From f3bda74d363a060ade5e5caeb654ba59bfed51a4 Mon Sep 17 00:00:00 2001 From: Nicholas Nethercote Date: Fri, 26 Nov 2021 13:39:05 +1100 Subject: [PATCH] Optimize `Layout::array`. The current implementation is much more conservative than it needs to be, because it's dealing with the size and alignment of a given `T`, which are more restricted than an arbitrary `Layout`. For example, imagine a struct with a `u32` and a `u4`. You can safely create a `Layout { size_: 5, align_: 4 }` by hand, but `Layout::new::` will give `Layout { size_: 8, align_: 4}`, where the size already has padding that accounts for the alignment. (And the existing `debug_assert_eq!` in `Layout::array` already demonstrates that no additional padding is required.) --- library/core/src/alloc/layout.rs | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/library/core/src/alloc/layout.rs b/library/core/src/alloc/layout.rs index a9abb8d2d59..9df0b5c5519 100644 --- a/library/core/src/alloc/layout.rs +++ b/library/core/src/alloc/layout.rs @@ -405,9 +405,17 @@ impl Layout { #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")] #[inline] pub fn array(n: usize) -> Result { - let (layout, offset) = Layout::new::().repeat(n)?; - debug_assert_eq!(offset, mem::size_of::()); - Ok(layout.pad_to_align()) + let array_size = mem::size_of::().checked_mul(n).ok_or(LayoutError)?; + + // SAFETY: + // - Size: `array_size` cannot be too big because `size_of::()` must + // be a multiple of `align_of::()`. Therefore, `array_size` + // rounded up to the nearest multiple of `align_of::()` is just + // `array_size`. And `array_size` cannot be too big because it was + // just checked by the `checked_mul()`. + // - Alignment: `align_of::()` will always give an acceptable + // (non-zero, power of two) alignment. + Ok(unsafe { Layout::from_size_align_unchecked(array_size, mem::align_of::()) }) } }