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Auto merge of #111850 - the8472:external-step-by, r=scottmcm
Specialize `StepBy<Range<{integer}>>`
OLD
iter::bench_range_step_by_fold_u16 700.00ns/iter +/- 10.00ns
iter::bench_range_step_by_fold_usize 519.00ns/iter +/- 6.00ns
iter::bench_range_step_by_loop_u32 555.00ns/iter +/- 7.00ns
iter::bench_range_step_by_sum_reducible 37.00ns/iter +/- 0.00ns
NEW
iter::bench_range_step_by_fold_u16 49.00ns/iter +/- 0.00ns
iter::bench_range_step_by_fold_usize 194.00ns/iter +/- 1.00ns
iter::bench_range_step_by_loop_u32 98.00ns/iter +/- 0.00ns
iter::bench_range_step_by_sum_reducible 1.00ns/iter +/- 0.00ns
NEW + `-Ctarget-cpu=x86-64-v3`
iter::bench_range_step_by_fold_u16 22.00ns/iter +/- 0.00ns
iter::bench_range_step_by_fold_usize 80.00ns/iter +/- 1.00ns
iter::bench_range_step_by_loop_u32 41.00ns/iter +/- 0.00ns
iter::bench_range_step_by_sum_reducible 1.00ns/iter +/- 0.00ns
I have only optimized for walltime of those methods, I haven't tested whether it eliminates bounds checks when indexing into slices via things like `(0..slice.len()).step_by(16)`.
This commit is contained in:
bors
commit
ae8ffa663c
@ -2,6 +2,7 @@ use core::borrow::Borrow;
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use core::iter::*;
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use core::mem;
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use core::num::Wrapping;
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use core::ops::Range;
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use test::{black_box, Bencher};
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#[bench]
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@ -69,6 +70,57 @@ fn bench_max(b: &mut Bencher) {
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})
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}
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#[bench]
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fn bench_range_step_by_sum_reducible(b: &mut Bencher) {
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let r = 0u32..1024;
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b.iter(|| {
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let r = black_box(r.clone()).step_by(8);
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let mut sum: u32 = 0;
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for i in r {
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sum += i;
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}
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sum
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})
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}
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#[bench]
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fn bench_range_step_by_loop_u32(b: &mut Bencher) {
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let r = 0..(u16::MAX as u32);
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b.iter(|| {
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let r = black_box(r.clone()).step_by(64);
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let mut sum: u32 = 0;
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for i in r {
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let i = i ^ i.wrapping_sub(1);
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sum = sum.wrapping_add(i);
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}
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sum
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})
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}
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#[bench]
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fn bench_range_step_by_fold_usize(b: &mut Bencher) {
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let r: Range<usize> = 0..(u16::MAX as usize);
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b.iter(|| {
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let r = black_box(r.clone());
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r.step_by(64)
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.map(|x: usize| x ^ (x.wrapping_sub(1)))
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.fold(0usize, |acc, i| acc.wrapping_add(i))
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})
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}
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#[bench]
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fn bench_range_step_by_fold_u16(b: &mut Bencher) {
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let r: Range<u16> = 0..u16::MAX;
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b.iter(|| {
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let r = black_box(r.clone());
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r.step_by(64).map(|x: u16| x ^ (x.wrapping_sub(1))).fold(0u16, |acc, i| acc.wrapping_add(i))
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})
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}
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pub fn copy_zip(xs: &[u8], ys: &mut [u8]) {
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for (a, b) in ys.iter_mut().zip(xs) {
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*a = *b;
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@ -1,4 +1,9 @@
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use crate::{intrinsics, iter::from_fn, ops::Try};
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use crate::convert::TryFrom;
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use crate::{
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intrinsics,
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iter::{from_fn, TrustedLen},
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ops::{Range, Try},
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};
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/// An iterator for stepping iterators by a custom amount.
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///
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@ -11,14 +16,22 @@ use crate::{intrinsics, iter::from_fn, ops::Try};
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#[stable(feature = "iterator_step_by", since = "1.28.0")]
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#[derive(Clone, Debug)]
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pub struct StepBy<I> {
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/// This field is guaranteed to be preprocessed by the specialized `SpecRangeSetup::setup`
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/// in the constructor.
