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optimize slice::ptr_rotate for compile-time-constant small rotates
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@edwloef
edwloef authored and gitbot committed Feb 20, 2025
1 parent 6127357 commit 4b82818
Showing 1 changed file with 166 additions and 161 deletions.
327 changes: 166 additions & 161 deletions core/src/slice/rotate.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,11 +11,18 @@ use crate::{cmp, ptr};
///
/// # Algorithm
///
/// 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:
/// Algorithm 1 is used if `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 2 is otherwise 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
/// left = 10, right = 6
/// the `^` indicates an element in its final place
Expand All @@ -39,13 +46,7 @@ use crate::{cmp, ptr};
/// `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
/// Algorithm 2 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.
Expand All @@ -65,172 +66,176 @@ pub(super) unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right:
if T::IS_ZST {
return;
}
loop {
// N.B. the below algorithms can fail if these cases are not checked
if (right == 0) || (left == 0) {
return;
// N.B. the below algorithms can fail if these cases are not checked
if (right == 0) || (left == 0) {
return;
}
// `T` is not a zero-sized type, so it's okay to divide by its size.
if !cfg!(feature = "optimize_for_size")
&& cmp::min(left, right) <= mem::size_of::<BufType>() / mem::size_of::<T>()
{
// Algorithm 1
// 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;
// SAFETY: `mid-left <= mid-left+right < mid+right`
let dim = unsafe { mid.sub(left).add(right) };
if left <= right {
// SAFETY:
//
// 1) The `if` condition about the sizes ensures `[mid-left; left]` will fit in
// `buf` without overflow and `buf` was created just above and so cannot be
// overlapped with any value of `[mid-left; left]`
// 2) [mid-left, mid+right) are all valid for reading and writing and we don't care
// about overlaps here.
// 3) The `if` condition about `left <= right` ensures writing `left` elements to
// `dim = mid-left+right` is valid because:
// - `buf` is valid and `left` elements were written in it in 1)
// - `dim+left = mid-left+right+left = mid+right` and we write `[dim, dim+left)`
unsafe {
// 1)
ptr::copy_nonoverlapping(mid.sub(left), buf, left);
// 2)
ptr::copy(mid, mid.sub(left), right);
// 3)
ptr::copy_nonoverlapping(buf, dim, left);
}
} else {
// SAFETY: same reasoning as above but with `left` and `right` reversed
unsafe {
ptr::copy_nonoverlapping(mid, buf, right);
ptr::copy(mid.sub(left), dim, left);
ptr::copy_nonoverlapping(buf, mid.sub(left), right);
}
}
if !cfg!(feature = "optimize_for_size")
&& ((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.
// SAFETY: callers must ensure `mid - left` is valid for reading and writing.
let x = unsafe { mid.sub(left) };
// beginning of first round
// SAFETY: see previous comment.
let mut tmp: T = unsafe { 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.
} else if !cfg!(feature = "optimize_for_size")
&& ((left + right < 24) || (mem::size_of::<T>() > mem::size_of::<[usize; 4]>()))
{
// Algorithm 2
// 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.
// SAFETY: callers must ensure `mid - left` is valid for reading and writing.
let x = unsafe { mid.sub(left) };
// beginning of first round
// SAFETY: see previous comment.
let mut tmp: T = unsafe { 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 {
// [long-safety-expl]
// SAFETY: callers must ensure `[left, left+mid+right)` are all valid for reading and
// writing.
//
// - `i` start with `right` so `mid-left <= x+i = x+right = mid-left+right < mid+right`
// - `i <= left+right-1` is always true
// - if `i < left`, `right` is added so `i < left+right` and on the next
// iteration `left` is removed from `i` so it doesn't go further
// - if `i >= left`, `left` is removed immediately and so it doesn't go further.
// - overflows cannot happen for `i` since the function's safety contract ask for
// `mid+right-1 = x+left+right` to be valid for writing
// - underflows cannot happen because `i` must be bigger or equal to `left` for
// a subtraction of `left` to happen.
//
// So `x+i` is valid for reading and writing if the caller respected the contract
tmp = unsafe { 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
// SAFETY: tmp has been read from a valid source and x is valid for writing
// according to the caller.
unsafe { 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 {
// SAFETY: `gcd` is at most equal to `right` so all values in `1..gcd` are valid for
// reading and writing as per the function's safety contract, see [long-safety-expl]
// above
tmp = unsafe { x.add(start).read() };
// [safety-expl-addition]
//
// Here `start < gcd` so `start < right` so `i < right+right`: `right` being the
// greatest common divisor of `(left+right, right)` means that `left = right` so
// `i < left+right` so `x+i = mid-left+i` is always valid for reading and writing
// according to the function's safety contract.
i = start + right;
loop {
// [long-safety-expl]
// SAFETY: callers must ensure `[left, left+mid+right)` are all valid for reading and
// writing.
