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maybe expose a way to pass pre-calculated hashes along with hash table keys #10661

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thestinger opened this issue Nov 26, 2013 · 2 comments
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maybe expose a way to pass pre-calculated hashes along with hash table keys #10661

thestinger opened this issue Nov 26, 2013 · 2 comments
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C-enhancement Category: An issue proposing an enhancement or a PR with one. I-slow Issue: Problems and improvements with respect to performance of generated code.

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@thestinger
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I found this was necessarily to avoid double-hashing with a shard-based concurrent hash table like Java's ConcurrentHashMap as the hash needs to be calculated to pick the correct shard.

https://github.com/thestinger/rust-core/blob/master/core/concurrent.rs#L299

This could be done by hoisting out the open-addressing implementation, but it will result in it requiring the hashes to be stored in the table rather than it just being an implementation detail.
@cgaebel
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cgaebel commented Feb 15, 2014

It'd probably need an unsafe tag, and a debug assertation from #12108, but given those I don't see why the HashMap can't support unsafe pub fn insert_hashed(hash: u64, k: K, v: V)

@steveklabnik steveklabnik mentioned this issue Feb 2, 2015
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I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#794

flip1995 pushed a commit to flip1995/rust that referenced this issue Apr 23, 2023
@bors
Auto merge of rust-lang#10661 - timvisee:manual-slice-size-calc-sugge…
203c909 
…stion, r=Manishearth

Add `manual_slice_size_calculation` applicable suggestion

Continuation of rust-lang/rust-clippy#10659 (comment).

This adds applicable suggestions to the `manual_slice_size_calculation` lint:

```
error: manual slice size calculation
  --> $DIR/manual_slice_size_calculation.rs:11:13
   |
LL |     let _ = s_i32.len() * size_of::<i32>(); // WARNING
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::mem::size_of_val(s_i32)`
   |
   = note: `-D clippy::manual-slice-size-calculation` implied by `-D warnings`
```

changelog: [`manual_slice_size_calculation`]: add machine applicable suggestion
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C-enhancement Category: An issue proposing an enhancement or a PR with one. I-slow Issue: Problems and improvements with respect to performance of generated code.
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