optimize slice::ptr_rotate for compile-time-constant small rotates

1 files changed, 166 insertions, 161 deletions

diff --git a/library/core/src/slice/rotate.rs b/library/core/src/slice/rotate.rs
index d8e0acb565c..20833dc31aa 100644
--- a/library/core/src/slice/rotate.rs
+++ b/library/core/src/slice/rotate.rs
@@ -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
@@ -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.
@@ -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;
             }
         }
     }