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Auto merge of #100214 - scottmcm:strict-range, r=thomcc
Optimize `array::IntoIter` `.into_iter()` on arrays was slower than it needed to be (especially compared to slice iterator) since it uses `Range<usize>`, which needs to handle degenerate ranges like `10..4`. This PR adds an internal `IndexRange` type that's like `Range<usize>` but with a safety invariant that means it doesn't need to worry about those cases -- it only handles `start <= end` -- and thus can give LLVM more information to optimize better. I added one simple demonstration of the improvement as a codegen test. (`vec::IntoIter` uses pointers instead of indexes, so doesn't have this problem, but that only works because its elements are boxed. `array::IntoIter` can't use pointers because that would keep it from being movable.)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,166 @@ | ||
| use crate::intrinsics::{assert_unsafe_precondition, unchecked_add, unchecked_sub}; | ||
| use crate::iter::{FusedIterator, TrustedLen}; | ||
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| /// Like a `Range<usize>`, but with a safety invariant that `start <= end`. | ||
| /// | ||
| /// This means that `end - start` cannot overflow, allowing some μoptimizations. | ||
| /// | ||
| /// (Normal `Range` code needs to handle degenerate ranges like `10..0`, | ||
| /// which takes extra checks compared to only handling the canonical form.) | ||
| #[derive(Clone, Debug, PartialEq, Eq)] | ||
| pub(crate) struct IndexRange { | ||
| start: usize, | ||
| end: usize, | ||
| } | ||
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|
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| impl IndexRange { | ||
| /// # Safety | ||
| /// - `start <= end` | ||
| #[inline] | ||
| pub const unsafe fn new_unchecked(start: usize, end: usize) -> Self { | ||
| // SAFETY: comparisons on usize are pure | ||
| unsafe { assert_unsafe_precondition!((start: usize, end: usize) => start <= end) }; | ||
| IndexRange { start, end } | ||
| } | ||
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| #[inline] | ||
| pub const fn zero_to(end: usize) -> Self { | ||
| IndexRange { start: 0, end } | ||
| } | ||
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| #[inline] | ||
| pub const fn start(&self) -> usize { | ||
| self.start | ||
| } | ||
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| #[inline] | ||
| pub const fn end(&self) -> usize { | ||
| self.end | ||
| } | ||
|
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| #[inline] | ||
| pub const fn len(&self) -> usize { | ||
| // SAFETY: By invariant, this cannot wrap | ||
| unsafe { unchecked_sub(self.end, self.start) } | ||
| } | ||
|
|
||
| /// # Safety | ||
| /// - Can only be called when `start < end`, aka when `len > 0`. | ||
| #[inline] | ||
| unsafe fn next_unchecked(&mut self) -> usize { | ||
| debug_assert!(self.start < self.end); | ||
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| let value = self.start; | ||
| // SAFETY: The range isn't empty, so this cannot overflow | ||
| self.start = unsafe { unchecked_add(value, 1) }; | ||
| value | ||
| } | ||
|
|
||
| /// # Safety | ||
| /// - Can only be called when `start < end`, aka when `len > 0`. | ||
| #[inline] | ||
| unsafe fn next_back_unchecked(&mut self) -> usize { | ||
| debug_assert!(self.start < self.end); | ||
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| // SAFETY: The range isn't empty, so this cannot overflow | ||
| let value = unsafe { unchecked_sub(self.end, 1) }; | ||
| self.end = value; | ||
| value | ||
| } | ||
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| /// Removes the first `n` items from this range, returning them as an `IndexRange`. | ||
| /// If there are fewer than `n`, then the whole range is returned and | ||
| /// `self` is left empty. | ||
| /// | ||
| /// This is designed to help implement `Iterator::advance_by`. | ||
| #[inline] | ||
| pub fn take_prefix(&mut self, n: usize) -> Self { | ||
| let mid = if n <= self.len() { | ||
| // SAFETY: We just checked that this will be between start and end, | ||
| // and thus the addition cannot overflow. | ||
| unsafe { unchecked_add(self.start, n) } | ||
| } else { | ||
| self.end | ||
| }; | ||
| let prefix = Self { start: self.start, end: mid }; | ||
| self.start = mid; | ||
| prefix | ||
| } | ||
|
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||
| /// Removes the last `n` items from this range, returning them as an `IndexRange`. | ||
| /// If there are fewer than `n`, then the whole range is returned and | ||
| /// `self` is left empty. | ||
| /// | ||
| /// This is designed to help implement `Iterator::advance_back_by`. | ||
| #[inline] | ||
| pub fn take_suffix(&mut self, n: usize) -> Self { | ||
| let mid = if n <= self.len() { | ||
| // SAFETY: We just checked that this will be between start and end, | ||
| // and thus the addition cannot overflow. | ||
| unsafe { unchecked_sub(self.end, n) } | ||
| } else { | ||
| self.start | ||
| }; | ||
| let suffix = Self { start: mid, end: self.end }; | ||
| self.end = mid; | ||
| suffix | ||
| } | ||
| } | ||
|
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| impl Iterator for IndexRange { | ||
| type Item = usize; | ||
|
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| #[inline] | ||
| fn next(&mut self) -> Option<usize> { | ||
| if self.len() > 0 { | ||
| // SAFETY: We just checked that the range is non-empty | ||
| unsafe { Some(self.next_unchecked()) } | ||
| } else { | ||
| None | ||
| } | ||
| } | ||
|
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||
| #[inline] | ||
| fn size_hint(&self) -> (usize, Option<usize>) { | ||
| let len = self.len(); | ||
| (len, Some(len)) | ||
| } | ||
|
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||
| #[inline] | ||
| fn advance_by(&mut self, n: usize) -> Result<(), usize> { | ||
| let original_len = self.len(); | ||
| self.take_prefix(n); | ||
| if n > original_len { Err(original_len) } else { Ok(()) } | ||
| } | ||
| } | ||
|
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| impl DoubleEndedIterator for IndexRange { | ||
| #[inline] | ||
| fn next_back(&mut self) -> Option<usize> { | ||
| if self.len() > 0 { | ||
| // SAFETY: We just checked that the range is non-empty | ||
| unsafe { Some(self.next_back_unchecked()) } | ||
| } else { | ||
| None | ||
| } | ||
| } | ||
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| #[inline] | ||
| fn advance_back_by(&mut self, n: usize) -> Result<(), usize> { | ||
| let original_len = self.len(); | ||
| self.take_suffix(n); | ||
| if n > original_len { Err(original_len) } else { Ok(()) } | ||
| } | ||
| } | ||
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| impl ExactSizeIterator for IndexRange { | ||
| #[inline] | ||
| fn len(&self) -> usize { | ||
| self.len() | ||
| } | ||
| } | ||
|
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| // SAFETY: Because we only deal in `usize`, our `len` is always perfect. | ||
| unsafe impl TrustedLen for IndexRange {} | ||
|
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| impl FusedIterator for IndexRange {} |
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