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Improve ptr_rotate performance, tests, and benchmarks
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@AaronKutch
AaronKutch committed Aug 6, 2019
1 parent 890881f commit ad7fdb6
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26 changes: 26 additions & 0 deletions src/libcore/benches/slice.rs
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Expand Up @@ -55,3 +55,29 @@ fn binary_search_l2_with_dups(b: &mut Bencher) {
fn binary_search_l3_with_dups(b: &mut Bencher) {
binary_search(b, Cache::L3, |i| i / 16 * 16);
}

macro_rules! rotate {
($fn:ident, $n:expr, $mapper:expr) => {
#[bench]
fn $fn(b: &mut Bencher) {
let mut x = (0usize..$n).map(&$mapper).collect::<Vec<_>>();
b.iter(|| {
for s in 0..x.len() {
x[..].rotate_right(s);
}
black_box(x[0].clone())
})
}
};
}

#[derive(Clone)]
struct Rgb(u8, u8, u8);

rotate!(rotate_u8, 32, |i| i as u8);
rotate!(rotate_rgb, 32, |i| Rgb(i as u8, (i as u8).wrapping_add(7), (i as u8).wrapping_add(42)));
rotate!(rotate_usize, 32, |i| i);
rotate!(rotate_16_usize_4, 16, |i| [i; 4]);
rotate!(rotate_16_usize_5, 16, |i| [i; 5]);
rotate!(rotate_64_usize_4, 64, |i| [i; 4]);
rotate!(rotate_64_usize_5, 64, |i| [i; 5]);
221 changes: 152 additions & 69 deletions src/libcore/slice/rotate.rs
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Expand Up @@ -2,88 +2,171 @@ use crate::cmp;
use crate::mem::{self, MaybeUninit};
use crate::ptr;

/// Rotation is much faster if it has access to a little bit of memory. This
/// union provides a RawVec-like interface, but to a fixed-size stack buffer.
#[allow(unions_with_drop_fields)]
union RawArray<T> {
/// Ensure this is appropriately aligned for T, and is big
/// enough for two elements even if T is enormous.
typed: [T; 2],
/// For normally-sized types, especially things like u8, having more
/// than 2 in the buffer is necessary for usefulness, so pad it out
/// enough to be helpful, but not so big as to risk overflow.
_extra: [usize; 32],
}

impl<T> RawArray<T> {
fn capacity() -> usize {
if mem::size_of::<T>() == 0 {
usize::max_value()
} else {
mem::size_of::<Self>() / mem::size_of::<T>()
}
}
}

