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remove alignment-changing in-place collect

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Currently stable users can't benefit from this because GlobaAlloc doesn't support
alignment-changing realloc and neither do most posix allocators.
So in practice it always results in an extra memcpy.
This commit is contained in:
The 8472 2024-01-18 22:50:14 +01:00
parent b28a95391b
commit 85d1787962
1 changed files with 8 additions and 5 deletions
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13
library/alloc/src/vec/in_place_collect.rs
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@ -168,7 +168,9 @@ const fn in_place_collectible<DEST, SRC>(
step_merge: Option<NonZeroUsize>,
step_expand: Option<NonZeroUsize>,
) -> bool {
if const { SRC::IS_ZST || DEST::IS_ZST || mem::align_of::<SRC>() < mem::align_of::<DEST>() } {
// Require matching alignments because an alignment-changing realloc is inefficient on many
// system allocators and better implementations would require the unstable Allocator trait.
if const { SRC::IS_ZST || DEST::IS_ZST || mem::align_of::<SRC>() != mem::align_of::<DEST>() } {
return false;
}
@ -188,7 +190,8 @@ const fn in_place_collectible<DEST, SRC>(
const fn needs_realloc<SRC, DEST>(src_cap: usize, dst_cap: usize) -> bool {
if const { mem::align_of::<SRC>() != mem::align_of::<DEST>() } {
return src_cap > 0;
// FIXME: use unreachable! once that works in const
panic!("in_place_collectible() prevents this");
}
// If src type size is an integer multiple of the destination type size then
@ -276,8 +279,8 @@ where
let dst_guard = InPlaceDstBufDrop { ptr: dst_buf, len, cap: dst_cap };
src.forget_allocation_drop_remaining();
// Adjust the allocation if the alignment didn't match or the source had a capacity in bytes
// that wasn't a multiple of the destination type size.
// Adjust the allocation if the source had a capacity in bytes that wasn't a multiple
// of the destination type size.
// Since the discrepancy should generally be small this should only result in some
// bookkeeping updates and no memmove.
if needs_realloc::<I::Src, T>(src_cap, dst_cap) {
@ -290,7 +293,7 @@ where
let src_size = mem::size_of::<I::Src>().unchecked_mul(src_cap);
let old_layout = Layout::from_size_align_unchecked(src_size, src_align);
// The must be equal or smaller for in-place iteration to be possible
// The allocation must be equal or smaller for in-place iteration to be possible
// therefore the new layout must be ≤ the old one and therefore valid.
let dst_align = mem::align_of::<T>();
let dst_size = mem::size_of::<T>().unchecked_mul(dst_cap);