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/// For most iterators that processing is a no-op, but for Range<{integer}> types it is lossy
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/// which means the inner iterator cannot be returned to user code.
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/// Additionally this type-dependent preprocessing means specialized implementations
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/// cannot be used interchangeably.
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iter: I,
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step: usize,
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first_take: bool,
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}
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impl<I> StepBy<I> {
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#[inline]
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pub(in crate::iter) fn new(iter: I, step: usize) -> StepBy<I> {
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assert!(step != 0);
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let iter = <I as SpecRangeSetup<I>>::setup(iter, step);
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StepBy { iter, step: step - 1, first_take: true }
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}
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}
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@ -32,16 +45,174 @@ where
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#[inline]
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fn next(&mut self) -> Option<Self::Item> {
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if self.first_take {
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self.first_take = false;
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self.iter.next()
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} else {
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self.iter.nth(self.step)
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}
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self.spec_next()
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}
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#[inline]
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fn size_hint(&self) -> (usize, Option<usize>) {
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self.spec_size_hint()
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}
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#[inline]
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fn nth(&mut self, n: usize) -> Option<Self::Item> {
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self.spec_nth(n)
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}
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fn try_fold<Acc, F, R>(&mut self, acc: Acc, f: F) -> R
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where
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F: FnMut(Acc, Self::Item) -> R,
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R: Try<Output = Acc>,
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{
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self.spec_try_fold(acc, f)
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}
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#[inline]
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fn fold<Acc, F>(self, acc: Acc, f: F) -> Acc
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where
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F: FnMut(Acc, Self::Item) -> Acc,
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{
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self.spec_fold(acc, f)
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}
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}
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impl<I> StepBy<I>
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where
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I: ExactSizeIterator,
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{
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// The zero-based index starting from the end of the iterator of the
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// last element. Used in the `DoubleEndedIterator` implementation.
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fn next_back_index(&self) -> usize {
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let rem = self.iter.len() % (self.step + 1);
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if self.first_take {
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if rem == 0 { self.step } else { rem - 1 }
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} else {
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rem
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}
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}
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}
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#[stable(feature = "double_ended_step_by_iterator", since = "1.38.0")]
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impl<I> DoubleEndedIterator for StepBy<I>
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where
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I: DoubleEndedIterator + ExactSizeIterator,
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{
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#[inline]
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fn next_back(&mut self) -> Option<Self::Item> {
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self.spec_next_back()
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}
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#[inline]
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fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
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self.spec_nth_back(n)
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}
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fn try_rfold<Acc, F, R>(&mut self, init: Acc, f: F) -> R
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where
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F: FnMut(Acc, Self::Item) -> R,
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R: Try<Output = Acc>,
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{
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self.spec_try_rfold(init, f)
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}
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#[inline]
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fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
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where
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Self: Sized,
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F: FnMut(Acc, Self::Item) -> Acc,
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{
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self.spec_rfold(init, f)
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}
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}
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// StepBy can only make the iterator shorter, so the len will still fit.
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#[stable(feature = "iterator_step_by", since = "1.28.0")]
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impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}
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trait SpecRangeSetup<T> {
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fn setup(inner: T, step: usize) -> T;
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}
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impl<T> SpecRangeSetup<T> for T {
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#[inline]
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default fn setup(inner: T, _step: usize) -> T {
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inner
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}
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}
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/// Specialization trait to optimize `StepBy<Range<{integer}>>` iteration.
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///
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/// # Safety
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///
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/// Technically this is safe to implement (look ma, no unsafe!), but in reality
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/// a lot of unsafe code relies on ranges over integers being correct.
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///
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/// For correctness *all* public StepBy methods must be specialized
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/// because `setup` drastically alters the meaning of the struct fields so that mixing
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/// different implementations would lead to incorrect results.
