dyn_stack/stack_req.rs
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use core::num::NonZeroUsize;
/// Stack allocation requirements.
#[derive(Debug, Clone, Copy, Eq, PartialEq)]
pub struct StackReq {
align: NonZeroUsize,
size: usize,
}
impl Default for StackReq {
#[inline]
fn default() -> Self {
Self::empty()
}
}
const fn unwrap(o: Option<usize>) -> usize {
match o {
Some(x) => x,
None => panic!(),
}
}
const fn round_up_pow2(a: usize, b: usize) -> usize {
unwrap(a.checked_add(!b.wrapping_neg())) & b.wrapping_neg()
}
const fn try_round_up_pow2(a: usize, b: usize) -> Option<usize> {
match a.checked_add(!b.wrapping_neg()) {
None => None,
Some(x) => Some(x & b.wrapping_neg()),
}
}
const fn max(a: usize, b: usize) -> usize {
if a > b {
a
} else {
b
}
}
/// Size overflow error during the computation of stack allocation requirements.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub struct SizeOverflow;
impl core::fmt::Display for SizeOverflow {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> Result<(), core::fmt::Error> {
f.write_str("size computation overflowed")
}
}
impl core::error::Error for SizeOverflow {}
impl StackReq {
/// Allocation requirements for an empty unaligned buffer.
#[inline]
pub const fn empty() -> StackReq {
Self {
align: unsafe { NonZeroUsize::new_unchecked(1) },
size: 0,
}
}
/// Allocation requirements sufficient for `n` elements of type `T`, overaligned with alignment
/// `align`.
///
/// # Panics
///
/// * if `align` is smaller than the minimum required alignment for an object of type `T`.
/// * if `align` is not a power of two.
/// * if the size computation overflows
#[inline]
pub const fn new_aligned<T>(n: usize, align: usize) -> StackReq {
assert!(align >= core::mem::align_of::<T>());
assert!(align.is_power_of_two());
StackReq {
// SAFETY: because align >= alignof::<T>() > 0
align: unsafe { NonZeroUsize::new_unchecked(align) },
size: unwrap(core::mem::size_of::<T>().checked_mul(n)),
}
}
/// Allocation requirements sufficient for `n` elements of type `T`.
///
/// # Panics
///
/// * if the size computation overflows
#[inline]
pub const fn new<T>(n: usize) -> StackReq {
StackReq::new_aligned::<T>(n, core::mem::align_of::<T>())
}
/// Same as [`StackReq::new_aligned`], but returns an error in case the size computation
/// overflows.
///
/// # Panics
///
/// * if `align` is smaller than the minimum required alignment for an object of type `T`.
/// * if `align` is not a power of two.
#[inline]
pub const fn try_new_aligned<T>(n: usize, align: usize) -> Result<StackReq, SizeOverflow> {
assert!(align >= core::mem::align_of::<T>());
assert!(align.is_power_of_two());
match core::mem::size_of::<T>().checked_mul(n) {
Some(x) => Ok(StackReq {
// SAFETY: same as above
align: unsafe { NonZeroUsize::new_unchecked(align) },
size: x,
}),
None => Err(SizeOverflow),
}
}
/// Same as [`StackReq::new`], but returns an error in case the size computation
/// overflows.
#[inline]
pub const fn try_new<T>(n: usize) -> Result<StackReq, SizeOverflow> {
StackReq::try_new_aligned::<T>(n, core::mem::align_of::<T>())
}
/// The number of allocated bytes required, aligned to `self.align_bytes()`.
#[inline]
pub const fn size_bytes(&self) -> usize {
self.size
}
/// The alignment of allocated bytes required.
#[inline]
pub const fn align_bytes(&self) -> usize {
self.align.get()
}
/// The number of allocated bytes required, with no alignment constraints.
///
/// # Panics
///
/// * if the size computation overflows
#[inline]
pub const fn unaligned_bytes_required(&self) -> usize {
unwrap(self.size.checked_add(self.align.get() - 1))
}
/// Same as [`StackReq::unaligned_bytes_required`], but returns an error if the size computation
/// overflows.
#[inline]
pub const fn try_unaligned_bytes_required(&self) -> Result<usize, SizeOverflow> {
match self.size.checked_add(self.align.get() - 1) {
Some(x) => Ok(x),
None => Err(SizeOverflow),
}
}
/// The required allocation to allocate storage sufficient for both of `self` and `other`,
/// simultaneously and in any order.
///
/// # Panics
///
/// * if the allocation requirement computation overflows.
#[inline]
pub const fn and(self, other: StackReq) -> StackReq {
let align = max(self.align.get(), other.align.get());
StackReq {
// SAFETY: align is either self.align or other.align, both of which are non zero
align: unsafe { NonZeroUsize::new_unchecked(align) },
size: unwrap(
round_up_pow2(self.size, align).checked_add(round_up_pow2(other.size, align)),
),
}
}
/// The required allocation to allocate storage sufficient for all the requirements produced by
/// the given iterator, simultaneously and in any order.
