1 files changed, 37 insertions, 39 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>()) })
     }
 }