/// Rotates the range `[mid-left, mid+right)` such that the element at `mid`
/// becomes the first element. Equivalently, rotates the range `left`
/// elements to the left or `right` elements to the right.
/// Rotates the range `[mid-left, mid+right)` such that the element at `mid` becomes the first
/// element. Equivalently, rotates the range `left` elements to the left or `right` elements to the
/// right.
///
/// # Safety
///
/// The specified range must be valid for reading and writing.
///
/// # Algorithm
///
/// For longer rotations, swap the left-most `delta = min(left, right)`
/// elements with the right-most `delta` elements. LLVM vectorizes this,
/// which is profitable as we only reach this step for a "large enough"
/// rotation. Doing this puts `delta` elements on the larger side into the
/// correct position, leaving a smaller rotate problem. Demonstration:
///
/// Algorithm 1 is used for small values of `left + right` or for large `T`. The elements are moved
/// into their final positions one at a time starting at `mid - left` and advancing by `right` steps
/// modulo `left + right`, such that only one temporary is needed. Eventually, we arrive back at
/// `mid - left`. However, if `gcd(left + right, right)` is not 1, the above steps skipped over
/// elements. For example:
/// ```text
/// [ 6 7 8 9 10 11 12 13 . 1 2 3 4 5 ]
/// 1 2 3 4 5 [ 11 12 13 . 6 7 8 9 10 ]
/// 1 2 3 4 5 [ 8 9 10 . 6 7 ] 11 12 13
/// 1 2 3 4 5 6 7 [ 10 . 8 9 ] 11 12 13
/// 1 2 3 4 5 6 7 [ 9 . 8 ] 10 11 12 13
/// 1 2 3 4 5 6 7 8 [ . ] 9 10 11 12 13
/// left = 10, right = 6
/// the `^` indicates an element in its final place
/// 6 7 8 9 10 11 12 13 14 15 . 0 1 2 3 4 5
/// after using one step of the above algorithm (The X will be overwritten at the end of the round,
/// and 12 is stored in a temporary):
/// X 7 8 9 10 11 6 13 14 15 . 0 1 2 3 4 5
/// ^
/// after using another step (now 2 is in the temporary):
/// X 7 8 9 10 11 6 13 14 15 . 0 1 12 3 4 5
/// ^ ^
/// after the third step (the steps wrap around, and 8 is in the temporary):
/// X 7 2 9 10 11 6 13 14 15 . 0 1 12 3 4 5
/// ^ ^ ^
/// after 7 more steps, the round ends with the temporary 0 getting put in the X:
/// 0 7 2 9 4 11 6 13 8 15 . 10 1 12 3 14 5
/// ^ ^ ^ ^ ^ ^ ^ ^
/// ```
/// Fortunately, the number of skipped over elements between finalized elements is always equal, so
/// we can just offset our starting position and do more rounds (the total number of rounds is the
/// `gcd(left + right, right)` value). The end result is that all elements are finalized once and
/// only once.
///
/// Algorithm 2 is used if `left + right` is large but `min(left, right)` is small enough to
/// fit onto a stack buffer. The `min(left, right)` elements are copied onto the buffer, `memmove`
/// is applied to the others, and the ones on the buffer are moved back into the hole on the
/// opposite side of where they originated.
///
/// Algorithms that can be vectorized outperform the above once `left + right` becomes large enough.
/// Algorithm 1 can be vectorized by chunking and performing many rounds at once, but there are too
/// few rounds on average until `left + right` is enormous, and the worst case of a single
/// round is always there. Instead, algorithm 3 utilizes repeated swapping of
/// `min(left, right)` elements until a smaller rotate problem is left.
///
/// Once the rotation is small enough, copy some elements into a stack
/// buffer, `memmove` the others, and move the ones back from the buffer.
pub unsafe fn ptr_rotate<T>(mut left: usize, mid: *mut T, mut right: usize) {
/// ```text
/// left = 11, right = 4
/// [4 5 6 7 8 9 10 11 12 13 14 . 0 1 2 3]
/// ^ ^ ^ ^ ^ ^ ^ ^ swapping the right most elements with elements to the left
/// [4 5 6 7 8 9 10 . 0 1 2 3] 11 12 13 14
/// ^ ^ ^ ^ ^ ^ ^ ^ swapping these
/// [4 5 6 . 0 1 2 3] 7 8 9 10 11 12 13 14
/// we cannot swap any more, but a smaller rotation problem is left to solve
/// ```
/// when `left < right` the swapping happens from the left instead.
pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize) {
type BufType = [usize; 32];
if mem::size_of::<T>() == 0 {
return;
}
loop {
let delta = cmp::min(left, right);
if delta <= RawArray::<T>::capacity() {
// We will always hit this immediately for ZST.
break;
// N.B. the below algorithms can fail if these cases are not checked
if (right == 0) || (left == 0) {
return;
}

ptr::swap_nonoverlapping(
mid.sub(left),
mid.add(right - delta),
delta);