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unsafe trait StepByImpl<I> {
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type Item;
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fn spec_next(&mut self) -> Option<Self::Item>;
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fn spec_size_hint(&self) -> (usize, Option<usize>);
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fn spec_nth(&mut self, n: usize) -> Option<Self::Item>;
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fn spec_try_fold<Acc, F, R>(&mut self, acc: Acc, f: F) -> R
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where
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F: FnMut(Acc, Self::Item) -> R,
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R: Try<Output = Acc>;
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fn spec_fold<Acc, F>(self, acc: Acc, f: F) -> Acc
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where
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F: FnMut(Acc, Self::Item) -> Acc;
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}
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/// Specialization trait for double-ended iteration.
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///
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/// See also: `StepByImpl`
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///
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/// # Safety
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///
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/// The specializations must be implemented together with `StepByImpl`
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/// where applicable. I.e. if `StepBy` does support backwards iteration
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/// for a given iterator and that is specialized for forward iteration then
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/// it must also be specialized for backwards iteration.
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unsafe trait StepByBackImpl<I> {
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type Item;
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fn spec_next_back(&mut self) -> Option<Self::Item>
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where
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I: DoubleEndedIterator + ExactSizeIterator;
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fn spec_nth_back(&mut self, n: usize) -> Option<Self::Item>
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where
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I: DoubleEndedIterator + ExactSizeIterator;
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fn spec_try_rfold<Acc, F, R>(&mut self, init: Acc, f: F) -> R
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where
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I: DoubleEndedIterator + ExactSizeIterator,
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F: FnMut(Acc, Self::Item) -> R,
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R: Try<Output = Acc>;
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fn spec_rfold<Acc, F>(self, init: Acc, f: F) -> Acc
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where
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I: DoubleEndedIterator + ExactSizeIterator,
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F: FnMut(Acc, Self::Item) -> Acc;
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}
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unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
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type Item = I::Item;
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#[inline]
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default fn spec_next(&mut self) -> Option<I::Item> {
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let step_size = if self.first_take { 0 } else { self.step };
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self.first_take = false;
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self.iter.nth(step_size)
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}
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#[inline]
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default fn spec_size_hint(&self) -> (usize, Option<usize>) {
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#[inline]
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fn first_size(step: usize) -> impl Fn(usize) -> usize {
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move |n| if n == 0 { 0 } else { 1 + (n - 1) / (step + 1) }
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@ -64,7 +235,7 @@ where
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}
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#[inline]
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fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
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default fn spec_nth(&mut self, mut n: usize) -> Option<I::Item> {
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if self.first_take {
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self.first_take = false;
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let first = self.iter.next();
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@ -108,7 +279,7 @@ where
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}
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}
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fn try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
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default fn spec_try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
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where
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F: FnMut(Acc, Self::Item) -> R,
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R: Try<Output = Acc>,
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@ -128,7 +299,7 @@ where
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from_fn(nth(&mut self.iter, self.step)).try_fold(acc, f)
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}
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fn fold<Acc, F>(mut self, mut acc: Acc, mut f: F) -> Acc
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default fn spec_fold<Acc, F>(mut self, mut acc: Acc, mut f: F) -> Acc
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where
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F: FnMut(Acc, Self::Item) -> Acc,
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{
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@ -148,34 +319,16 @@ where
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}
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}
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impl<I> StepBy<I>
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where
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I: ExactSizeIterator,
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{
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// The zero-based index starting from the end of the iterator of the
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// last element. Used in the `DoubleEndedIterator` implementation.
