Re: Formal review request for Hash2

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2024-12-05 21:53:34

On 12/6/24 00:18, Ivan Matek via Boost wrote:
> 
> I did not paste std::string results since as mentioned they are mostly
> influenced by clang inlining decisions, but you get the idea: there seems
> to be small but real performance difference when hashing size. I checked
> what some other popular C++ non std library does for std::string_view  when
> it is implementing replacement for std::hash and it also hashes size, so I
> presume it is not unreasonable that boost does same, but I wonder if we
> could do something faster.

Technically, it doesn't need to be size to separate the two fields. You
can use any kind of separator, including a fixed one (i.e. not depending
on either of the fields it separates). I.e. instead of this:

  "ab" | 2 | "c" | 1

you could have this:

  "ab" | separator_t{} | "c" | separator_t{}

An empty sequence could simply be hashed as separator_t{}.

The question is how separator_t would hash and whether it could be made
any faster than just hashing the size. I suppose, it could be smaller
(e.g. 1 byte of data instead of sizeof(size_t)), but I'm not sure if
that would mean anything in terms of performance.