Guillaume Gomez b917753d79 Rollup merge of #120116 - the8472:only-same-alignments, r=cuviper ... Remove alignment-changing in-place collect This removes the alignment-changing in-place collect optimization introduced in #110353 Currently stable users can't benefit from the optimization because GlobaAlloc doesn't support alignment-changing realloc and neither do most posix allocators. So in practice it has a negative impact on performance. Explanation from https://github.com/rust-lang/rust/issues/120091#issuecomment-1899071681: > > You mention that in case of alignment mismatch -- when the new alignment is less than the old -- the implementation calls `mremap`. > > I was trying to note that this isn't really the case in practice, due to the semantics of Rust's allocator APIs. The only use of the allocator within the `in_place_collect` implementation itself is [a call to `Allocator::shrink()`](db7125f008/library/alloc/src/vec/in_place_collect.rs (L299-L303)), which per its documentation [allows decreasing the required alignment](https://doc.rust-lang.org/1.75.0/core/alloc/trait.Allocator.html). However, in stable Rust, the only available `Allocator` is [`Global`](https://doc.rust-lang.org/1.75.0/alloc/alloc/struct.Global.html), which delegates to the registered `GlobalAlloc`. Since `GlobalAlloc::realloc()` [cannot change the required alignment](https://doc.rust-lang.org/1.75.0/core/alloc/trait.GlobalAlloc.html#method.realloc), the implementation of [`<Global as Allocator>::shrink()`](db7125f008/library/alloc/src/alloc.rs (L280-L321)) must fall back to creating a brand-new allocation, `memcpy`ing the data into it, and freeing the old allocation, whenever the alignment doesn't remain exactly the same. > > Therefore, the underlying allocator, provided by libc or some other source, has no opportunity to internally `mremap()` the data when the alignment is changed, since it has no way of knowing that the allocation is the same.		2024-01-20 20:06:35 +01:00
..
benches	remove redundant imports	2023-12-10 10:56:22 +08:00
src	Rollup merge of #120116 - the8472:only-same-alignments, r=cuviper	2024-01-20 20:06:35 +01:00
tests	apply fmt	2024-01-11 15:04:48 +03:00
Cargo.toml	Rename {collections=>alloc}{tests,benches}	2023-11-26 12:04:56 +00:00