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fn next_back_index(&self) -> usize {
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let rem = self.iter.len() % (self.step + 1);
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if self.first_take {
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if rem == 0 { self.step } else { rem - 1 }
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} else {
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rem
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}
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}
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}
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unsafe impl<I: DoubleEndedIterator + ExactSizeIterator> StepByBackImpl<I> for StepBy<I> {
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type Item = I::Item;
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#[stable(feature = "double_ended_step_by_iterator", since = "1.38.0")]
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impl<I> DoubleEndedIterator for StepBy<I>
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where
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I: DoubleEndedIterator + ExactSizeIterator,
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{
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#[inline]
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fn next_back(&mut self) -> Option<Self::Item> {
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default fn spec_next_back(&mut self) -> Option<Self::Item> {
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self.iter.nth_back(self.next_back_index())
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}
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#[inline]
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fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
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default fn spec_nth_back(&mut self, n: usize) -> Option<I::Item> {
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// `self.iter.nth_back(usize::MAX)` does the right thing here when `n`
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// is out of bounds because the length of `self.iter` does not exceed
|
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// `usize::MAX` (because `I: ExactSizeIterator`) and `nth_back` is
|
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@ -184,7 +337,7 @@ where
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self.iter.nth_back(n)
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}
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|
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fn try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
|
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default fn spec_try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
|
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where
|
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F: FnMut(Acc, Self::Item) -> R,
|
||||
R: Try<Output = Acc>,
|
||||
@ -207,10 +360,10 @@ where
|
||||
}
|
||||
|
||||
#[inline]
|
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fn rfold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
|
||||
default fn spec_rfold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
|
||||
where
|
||||
Self: Sized,
|
||||
F: FnMut(Acc, Self::Item) -> Acc,
|
||||
F: FnMut(Acc, I::Item) -> Acc,
|
||||
{
|
||||
#[inline]
|
||||
fn nth_back<I: DoubleEndedIterator>(
|
||||
@ -230,6 +383,192 @@ where
|
||||
}
|
||||
}
|
||||
|
||||
// StepBy can only make the iterator shorter, so the len will still fit.
|
||||
#[stable(feature = "iterator_step_by", since = "1.28.0")]
|
||||
impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}
|
||||
/// For these implementations, `SpecRangeSetup` calculates the number
|
||||
/// of iterations that will be needed and stores that in `iter.end`.
|
||||
///
|
||||
/// The various iterator implementations then rely on that to not need
|
||||
/// overflow checking, letting loops just be counted instead.
|
||||
///
|
||||
/// These only work for unsigned types, and will need to be reworked
|
||||
/// if you want to use it to specialize on signed types.
|
||||
///
|
||||
/// Currently these are only implemented for integers up to usize due to
|
||||
/// correctness issues around ExactSizeIterator impls on 16bit platforms.
|
||||
/// And since ExactSizeIterator is a prerequisite for backwards iteration
|
||||
/// and we must consistently specialize backwards and forwards iteration
|
||||
/// that makes the situation complicated enough that it's not covered
|
||||
/// for now.
|
||||
macro_rules! spec_int_ranges {
|
||||
($($t:ty)*) => ($(
|
||||
|
||||
const _: () = assert!(usize::BITS >= <$t>::BITS);
|
||||
|
||||
impl SpecRangeSetup<Range<$t>> for Range<$t> {
|
||||
#[inline]
|
||||
fn setup(mut r: Range<$t>, step: usize) -> Range<$t> {
|
||||
let inner_len = r.size_hint().0;
|
||||
// If step exceeds $t::MAX, then the count will be at most 1 and
|
||||
// thus always fit into $t.
|
||||
let yield_count = inner_len.div_ceil(step);
|
||||
// Turn the range end into an iteration counter
|
||||
r.end = yield_count as $t;
|
||||
r
|
||||
}
|
||||
}
|
||||
|
||||
unsafe impl StepByImpl<Range<$t>> for StepBy<Range<$t>> {
|
||||
#[inline]
|
||||
fn spec_next(&mut self) -> Option<$t> {
|
||||
// if a step size larger than the type has been specified fall back to
|
||||
// t::MAX, in which case remaining will be at most 1.
|
||||
// The `+ 1` can't overflow since the constructor substracted 1 from the original value.
