[all-commits] [llvm/llvm-project] ad79a1: [ADT] Update hash function of uint64_t for DenseMa...

[Chuanqi Xu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ad79a14c9e5ec4a369eed4adf567c22cc029863f
      https://github.com/llvm/llvm-project/commit/ad79a14c9e5ec4a369eed4adf567c22cc029863f
  Author: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
  Date:   2024-06-20 (Thu, 20 Jun 2024)

  Changed paths:
    M llvm/include/llvm/ADT/DenseMapInfo.h

  Log Message:
  -----------
  [ADT] Update hash function of uint64_t for DenseMap (#95734)

(Background: See the comment:
https://github.com/llvm/llvm-project/pull/92083#issuecomment-2168121729)

It looks like the hash function for 64bits integers are not very good:

```
  static unsigned getHashValue(const unsigned long long& Val) {
    return (unsigned)(Val * 37ULL);
  }
```

Since the result is truncated to 32 bits. It looks like the higher 32
bits won't contribute to the result. So that `0x1'00000001` will have
the the same results to `0x2'00000001`, `0x3'00000001`, ...

Then we may meet a lot collisions in such cases. I feel it should
generally good to include higher 32 bits for hashing functions.

Not sure who's the appropriate reviewer, adding some people by
impressions.



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