diff options
Diffstat (limited to 'library/core')
| -rw-r--r-- | library/core/src/alloc/layout.rs | 76 | ||||
| -rw-r--r-- | library/core/src/mem/valid_align.rs | 11 |
2 files changed, 38 insertions, 49 deletions
diff --git a/library/core/src/alloc/layout.rs b/library/core/src/alloc/layout.rs index 59ebe5fbe02..2f378836cbb 100644 --- a/library/core/src/alloc/layout.rs +++ b/library/core/src/alloc/layout.rs @@ -1,9 +1,3 @@ -// Seemingly inconsequential code changes to this file can lead to measurable -// performance impact on compilation times, due at least in part to the fact -// that the layout code gets called from many instantiations of the various -// collections, resulting in having to optimize down excess IR multiple times. -// Your performance intuition is useless. Run perf. - use crate::cmp; use crate::fmt; use crate::mem::{self, ValidAlign}; @@ -58,8 +52,8 @@ impl Layout { /// * `align` must be a power of two, /// /// * `size`, when rounded up to the nearest multiple of `align`, - /// must not overflow isize (i.e., the rounded value must be - /// less than or equal to `isize::MAX`). + /// must not overflow (i.e., the rounded value must be less than + /// or equal to `usize::MAX`). #[stable(feature = "alloc_layout", since = "1.28.0")] #[rustc_const_stable(feature = "const_alloc_layout_size_align", since = "1.50.0")] #[inline] @@ -68,13 +62,6 @@ impl Layout { return Err(LayoutError); } - // SAFETY: just checked that align is a power of two. - Layout::from_size_valid_align(size, unsafe { ValidAlign::new_unchecked(align) }) - } - - /// Internal helper constructor to skip revalidating alignment validity. - #[inline] - const fn from_size_valid_align(size: usize, align: ValidAlign) -> Result<Self, LayoutError> { // (power-of-two implies align != 0.) // Rounded up size is: @@ -89,12 +76,13 @@ impl Layout { // // Above implies that checking for summation overflow is both // necessary and sufficient. - if size > isize::MAX as usize - (align.as_nonzero().get() - 1) { + if size > usize::MAX - (align - 1) { return Err(LayoutError); } - // SAFETY: Layout::size invariants checked above. - Ok(Layout { size, align }) + // SAFETY: the conditions for `from_size_align_unchecked` have been + // checked above. + unsafe { Ok(Layout::from_size_align_unchecked(size, align)) } } /// Creates a layout, bypassing all checks. @@ -108,8 +96,8 @@ impl Layout { #[must_use] #[inline] pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self { - // SAFETY: the caller is required to uphold the preconditions. - unsafe { Layout { size, align: ValidAlign::new_unchecked(align) } } + // SAFETY: the caller must ensure that `align` is a power of two. + Layout { size, align: unsafe { ValidAlign::new_unchecked(align) } } } /// The minimum size in bytes for a memory block of this layout. @@ -138,9 +126,10 @@ impl Layout { #[inline] pub const fn new<T>() -> Self { let (size, align) = size_align::<T>(); - // SAFETY: if the type is instantiated, rustc already ensures that its - // layout is valid. Use the unchecked constructor to avoid inserting a - // panicking codepath that needs to be optimized out. + // SAFETY: the align is guaranteed by Rust to be a power of two and + // the size+align combo is guaranteed to fit in our address space. As a + // result use the unchecked constructor here to avoid inserting code + // that panics if it isn't optimized well enough. unsafe { Layout::from_size_align_unchecked(size, align) } } @@ -152,6 +141,7 @@ impl Layout { #[inline] pub fn for_value<T: ?Sized>(t: &T) -> Self { let (size, align) = (mem::size_of_val(t), mem::align_of_val(t)); + debug_assert!(Layout::from_size_align(size, align).is_ok()); // SAFETY: see rationale in `new` for why this is using the unsafe variant unsafe { Layout::from_size_align_unchecked(size, align) } } @@ -186,6 +176,7 @@ impl Layout { pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self { // SAFETY: we pass along the prerequisites of these functions to the caller let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) }; + debug_assert!