|
||||
let step = <$t>::try_from(self.step + 1).unwrap_or(<$t>::MAX);
|
||||
let remaining = self.iter.end;
|
||||
if remaining > 0 {
|
||||
let val = self.iter.start;
|
||||
// this can only overflow during the last step, after which the value
|
||||
// will not be used
|
||||
self.iter.start = val.wrapping_add(step);
|
||||
self.iter.end = remaining - 1;
|
||||
Some(val)
|
||||
} else {
|
||||
None
|
||||
}
|
||||
}
|
||||
|
||||
#[inline]
|
||||
fn spec_size_hint(&self) -> (usize, Option<usize>) {
|
||||
let remaining = self.iter.end as usize;
|
||||
(remaining, Some(remaining))
|
||||
}
|
||||
|
||||
// The methods below are all copied from the Iterator trait default impls.
|
||||
// We have to repeat them here so that the specialization overrides the StepByImpl defaults
|
||||
|
||||
#[inline]
|
||||
fn spec_nth(&mut self, n: usize) -> Option<Self::Item> {
|
||||
self.advance_by(n).ok()?;
|
||||
self.next()
|
||||
}
|
||||
|
||||
#[inline]
|
||||
fn spec_try_fold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
|
||||
where
|
||||
F: FnMut(Acc, Self::Item) -> R,
|
||||
R: Try<Output = Acc>
|
||||
{
|
||||
let mut accum = init;
|
||||
while let Some(x) = self.next() {
|
||||
accum = f(accum, x)?;
|
||||
}
|
||||
try { accum }
|
||||
}
|
||||
|
||||
#[inline]
|
||||
fn spec_fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
|
||||
where
|
||||
F: FnMut(Acc, Self::Item) -> Acc
|
||||
{
|
||||
// if a step size larger than the type has been specified fall back to
|
||||
// t::MAX, in which case remaining will be at most 1.
|
||||
let step = <$t>::try_from(self.step + 1).unwrap_or(<$t>::MAX);
|
||||
let remaining = self.iter.end;
|
||||
let mut acc = init;
|
||||
let mut val = self.iter.start;
|
||||
for _ in 0..remaining {
|
||||
acc = f(acc, val);
|
||||
// this can only overflow during the last step, after which the value
|
||||
// will no longer be used
|
||||
val = val.wrapping_add(step);
|
||||
}
|
||||
acc
|
||||
}
|
||||
}
|
||||
|
||||
/// Safety: This macro is only applied to ranges over types <= usize
|
||||
/// which means the inner length is guaranteed to fit into a usize and so
|
||||
/// the outer length calculation won't encounter clamped values
|
||||
#[unstable(feature = "trusted_len", issue = "37572")]
|
||||
unsafe impl TrustedLen for StepBy<Range<$t>> {}
|
||||
)*)
|
||||
}
|
||||
|
||||
macro_rules! spec_int_ranges_r {
|
||||
($($t:ty)*) => ($(
|
||||
const _: () = assert!(usize::BITS >= <$t>::BITS);
|
||||
|
||||
unsafe impl StepByBackImpl<Range<$t>> for StepBy<Range<$t>> {
|
||||
|
||||
#[inline]
|
||||
fn spec_next_back(&mut self) -> Option<Self::Item>
|
||||
where Range<$t>: DoubleEndedIterator + ExactSizeIterator,
|
||||
{
|
||||
let step = (self.step + 1) as $t;
|
||||
let remaining = self.iter.end;
|
||||
if remaining > 0 {
|
||||
let start = self.iter.start;
|
||||
self.iter.end = remaining - 1;
|
||||
Some(start + step * (remaining - 1))
|
||||
} else {
|
||||
None
|
||||
}
|
||||
}
|
||||
|
||||
// The methods below are all copied from the Iterator trait default impls.
