ChibiHash: Small, Fast 64 bit hash function

While the benefit of processing chunks of 8 bytes is obvious, what's the purpose of grouping those into macrogroups of 4? Does it trigger any implicit parallelism I failed to spot? Or is it just to end this phase with the 4 h[] having had the same amount of entropy, and thus starting the next one with h[0]?

> The way it loads the 8 bytes is also important. The correct way is to load via shift+or > This is free of any UB, works on any alignment and on any machine regardless of it's endianness. It's also fast, gcc and clang recognize this pattern and optimize it into a single mov instruction on x86 targets.

Is a single MOV instruction still fast when the 8 bytes begin on an odd address?

> Is a single MOV instruction still fast when the 8 bytes begin on an odd address?

On x86, yes. There is no performance penalty for misaligned loads, except when the misaligned load also happens to straddle a cache line boundary, in which case it is slower, but only marginally so.

like mentioned x86/64 is quite generous with non-aligned access, yet on architectures that require aligned loads, they will be aligned (all lowest bits being zero at the start), so it will continue being aligned with each 8 byte load.

> they will be aligned (all lowest bits being zero at the start

I don't think so. At the start k will be equal to keyIn, which can point to any arbitrary memory location.

But in practice, will always point to the start of a buffer allocated from the heap which will be aligned.

Why? If you are hashing names, for example, or keywords, they can begin at whatever position inside the buffer.

The case where you allocate a buffer, populate it with random contents and hash the whole of it is an artificial setup for benchmarking, but far from real use cases.

i was about to answer pretty much as 'rerdavies' did. If you have such requirements the specific architectures would need wrappers to copy the data, or do pref/post fix up (ala crc32 & avx implementations on non perfect boundary)