///
/// # Panics
///
/// * if the allocation requirement computation overflows.
#[inline]
pub fn all_of(reqs: impl IntoIterator<Item = StackReq>) -> StackReq {
fn all_of_impl(mut reqs: impl Iterator<Item = StackReq>) -> StackReq {
let mut total = StackReq::empty();
while let Some(req) = reqs.next() {
total = total.and(req);
}
total
}
all_of_impl(reqs.into_iter())
}
/// The required allocation to allocate storage sufficient for either of `self` and `other`,
/// with only one being active at a time.
///
/// # Panics
///
/// * if the allocation requirement computation overflows.
#[inline]
pub const fn or(self, other: StackReq) -> StackReq {
let align = max(self.align.get(), other.align.get());
StackReq {
// SAFETY: same as above
align: unsafe { NonZeroUsize::new_unchecked(align) },
size: max(
round_up_pow2(self.size, align),
round_up_pow2(other.size, align),
),
}
}
/// The required allocation to allocate storage sufficient for any of the requirements produced
/// by the given iterator, with at most one being active at a time.
///
/// # Panics
///
/// * if the allocation requirement computation overflows.
#[inline]
pub fn any_of(reqs: impl IntoIterator<Item = StackReq>) -> StackReq {
fn any_of_impl(mut reqs: impl Iterator<Item = StackReq>) -> StackReq {
let mut total = StackReq::empty();
while let Some(req) = reqs.next() {
total = total.or(req);
}
total
}
any_of_impl(reqs.into_iter())
}
/// Same as [`StackReq::and`], but returns an error if the size computation overflows.
#[inline]
pub const fn try_and(self, other: StackReq) -> Result<StackReq, SizeOverflow> {
let align = max(self.align.get(), other.align.get());
Ok(StackReq {
// SAFETY: same as above
align: unsafe { NonZeroUsize::new_unchecked(align) },
size: match match try_round_up_pow2(self.size, align) {
Some(x) => x,
None => return Err(SizeOverflow),
}
.checked_add(match try_round_up_pow2(other.size, align) {
Some(x) => x,
None => return Err(SizeOverflow),
}) {
Some(x) => x,
None => return Err(SizeOverflow),
},
})
}
/// Same as [`StackReq::all_of`], but returns an error if the size computation overflows.
#[inline]
pub fn try_all_of(reqs: impl IntoIterator<Item = StackReq>) -> Result<StackReq, SizeOverflow> {
fn try_all_of_impl(
mut reqs: impl Iterator<Item = StackReq>,
) -> Result<StackReq, SizeOverflow> {
let mut total = StackReq::empty();
while let Some(req) = reqs.next() {
total = total.try_and(req)?;
}
Ok(total)
}
try_all_of_impl(reqs.into_iter())
}
/// Same as [`StackReq::or`], but returns an error if the size computation overflows.
#[inline]
pub const fn try_or(self, other: StackReq) -> Result<StackReq, SizeOverflow> {
let align = max(self.align.get(), other.align.get());
Ok(StackReq {
// SAFETY: same as above
align: unsafe { NonZeroUsize::new_unchecked(align) },
size: max(
match try_round_up_pow2(self.size, align) {
Some(x) => x,
None => return Err(SizeOverflow),
},
match try_round_up_pow2(other.size, align) {
Some(x) => x,
None => return Err(SizeOverflow),
},
),
})
}
/// Same as [`StackReq::any_of`], but returns an error if the size computation overflows.
#[inline]
pub fn try_any_of(reqs: impl IntoIterator<Item = StackReq>) -> Result<StackReq, SizeOverflow> {
fn try_any_of_impl(
mut reqs: impl Iterator<Item = StackReq>,
) -> Result<StackReq, SizeOverflow> {
let mut total = StackReq::empty();
while let Some(req) = reqs.next() {
total = total.try_or(req)?;
}
Ok(total)
}
try_any_of_impl(reqs.into_iter())
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn size_of() {
let size = core::mem::size_of::<usize>() * 2;
assert_eq!(core::mem::size_of::<StackReq>(), size);
assert_eq!(core::mem::size_of::<Option<StackReq>>(), size);
}
#[test]
fn round_up() {
assert_eq!(round_up_pow2(0, 4), 0);
assert_eq!(round_up_pow2(1, 4), 4);
assert_eq!(round_up_pow2(2, 4), 4);
assert_eq!(round_up_pow2(3, 4), 4);
assert_eq!(round_up_pow2(4, 4), 4);
}
#[test]
#[should_panic]
fn overflow() {
let _ = StackReq::new::<i32>(usize::MAX);
}
#[test]
#[should_panic]
fn and_overflow() {
let _ = StackReq::new::<u8>(usize::MAX).and(StackReq::new::<u8>(1));
}
}