|
||||
// We have to repeat them here so that the specialization overrides the StepByImplBack defaults
|
||||
|
||||
#[inline]
|
||||
fn spec_nth_back(&mut self, n: usize) -> Option<Self::Item>
|
||||
where Self: DoubleEndedIterator,
|
||||
{
|
||||
if self.advance_back_by(n).is_err() {
|
||||
return None;
|
||||
}
|
||||
self.next_back()
|
||||
}
|
||||
|
||||
#[inline]
|
||||
fn spec_try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
|
||||
where
|
||||
Self: DoubleEndedIterator,
|
||||
F: FnMut(Acc, Self::Item) -> R,
|
||||
R: Try<Output = Acc>
|
||||
{
|
||||
let mut accum = init;
|
||||
while let Some(x) = self.next_back() {
|
||||
accum = f(accum, x)?;
|
||||
}
|
||||
try { accum }
|
||||
}
|
||||
|
||||
#[inline]
|
||||
fn spec_rfold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
|
||||
where
|
||||
Self: DoubleEndedIterator,
|
||||
F: FnMut(Acc, Self::Item) -> Acc
|
||||
{
|
||||
let mut accum = init;
|
||||
while let Some(x) = self.next_back() {
|
||||
accum = f(accum, x);
|
||||
}
|
||||
accum
|
||||
}
|
||||
}
|
||||
)*)
|
||||
}
|
||||
|
||||
#[cfg(target_pointer_width = "64")]
|
||||
spec_int_ranges!(u8 u16 u32 u64 usize);
|
||||
// DoubleEndedIterator requires ExactSizeIterator, which isn't implemented for Range<u64>
|
||||
#[cfg(target_pointer_width = "64")]
|
||||
spec_int_ranges_r!(u8 u16 u32 usize);
|
||||
|
||||
#[cfg(target_pointer_width = "32")]
|
||||
spec_int_ranges!(u8 u16 u32 usize);
|
||||
#[cfg(target_pointer_width = "32")]
|
||||
spec_int_ranges_r!(u8 u16 u32 usize);
|
||||
|
||||
#[cfg(target_pointer_width = "16")]
|
||||
spec_int_ranges!(u8 u16 usize);
|
||||
#[cfg(target_pointer_width = "16")]
|
||||
spec_int_ranges_r!(u8 u16 usize);
|
||||
|
||||
@ -244,3 +244,58 @@ fn test_step_by_skip() {
|
||||
assert_eq!((0..=50).step_by(10).nth(3), Some(30));
|
||||
assert_eq!((200..=255u8).step_by(10).nth(3), Some(230));
|
||||
}
|
||||
|
||||
|
||||
struct DeOpt<I: Iterator>(I);
|
||||
|
||||
impl<I: Iterator> Iterator for DeOpt<I> {
|
||||
type Item = I::Item;
|
||||
|
||||
fn next(&mut self) -> core::option::Option<Self::Item> {
|
||||
self.0.next()
|
||||
}
|
||||
}
|
||||
|
||||
impl<I: DoubleEndedIterator> DoubleEndedIterator for DeOpt<I> {
|
||||
fn next_back(&mut self) -> core::option::Option<Self::Item> {
|
||||
self.0.next_back()
|
||||
}
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn test_step_by_fold_range_specialization() {
|
||||
macro_rules! t {
|
||||
($range:expr, $var: ident, $body:tt) => {
|
||||
{
|
||||
// run the same tests for the non-optimized version
|
||||
let mut $var = DeOpt($range);
|
||||
$body
|
||||
}
|
||||
{
|
||||
let mut $var = $range;
|
||||
$body
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
t!((1usize..5).step_by(1), r, {
|
||||
assert_eq!(r.next_back(), Some(4));
|
||||
assert_eq!(r.sum::<usize>(), 6);
|
||||
});
|
||||
|
||||
t!((0usize..4).step_by(2), r, {
|
||||
assert_eq!(r.next(), Some(0));
|
||||
assert_eq!(r.sum::<usize>(), 2);
|
||||
});
|
||||
|
||||
|
||||
t!((0usize..5).step_by(2), r, {
|
||||
assert_eq!(r.next(), Some(0));
|
||||
assert_eq!(r.sum::<usize>(), 6);
|
||||
});
|
||||
|
||||
t!((usize::MAX - 6 .. usize::MAX).step_by(5), r, {
|
||||
assert_eq!(r.next(), Some(usize::MAX - 6));
|
||||
assert_eq!(r.sum::<usize>(), usize::MAX - 1);
|
||||
});
|
||||
}
|
||||
|
||||
Loading…
Reference in New Issue
Block a user