(Layout::from_size_align(size, align).is_ok()); // SAFETY: see rationale in `new` for why this is using the unsafe variant unsafe { Layout::from_size_align_unchecked(size, align) } } @@ -285,11 +276,12 @@ impl Layout { let pad = self.padding_needed_for(self.align()); // This cannot overflow. Quoting from the invariant of Layout: // > `size`, when rounded up to the nearest multiple of `align`, - // > must not overflow isize (i.e., the rounded value must be - // > less than or equal to `isize::MAX`) + // > must not overflow (i.e., the rounded value must be less than + // > `usize::MAX`) let new_size = self.size() + pad; - // SAFETY: padded size is guaranteed to not exceed `isize::MAX`. + // SAFETY: self.align is already known to be valid and new_size has been + // padded already. unsafe { Layout::from_size_align_unchecked(new_size, self.align()) } } @@ -306,13 +298,14 @@ impl Layout { pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> { // This cannot overflow. Quoting from the invariant of Layout: // > `size`, when rounded up to the nearest multiple of `align`, - // > must not overflow isize (i.e., the rounded value must be - // > less than or equal to `isize::MAX`) + // > must not overflow (i.e., the rounded value must be less than + // > `usize::MAX`) let padded_size = self.size() + self.padding_needed_for(self.align()); let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError)?; - // The safe constructor is called here to enforce the isize size limit. - Layout::from_size_valid_align(alloc_size, self.align).map(|layout| (layout, padded_size)) + // SAFETY: self.align is already known to be valid and alloc_size has been + // padded already. + unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) } } /// Creates a layout describing the record for `self` followed by @@ -363,14 +356,13 @@ impl Layout { #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")] #[inline] pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> { - let new_align = cmp::max(self.align, next.align); + let new_align = cmp::max(self.align(), next.align()); let pad = self.padding_needed_for(next.align()); let offset = self.size().checked_add(pad).ok_or(LayoutError)?; let new_size = offset.checked_add(next.size()).ok_or(LayoutError)?; - // The safe constructor is called here to enforce the isize size limit. - let layout = Layout::from_size_valid_align(new_size, new_align)?; + let layout = Layout::from_size_align(new_size, new_align)?; Ok((layout, offset)) } @@ -390,8 +382,7 @@ impl Layout { #[inline] pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> { let size = self.size().checked_mul(n).ok_or(LayoutError)?; - // The safe constructor is called here to enforce the isize size limit. - Layout::from_size_valid_align(size, self.align) + Layout::from_size_align(size, self.align()) } /// Creates a layout describing the record for `self` followed by @@ -404,8 +395,7 @@ impl Layout { #[inline] pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> { let new_size = self.size().checked_add(next.size()).ok_or(LayoutError)?; - // The safe constructor is called here to enforce the isize size limit. - Layout::from_size_valid_align(new_size, self.align) + Layout::from_size_align(new_size, self.align()) } /// Creates a layout describing the record for a `[T; n]`. @@ -415,8 +405,16 @@ impl Layout { #[inline] pub fn array<T>(n: usize) -> Result<Self, LayoutError> { let array_size = mem::size_of::<T>().checked_mul(n).ok_or(LayoutError)?; - // The safe constructor is called here to enforce the isize size limit. - Layout::from_size_valid_align(array_size, ValidAlign::of::<T>()) + + // SAFETY: + // - Size: `array_size` cannot be too big because `size_of::<T>()` must + // be a multiple of `align_of::<T>()`. Therefore, `array_size` + // rounded up to the nearest multiple of `align_of::<T>()` is just + // `array_size`. And `array_size` cannot be too big because it was + // just checked by the `checked_mul()`. + // - Alignment: `align_of::<T>()` will always give an acceptable + // (non-zero, power of two) alignment. + Ok(unsafe { Layout::from_size_align_unchecked(array_size, mem::align_of::<T>()) }) } } diff --git a/library/core/src/mem/valid_align.rs b/library/core/src/mem/valid_align.rs index 4ce6d13cf90..fcfa95120df 100644 --- a/library/core/src/mem/valid_align.rs +++ b/library/core/src/mem/valid_align.rs @@ -1,5 +1,4 @@ use crate::convert::TryFrom; -use crate::intrinsics::assert_unsafe_precondition; use crate::num::NonZeroUsize; use crate::{cmp, fmt, hash, mem, num}; @@ -27,8 +26,7 @@ impl ValidAlign { /// It must *not* be zero. #[inline] pub(crate) const unsafe fn new_unchecked(align: usize) -> Self { - // SAFETY: Precondition passed to the caller. - unsafe { assert_unsafe_precondition!(align.is_power_of_two()) }; + debug_assert!(align.is_power_of_two()); // SAFETY: By precondition, this must be a power of two, and // our variants encompass all possible powers of two. @@ -48,13 +46,6 @@ impl ValidAlign { pub(crate) fn log2(self) -> u32 { self.as_nonzero().trailing_zeros() } - - /// Returns the alignment for a type. - #[inline] - pub(crate) fn of<T>() -> Self { - // SAFETY: rustc ensures that type alignment is always a power of two. - unsafe { ValidAlign::new_unchecked(mem::align_of::<T>()) } - } } impl fmt::Debug for ValidAlign { |