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On 2017-09-28 12:10, Pedro Alves wrote:
On 09/12/2017 07:57 PM, Tom Tromey wrote:Change a couple of spots in mi-main.c to use std::set. This simplifies the code and removes some cleanups.std::set always gives me pause. For small objects like int, and when the use case is insertion phase + lookup phase + discard set, unsorted inserting into a vector, sorting, and then binary searching the vector for lookuups is very likely to have better performance, for cache locality reasons, and also because fewer allocations (with std::set being a node-based container...) But it's likely that in this case it doesn't really matter, so let's go with the simplicity argument. (At some point I may propose some data structure on top of std::vector for use cases like this.)
We have discussed something like this with Sergio in this thread: https://sourceware.org/ml/gdb-patches/2017-07/msg00434.html (you can ctrl-f "boilerplate")We managed to sneak in an std::set while you were not looking/on vacation :). The idea was that the std::set interface really helped to keep the code clean and short, and that we could later implement something with the same behavior and interface as std::set, but based on a vector. It's a bit different than what you propose, because to be a drop-in replacement for a set, we would always keep the vector sorted (insert the new elements where they belong). IIUC, what you propose is to insert everything and then sort it. I think the latter has a better worst-case performance (sorting at the end is n*log(n)) compared to the former (insertion in reverse order will become n^2). I don't think it matters much though, since this data structure would be explicitly for relatively small amount of items.
I am not sure if the reasoning is different for a set of string (as in common/environ.{h,c}). I assume it would be similar, since the actual string object is rather small, and when inserting a string in the middle, the following strings will be moved one position and not copied (I haven't verified though).
Simon
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