//
// - `i` start with `right` so `mid-left <= x+i = x+right = mid-left+right < mid+right`
// - `i <= left+right-1` is always true
// - if `i < left`, `right` is added so `i < left+right` and on the next
// iteration `left` is removed from `i` so it doesn't go further
// - if `i >= left`, `left` is removed immediately and so it doesn't go further.
// - overflows cannot happen for `i` since the function's safety contract ask for
// `mid+right-1 = x+left+right` to be valid for writing
// - underflows cannot happen because `i` must be bigger or equal to `left` for
// a subtraction of `left` to happen.
//
// So `x+i` is valid for reading and writing if the caller respected the contract
// SAFETY: see [long-safety-expl] and [safety-expl-addition]
tmp = unsafe { 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
// SAFETY: tmp has been read from a valid source and x is valid for writing
// according to the caller.
unsafe { x.write(tmp) };
if i == start {
// SAFETY: see [long-safety-expl] and [safety-expl-addition]
unsafe { x.add(start).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 {
// SAFETY: `gcd` is at most equal to `right` so all values in `1..gcd` are valid for
// reading and writing as per the function's safety contract, see [long-safety-expl]
// above
tmp = unsafe { x.add(start).read() };
// [safety-expl-addition]
//
// Here `start < gcd` so `start < right` so `i < right+right`: `right` being the
// greatest common divisor of `(left+right, right)` means that `left = right` so
// `i < left+right` so `x+i = mid-left+i` is always valid for reading and writing
// according to the function's safety contract.
i = start + right;
}
} else {
loop {
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 {
// SAFETY: see [long-safety-expl] and [safety-expl-addition]
tmp = unsafe { x.add(i).replace(tmp) };
if i >= left {
i -= left;
if i == start {
// SAFETY: see [long-safety-expl] and [safety-expl-addition]
unsafe { x.add(start).write(tmp) };
break;
}
} else {
i += right;
// SAFETY:
// `left >= right` so `[mid-right, mid+right)` is valid for reading and writing
// Subtracting `right` from `mid` each turn is counterbalanced by the addition and
// check after it.
unsafe {
ptr::swap_nonoverlapping(mid.sub(right), mid, right);
mid = mid.sub(right);
}
left -= right;
if left < right {
break;
}
}
}
return;
// `T` is not a zero-sized type, so it's okay to divide by its size.
} else if !cfg!(feature = "optimize_for_size")
&& 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;
// SAFETY: `mid-left <= mid-left+right < mid+right`
let dim = unsafe { mid.sub(left).add(right) };
if left <= right {
// SAFETY:
//
// 1) The `else if` condition about the sizes ensures `[mid-left; left]` will fit in
// `buf` without overflow and `buf` was created just above and so cannot be
// overlapped with any value of `[mid-left; left]`
// 2) [mid-left, mid+right) are all valid for reading and writing and we don't care
// about overlaps here.
// 3) The `if` condition about `left <= right` ensures writing `left` elements to
// `dim = mid-left+right` is valid because:
// - `buf` is valid and `left` elements were written in it in 1)
// - `dim+left = mid-left+right+left = mid+right` and we write `[dim, dim+left)`
unsafe {
// 1)
ptr::copy_nonoverlapping(mid.sub(left), buf, left);
// 2)
ptr::copy(mid, mid.sub(left), right);
// 3)
ptr::copy_nonoverlapping(buf, dim, left);
}
} else {
// SAFETY: same reasoning as above but with `left` and `right` reversed
unsafe {
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 {
// SAFETY:
// `left >= right` so `[mid-right, mid+right)` is valid for reading and writing
// Subtracting `right` from `mid` each turn is counterbalanced by the addition and
// check after it.
unsafe {
ptr::swap_nonoverlapping(mid.sub(right), mid, right);
mid = mid.sub(right);
}
left -= right;
if left < right {
break;
// Algorithm 3, `left < right`
loop {
// SAFETY: `[mid-left, mid+left)` is valid for reading and writing because
// `left < right` so `mid+left < mid+right`.
// Adding `left` to `mid` each turn is counterbalanced by the subtraction and check
// after it.
unsafe {
ptr::swap_nonoverlapping(mid.sub(left), mid, left);
mid = mid.add(left);
}
right -= left;
if right < left {
break;
}
}
}
} else {
// Algorithm 3, `left < right`
loop {
// SAFETY: `[mid-left, mid+left)` is valid for reading and writing because
// `left < right` so `mid+left < mid+right`.
// Adding `left` to `mid` each turn is counterbalanced by the subtraction and check
// after it.
unsafe {
ptr::swap_nonoverlapping(mid.sub(left), mid, left);
mid = mid.add(left);
}
right -= left;
if right < left {
break;
}

if (right == 0) || (left == 0) {
return;
}
}
}
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