40cf72108e
Change slice::to_vec to not use extend_from_slice I saw this [Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/String.3A.3Afrom%28.26str%29.20wonky.20codegen/near/216164455), and didn't see any update from it, so I thought I'd try to fix it. This converts `to_vec` to no longer use `extend_from_slice`, but relies on knowing that the allocated capacity is the same size as the input. [Godbolt new v1](https://rust.godbolt.org/z/1bcWKG) [Godbolt new v2 w/ drop guard](https://rust.godbolt.org/z/5jn76K) [Godbolt old version](https://rust.godbolt.org/z/e4ePav) After some amount of iteration, there are now two specializations for `to_vec`, one for `Copy` types that use memcpy, and one for clone types which is the original from this PR. This is then used inside of `impl<T: Clone> FromIterator<Iter::Slice<T>> for Vec<T>` which is essentially equivalent to `&[T] -> Vec<T>`, instead of previous specialization of the `extend` function. This is because extend has to reason more about existing capacity by calling `reserve` on an existing vec, and thus produces worse asm. Downsides: This allocates the exact capacity, so I think if many items are added to this `Vec` after, it might need to allocate whereas extending may not. I also noticed the number of faults went up in the benchmarks, but not sure where from exactly. |
||
|---|---|---|
| .. | ||
| alloc | ||
| backtrace@af078ecc0b | ||
| core | ||
| panic_abort | ||
| panic_unwind | ||
| proc_macro | ||
| profiler_builtins | ||
| rtstartup | ||
| rustc-std-workspace-alloc | ||
| rustc-std-workspace-core | ||
| rustc-std-workspace-std | ||
| std | ||
| stdarch@777efaf564 | ||
| term | ||
| test | ||
| unwind | ||