if left <= right {
right -= delta;
if (left + right < 24) || (mem::size_of::<T>() > mem::size_of::<[usize; 4]>()) {
// Algorithm 1
// Microbenchmarks indicate that the average performance for random shifts is better all
// the way until about `left + right == 32`, but the worst case performance breaks even
// around 16. 24 was chosen as middle ground. If the size of `T` is larger than 4
// `usize`s, this algorithm also outperforms other algorithms.
let x = mid.sub(left);
// beginning of first round
let mut tmp: T = x.read();
let mut i = right;
// `gcd` can be found before hand by calculating `gcd(left + right, right)`,
// but it is faster to do one loop which calculates the gcd as a side effect, then
// doing the rest of the chunk
let mut gcd = right;
// benchmarks reveal that it is faster to swap temporaries all the way through instead
// of reading one temporary once, copying backwards, and then writing that temporary at
// the very end. This is possibly due to the fact that swapping or replacing temporaries
// uses only one memory address in the loop instead of needing to manage two.
loop {
tmp = x.add(i).replace(tmp);
// instead of incrementing `i` and then checking if it is outside the bounds, we
// check if `i` will go outside the bounds on the next increment. This prevents
// any wrapping of pointers or `usize`.
if i >= left {
i -= left;
if i == 0 {
// end of first round
x.write(tmp);
break;
}
// this conditional must be here if `left + right >= 15`
if i < gcd {
gcd = i;
}
} else {
i += right;
}
}
// finish the chunk with more rounds
for start in 1..gcd {
tmp = x.add(start).read();
i = start + right;
loop {
tmp = x.add(i).replace(tmp);
if i >= left {
i -= left;
if i == start {
x.add(start).write(tmp);
break;
}
} else {
i += right;
}
}
}
return;
// `T` is not a zero-sized type, so it's okay to divide by its size.
} else if cmp::min(left, right) <= mem::size_of::<BufType>() / mem::size_of::<T>() {
// Algorithm 2
// The `[T; 0]` here is to ensure this is appropriately aligned for T
let mut rawarray = MaybeUninit::<(BufType, [T; 0])>::uninit();
let buf = rawarray.as_mut_ptr() as *mut T;
let dim = mid.sub(left).add(right);
if left <= right {
ptr::copy_nonoverlapping(mid.sub(left), buf, left);
ptr::copy(mid, mid.sub(left), right);
ptr::copy_nonoverlapping(buf, dim, left);
} else {
ptr::copy_nonoverlapping(mid, buf, right);
ptr::copy(mid.sub(left), dim, left);
ptr::copy_nonoverlapping(buf, mid.sub(left), right);
}
return;
} else if left >= right {
// Algorithm 3
// There is an alternate way of swapping that involves finding where the last swap
// of this algorithm would be, and swapping using that last chunk instead of swapping
// adjacent chunks like this algorithm is doing, but this way is still faster.
loop {
ptr::swap_nonoverlapping(mid.sub(right), mid, right);
mid = mid.sub(right);
left -= right;
if left < right {
break;
}
}
} else {
left -= delta;
// Algorithm 3, `left < right`
loop {
ptr::swap_nonoverlapping(mid.sub(left), mid, left);
mid = mid.add(left);
right -= left;
if right < left {
break;
}
}
}
}

let mut rawarray = MaybeUninit::<RawArray<T>>::uninit();
let buf = &mut (*rawarray.as_mut_ptr()).typed as *mut [T; 2] as *mut T;

let dim = mid.sub(left).add(right);
if left <= right {
ptr::copy_nonoverlapping(mid.sub(left), buf, left);
ptr::copy(mid, mid.sub(left), right);
ptr::copy_nonoverlapping(buf, dim, left);
}
else {
ptr::copy_nonoverlapping(mid, buf, right);
ptr::copy(mid.sub(left), dim, left);
ptr::copy_nonoverlapping(buf, mid.sub(left), right);
}
}
38 changes: 38 additions & 0 deletions src/libcore/tests/slice.rs
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Expand Up @@ -1130,6 +1130,44 @@ fn test_rotate_right() {
}
}

#[test]
#[cfg(not(miri))]
fn brute_force_rotate_test_0() {
// In case of edge cases involving multiple algorithms
let n = 300;
for len in 0..n {
for s in 0..len {
let mut v = Vec::with_capacity(len);
for i in 0..len {
v.push(i);
}
v[..].rotate_right(s);
for i in 0..v.len() {
assert_eq!(v[i], v.len().wrapping_add(i.wrapping_sub(s)) % v.len());
}
}
}
}

#[test]
fn brute_force_rotate_test_1() {
// `ptr_rotate` covers so many kinds of pointer usage, that this is just a good test for
// pointers in general. This uses a `[usize; 4]` to hit all algorithms without overwhelming miri
let n = 30;
for len in 0..n {
for s in 0..len {
let mut v: Vec<[usize; 4]> = Vec::with_capacity(len);
for i in 0..len {
v.push([i, 0, 0, 0]);
}
v[..].rotate_right(s);
for i in 0..v.len() {
assert_eq!(v[i][0], v.len().wrapping_add(i.wrapping_sub(s)) % v.len());
}
}
}
}

#[test]
#[cfg(not(target_arch = "wasm32"))]
fn sort_unstable() {
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