb 16, 2016

If a HashSet has Copy elements, it doesn't need to visit all elements on drop. The iteration in RawTable's drop still is computed even if the elements have no destructors, which leads to O(n) average complexity even then.
@bluss bluss added I-slow Issue: Problems and improvements with respect to performance of generated code. A-collections Area: `std::collections` labels Feb 16, 2016
@hanna-kruppe
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To be clear, this is also in release mode, right? A while back there was a similar issue for vec but AFAIK that one was only about debug mode.

@bluss
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bluss commented Feb 16, 2016

Yes, also in release mode.

@apasel422 apasel422 mentioned this issue Feb 16, 2016
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bors added a commit that referenced this issue Feb 17, 2016
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Auto merge of #31718 - apasel422:issue-31711, r=bluss
2051a92 
This changes the performance of `drop` from linear to constant time for such `HashMap`s.

Closes #31711.

r? @bluss
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A-collections Area: `std::collections` I-slow Issue: Problems and improvements with respect to performance of generated code.
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