Libc++'s Implementation of std::string

Re:Libc++'s Implementation of std::string by gpderetta on Friday January 31, 2020 @05:23AM (#22200523)

You shouldn't need a branch though. The size would always be the last element of the buffer. It doubles as the null terminator if the size is exactly 23. The only additional operation is adding the static offset to the stored value to get the actual size.

It is probably just a small win and not everybody wanted to pay for the extra complexity (SSO is already fairly complex).

Re your suggestion on large SSO, years ago, when doing document clustering, using large fixed size strings (64 chars if I remember correctly, longer strings were just truncated) was a huge win for me (even coping them around wasn't an issue). Not sure if it is appropriate for a general purpose string though.

edit: never mind, I see what you mean about having to check the size.

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Re:Libc++'s Implementation of std::string by OskarS on Friday January 31, 2020 @08:14AM (#22201368)

Yeah, exactly :) when in SSO mode, it stores the size as a delta occupying a single byte, but when in heap-allocated mode, it stores the size as a an 8-byte size_t, and you require a branch to check which case is the right one. You can see the branch here in the source [0], and godbolt confirms that for std::string it's just a memory load [1]. A shame Folly isn't available on Compiler Explorer, would be interesting to compare.

[0]: https://github.com/facebook/folly/blob/master/folly/FBString...

[1]: https://godbolt.org/z/U-LvGm

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