The Little Things: The Missing Performance in std::vector
The Little Things: The Missing Performance in std::vector
`std::vector` is often said to be the default container, because it provides good baseline performance for common operations. Recently, I experimented with a simple API change that can improve the performance of a common usage pattern by 10+ %.
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Couldn't this be solved by having
push_backbeing an inline function (or at least the check on capacity being inlined and the rest of the non-trivial part being in a sub non-inline function)?I don't know about C++, but in Rust the push is inline, and still doesn't always optimize checks away due to an annoying edge case: integer overflow. Reserving (old_len + new_len) could give you a smaller buffer than new_len. The optimizer sees it and is pedantic about it.
In C++ integer overflow is UB so this edge case cannot exist
Only signed overflow. size_t is unsigned.
That's totally right but I thought you were talking about signed numbers since you said “integer overflow”. I forgot that
lenis usually unsigned in C++.
Ok, but why did you not use emplace?
emplacecontrols the construction of the object added to the collection. It's also important but not related to the problem exposed by OP which is “how to remove the capacity check when we know statically that there is enough space”.