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Use align_offset to check alignment (#176)

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Co-authored-by: sarah <>
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sarah 2023-09-05 22:04:52 +02:00 committed by GitHub
parent b38d7d0e21
commit caff759066
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 19 additions and 6 deletions
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25
src/internal.rs
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@ -130,6 +130,19 @@ pub(crate) unsafe fn pod_read_unaligned<T: Copy>(bytes: &[u8]) -> T {
}
}
/// Checks if `ptr` is aligned to an `align` memory boundary.
///
/// ## Panics
/// * If `align` is not a power of two. This includes when `align` is zero.
#[inline]
pub(crate) fn is_aligned_to(ptr: *const (), align: usize) -> bool {
// This is in a way better than `ptr as usize % align == 0`,
// because casting a pointer to an integer has the side effect that it exposes
// the pointer's provenance, which may theoretically inhibit some compiler
// optimizations.
ptr.align_offset(align) == 0
}
/// Re-interprets `&[u8]` as `&T`.
///
/// ## Failure
@ -142,7 +155,7 @@ pub(crate) unsafe fn try_from_bytes<T: Copy>(
) -> Result<&T, PodCastError> {
if s.len() != size_of::<T>() {
Err(PodCastError::SizeMismatch)
} else if (s.as_ptr() as usize) % align_of::<T>() != 0 {
} else if !is_aligned_to(s.as_ptr() as *const (), align_of::<T>()) {
Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
} else {
Ok(unsafe { &*(s.as_ptr() as *const T) })
@ -161,7 +174,7 @@ pub(crate) unsafe fn try_from_bytes_mut<T: Copy>(
) -> Result<&mut T, PodCastError> {
if s.len() != size_of::<T>() {
Err(PodCastError::SizeMismatch)
} else if (s.as_ptr() as usize) % align_of::<T>() != 0 {
} else if !is_aligned_to(s.as_ptr() as *const (), align_of::<T>()) {
Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
} else {
Ok(unsafe { &mut *(s.as_mut_ptr() as *mut T) })
@ -284,7 +297,7 @@ pub(crate) unsafe fn try_cast_ref<A: Copy, B: Copy>(
// Note(Lokathor): everything with `align_of` and `size_of` will optimize away
// after monomorphization.
if align_of::<B>() > align_of::<A>()
&& (a as *const A as usize) % align_of::<B>() != 0
&& !is_aligned_to(a as *const A as *const (), align_of::<B>())
{
Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
} else if size_of::<B>() == size_of::<A>() {
@ -304,7 +317,7 @@ pub(crate) unsafe fn try_cast_mut<A: Copy, B: Copy>(
// Note(Lokathor): everything with `align_of` and `size_of` will optimize away
// after monomorphization.
if align_of::<B>() > align_of::<A>()
&& (a as *mut A as usize) % align_of::<B>() != 0
&& !is_aligned_to(a as *const A as *const (), align_of::<B>())
{
Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
} else if size_of::<B>() == size_of::<A>() {
@ -336,7 +349,7 @@ pub(crate) unsafe fn try_cast_slice<A: Copy, B: Copy>(
// Note(Lokathor): everything with `align_of` and `size_of` will optimize away
// after monomorphization.
if align_of::<B>() > align_of::<A>()
&& (a.as_ptr() as usize) % align_of::<B>() != 0
&& !is_aligned_to(a.as_ptr() as *const (), align_of::<B>())
{
Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
} else if size_of::<B>() == size_of::<A>() {
@ -362,7 +375,7 @@ pub(crate) unsafe fn try_cast_slice_mut<A: Copy, B: Copy>(
// Note(Lokathor): everything with `align_of` and `size_of` will optimize away
// after monomorphization.
if align_of::<B>() > align_of::<A>()
&& (a.as_mut_ptr() as usize) % align_of::<B>() != 0
&& !is_aligned_to(a.as_ptr() as *const (), align_of::<B>())
{
Err(PodCastError::TargetAlignmentGreaterAndInputNotAligned)
} else if size_of::<B>() == size_of::<A>() {